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A representation of the structure of myoglobin, showing alpha helices, represented by ribbons. This protein was the first to have its structure solved by X-ray crystallography by Max Perutz and John Kendrew in 1958, for which they received a Nobel Prize in Chemistry

A biomolecule or biological molecule is loosely defined as a molecule produced by a living organism and essential to one or more typically biological processes.[1] Biomolecules include large macromolecules such as proteins, carbohydrates, lipids, and nucleic acids, as well as small molecules such as vitamins and hormones. A general name for this class of material is biological materials. Biomolecules are an important element of living organisms. They are often endogenous,[2] i.e. produced within the organism,[3] but organisms usually also need exogenous biomolecules, for example certain nutrients, to survive.

Biomolecules and their reactions are studied in biology and its subfields of biochemistry and molecular biology. Most biomolecules are organic compounds, and just four elements—oxygen, carbon, hydrogen, and nitrogen—make up 96% of the human body's mass. But many other elements, such as the various biometals, are also present in small amounts.

The uniformity of both specific types of molecules (the biomolecules) and of certain metabolic pathways are invariant features among the wide diversity of life forms; thus these biomolecules and metabolic pathways are referred to as "biochemical universals"[4] or "theory of material unity of the living beings", a unifying concept in biology, along with cell theory and evolution theory.[5]

Types of biomolecules

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A diverse range of biomolecules exist, including:

Biomonomers Bio-oligo Biopolymers Polymerization process Covalent bond name between monomers
Amino acids Oligopeptides Polypeptides, proteins (hemoglobin...) Polycondensation Peptide bond
Monosaccharides Oligosaccharides Polysaccharides (cellulose...) Polycondensation Glycosidic bond
Isoprene Terpenes Polyterpenes: cis-1,4-polyisoprene natural rubber and trans-1,4-polyisoprene gutta-percha Polyaddition

Nucleosides and nucleotides

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Main articles: Nucleosides and Nucleotides

Nucleosides are molecules formed by attaching a nucleobase to a ribose or deoxyribose ring. Examples of these include cytidine (C), uridine (U), adenosine (A), guanosine (G), and thymidine (T).

Nucleosides can be phosphorylated by specific kinases in the cell, producing nucleotides. Both DNA and RNA are polymers, consisting of long, linear molecules assembled by polymerase enzymes from repeating structural units, or monomers, of mononucleotides. DNA uses the deoxynucleotides C, G, A, and T, while RNA uses the ribonucleotides (which have an extra hydroxyl(OH) group on the pentose ring) C, G, A, and U. Modified bases are fairly common (such as with methyl groups on the base ring), as found in ribosomal RNA or transfer RNAs or for discriminating the new from old strands of DNA after replication.[6]

Each nucleotide is made of an acyclic nitrogenous base, a pentose and one to three phosphate groups. They contain carbon, nitrogen, oxygen, hydrogen and phosphorus. They serve as sources of chemical energy (adenosine triphosphate and guanosine triphosphate), participate in cellular signaling (cyclic guanosine monophosphate and cyclic adenosine monophosphate), and are incorporated into important cofactors of enzymatic reactions (coenzyme A, flavin adenine dinucleotide, flavin mononucleotide, and nicotinamide adenine dinucleotide phosphate).[7]

DNA and RNA structure

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Main articles: DNA and Nucleic acid structure

DNA structure is dominated by the well-known double helix formed by Watson-Crick base-pairing of C with G and A with T. This is known as B-form DNA, and is overwhelmingly the most favorable and common state of DNA; its highly specific and stable base-pairing is the basis of reliable genetic information storage. DNA can sometimes occur as single strands (often needing to be stabilized by single-strand binding proteins) or as A-form or Z-form helices, and occasionally in more complex 3D structures such as the crossover at Holliday junctions during DNA replication.[7]

Stereo 3D image of a group I intron ribozyme (PDB file 1Y0Q); gray lines show base pairs; ribbon arrows show double-helix regions, blue to red from 5' to 3'[when defined as?] end; white ribbon is an RNA product.

RNA, in contrast, forms large and complex 3D tertiary structures reminiscent of proteins, as well as the loose single strands with locally folded regions that constitute messenger RNA molecules. Those RNA structures contain many stretches of A-form double helix, connected into definite 3D arrangements by single-stranded loops, bulges, and junctions.[8] Examples are tRNA, ribosomes, ribozymes, and riboswitches. These complex structures are facilitated by the fact that RNA backbone has less local flexibility than DNA but a large set of distinct conformations, apparently because of both positive and negative interactions of the extra OH on the ribose.[9] Structured RNA molecules can do highly specific binding of other molecules and can themselves be recognized specifically; in addition, they can perform enzymatic catalysis (when they are known as "ribozymes", as initially discovered by Tom Cech and colleagues).[10]

Saccharides

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Monosaccharides are the simplest form of carbohydrates with only one simple sugar. They essentially contain an aldehyde or ketone group in their structure.[11] The presence of an aldehyde group in a monosaccharide is indicated by the prefix aldo-. Similarly, a ketone group is denoted by the prefix keto-.[6] Examples of monosaccharides are the hexoses, glucose, fructose, Trioses, Tetroses, Heptoses, galactose, pentoses, ribose, and deoxyribose. Consumed fructose and glucose have different rates of gastric emptying, are differentially absorbed and have different metabolic fates, providing multiple opportunities for two different saccharides to differentially affect food intake.[11] Most saccharides eventually provide fuel for cellular respiration.

Disaccharides are formed when two monosaccharides, or two single simple sugars, form a bond with removal of water. They can be hydrolyzed to yield their saccharin building blocks by boiling with dilute acid or reacting them with appropriate enzymes.[6] Examples of disaccharides include sucrose, maltose, and lactose.

Polysaccharides are polymerized monosaccharides, or complex carbohydrates. They have multiple simple sugars. Examples are starch, cellulose, and glycogen. They are generally large and often have a complex branched connectivity. Because of their size, polysaccharides are not water-soluble, but their many hydroxy groups become hydrated individually when exposed to water, and some polysaccharides form thick colloidal dispersions when heated in water.[6] Shorter polysaccharides, with 3 to 10 monomers, are called oligosaccharides.[12] A fluorescent indicator-displacement molecular imprinting sensor was developed for discriminating saccharides. It successfully discriminated three brands of orange juice beverage.[13] The change in fluorescence intensity of the sensing films resulting is directly related to the saccharide concentration.[14]

Lignin

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Lignin is a complex polyphenolic macromolecule composed mainly of beta-O4-aryl linkages. After cellulose, lignin is the second most abundant biopolymer and is one of the primary structural components of most plants. It contains subunits derived from p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol,[15] and is unusual among biomolecules in that it is racemic. The lack of optical activity is due to the polymerization of lignin which occurs via free radical coupling reactions in which there is no preference for either configuration at a chiral center.

Lipid

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Lipids (oleaginous) are chiefly fatty acid esters, and are the basic building blocks of biological membranes. Another biological role is energy storage (e.g., triglycerides). Most lipids consist of a polar or hydrophilic head (typically glycerol) and one to three non polar or hydrophobic fatty acid tails, and therefore they are amphiphilic. Fatty acids consist of unbranched chains of carbon atoms that are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14–24 carbon groups long, but it is always an even number.

For lipids present in biological membranes, the hydrophilic head is from one of three classes:

  • Glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide residues.
  • Phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
  • Sterols, whose heads contain a planar steroid ring, for example, cholesterol.

Other lipids include prostaglandins and leukotrienes which are both 20-carbon fatty acyl units synthesized from arachidonic acid. They are also known as fatty acids

Amino acids

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Amino acids contain both amino and carboxylic acid functional groups. (In biochemistry, the term amino acid is used when referring to those amino acids in which the amino and carboxylate functionalities are attached to the same carbon, plus proline which is not actually an amino acid).

Modified amino acids are sometimes observed in proteins; this is usually the result of enzymatic modification after translation (protein synthesis). For example, phosphorylation of serine by kinases and dephosphorylation by phosphatases is an important control mechanism in the cell cycle. Only two amino acids other than the standard twenty are known to be incorporated into proteins during translation, in certain organisms:

Besides those used in protein synthesis, other biologically important amino acids include carnitine (used in lipid transport within a cell), ornithine, GABA and taurine.

Protein structure

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Main articles: Protein structure, Protein primary structure, Protein secondary structure, Protein tertiary structure, and Protein quaternary structure

The particular series of amino acids that form a protein is known as that protein's primary structure. This sequence is determined by the genetic makeup of the individual. It specifies the order of side-chain groups along the linear polypeptide "backbone".

Proteins have two types of well-classified, frequently occurring elements of local structure defined by a particular pattern of hydrogen bonds along the backbone: alpha helix and beta sheet. Their number and arrangement is called the secondary structure of the protein. Alpha helices are regular spirals stabilized by hydrogen bonds between the backbone CO group (carbonyl) of one amino acid residue and the backbone NH group (amide) of the i+4 residue. The spiral has about 3.6 amino acids per turn, and the amino acid side chains stick out from the cylinder of the helix. Beta pleated sheets are formed by backbone hydrogen bonds between individual beta strands each of which is in an "extended", or fully stretched-out, conformation. The strands may lie parallel or antiparallel to each other, and the side-chain direction alternates above and below the sheet. Hemoglobin contains only helices, natural silk is formed of beta pleated sheets, and many enzymes have a pattern of alternating helices and beta-strands. The secondary-structure elements are connected by "loop" or "coil" regions of non-repetitive conformation, which are sometimes quite mobile or disordered but usually adopt a well-defined, stable arrangement.[16]

The overall, compact, 3D structure of a protein is termed its tertiary structure or its "fold". It is formed as result of various attractive forces like hydrogen bonding, disulfide bridges, hydrophobic interactions, hydrophilic interactions, van der Waals force etc.

When two or more polypeptide chains (either of identical or of different sequence) cluster to form a protein, quaternary structure of protein is formed. Quaternary structure is an attribute of polymeric (same-sequence chains) or heteromeric (different-sequence chains) proteins like hemoglobin, which consists of two "alpha" and two "beta" polypeptide chains.

Apoenzymes

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An apoenzyme (or, generally, an apoprotein) is the protein without any small-molecule cofactors, substrates, or inhibitors bound. It is often important as an inactive storage, transport, or secretory form of a protein. This is required, for instance, to protect the secretory cell from the activity of that protein. Apoenzymes become active enzymes on addition of a cofactor. Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds, (e.g., [Flavin group|flavin] and heme). Organic cofactors can be either prosthetic groups, which are tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site during the reaction.

Isoenzymes

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Isoenzymes, or isozymes, are multiple forms of an enzyme, with slightly different protein sequence and closely similar but usually not identical functions. They are either products of different genes, or else different products of alternative splicing. They may either be produced in different organs or cell types to perform the same function, or several isoenzymes may be produced in the same cell type under differential regulation to suit the needs of changing development or environment. LDH (lactate dehydrogenase) has multiple isozymes, while fetal hemoglobin is an example of a developmentally regulated isoform of a non-enzymatic protein. The relative levels of isoenzymes in blood can be used to diagnose problems in the organ of secretion .

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Biomolecule

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Biomolecules, also known as biological molecules, are large organic macromolecules composed primarily of , , , and often , , or , that are produced by living organisms and are essential for cellular structure, function, and regulation. These molecules are built from smaller organic subunits through processes like , enabling the diverse and complex structures necessary for life. They are broadly classified into four major classes: carbohydrates, , proteins, and nucleic acids, each playing distinct yet interconnected roles in biological systems. Carbohydrates, often called saccharides, serve as primary energy sources for cells through molecules like glucose and provide structural support, such as in cellulose for plant cell walls or chitin in fungal and arthropod exoskeletons. Lipids, including fats, phospholipids, and steroids, function in long-term energy storage, form the hydrophobic barriers of cell membranes, and act as signaling molecules like hormones. Proteins, constructed from amino acid chains, exhibit remarkable versatility as enzymes that catalyze biochemical reactions, structural components like collagen, transporters such as hemoglobin, and regulatory elements including antibodies. Nucleic acids, namely DNA and RNA, store and transmit genetic information, with DNA maintaining the hereditary blueprint in the nucleus and RNA facilitating protein synthesis and other cellular processes. The study of biomolecules reveals their —from primary sequences to complex three-dimensional structures—that dictates function, influencing everything from metabolic pathways to disease mechanisms. Advances in techniques like and have elucidated these structures, underscoring biomolecules' role as that drive all aspects of life. Understanding their interactions is fundamental to fields like biochemistry, medicine, and , where disruptions in biomolecular function contribute to conditions such as metabolic disorders or genetic diseases.

Overview

Definition and Characteristics

Biomolecules are organic molecules produced by living organisms that are essential for maintaining processes, serving as the building blocks and functional components of cells. These molecules are primarily composed of carbon, , oxygen, , , and , which account for the vast majority of an organism's dry mass. A key characteristic of biomolecules is their distinction between small molecules and macromolecules based on molecular weight. Small biomolecules, such as vitamins and hormones, typically have low molecular weights (often 100–1000 daltons) and act as precursors, cofactors, or signaling agents in metabolic pathways. In contrast, macromolecules like proteins and nucleic acids have large molecular weights (thousands to millions of daltons) and form complex polymers that dominate cellular structure and function. Biomolecules often exhibit , where carbon atoms with four different substituents create asymmetric centers, leading to enantiomers; biological systems selectively utilize specific forms, such as L-amino acids in proteins and D-sugars in carbohydrates. Many biomolecules also display amphipathicity, featuring both polar (hydrophilic) and nonpolar (hydrophobic) regions that influence their interactions in aqueous environments. Additionally, they demonstrate stability under physiological conditions ( 6–8, 37°C) due to robust covalent bonds that resist or degradation. In terms of chemical properties, biomolecules vary in polarity, with polar groups like hydroxyl or amino enabling hydrogen bonding and hydrophilic solubility in , while nonpolar chains confer hydrophobicity. Their reactivity facilitates the formation of specific covalent linkages, such as peptide bonds between or glycosidic bonds between sugars, enabling and diverse functionalities. The major classes—nucleic acids, carbohydrates, , and proteins—exemplify this chemical diversity in supporting life's complexity.

Classification and Functions

Biomolecules are primarily classified into four main classes based on their and structure: nucleic acids, which are polymers of nucleotides; carbohydrates, defined as polyhydroxy aldehydes or ketones or compounds that yield such units upon ; , a diverse group of hydrophobic molecules not classified as polymers; and proteins, polymers of . This classification reflects their elemental makeup, primarily , , , , and , with variations in bonding and functional groups determining their properties. Classification can also occur by function, such as informational roles in nucleic acids for genetic storage and transfer, structural roles in proteins and carbohydrates for support in tissues and cell walls, catalytic roles in enzymes (proteins) for accelerating reactions, and energy storage in and carbohydrates. Additionally, biomolecules are categorized by , distinguishing monomers like or monosaccharides from polymers such as proteins or , where large macromolecules form through covalent linkages of smaller units. A hierarchical approach to begins with the primary four classes, then extends to secondary subtypes within them; for instance, proteins are subdivided into globular forms, which are compact and often soluble for enzymatic or transport functions, and fibrous forms, which are elongated for mechanical strength. In biological systems, these classes fulfill essential roles: nucleic acids enable information transfer from DNA to RNA to proteins; carbohydrates and lipids provide energy storage, with lipids yielding more energy per gram; proteins act in catalysis via enzymes and offer structural support; and carbohydrates contribute to cell wall integrity in plants and microbes. Biomolecules represent products of , with many core structures, such as protein folds and sequences, conserved across due to their critical roles in and , underscoring shared evolutionary ancestry.

Nucleic Acids

Nucleosides and

Nucleosides are organic molecules composed of a nitrogenous base covalently linked to a sugar through an N-glycosidic bond at the 1' carbon of the sugar. The nitrogenous bases are heterocyclic compounds classified as either purines, which have a fused double-ring structure, or pyrimidines, which have a single six-membered ring. The sugar component is either β-D-ribofuranose () in ribonucleosides or 2-deoxy-β-D-ribofuranose () in deoxyribonucleosides. Nucleotides are derived from nucleosides by the addition of one to three phosphate groups esterified to the 5' hydroxyl group of the sugar via phosphoester bonds. This imparts solubility and reactivity to the molecule, enabling its roles in cellular processes. For instance, , a ribonucleoside consisting of attached to , becomes (AMP), diphosphate (ADP), or triphosphate (ATP) upon sequential phosphorylation. Similarly, , the deoxyribonucleoside analog, forms (dAMP) as its nucleotide counterpart. The bases and feature in both ribonucleosides and deoxyribonucleosides, while the bases , uracil (in ribonucleosides), and (in deoxyribonucleosides) complete the set of canonical bases. These structures ensure specificity in biological recognition and bonding. and nucleotides are synthesized through two primary pathways: de novo biosynthesis, which assembles the base and sugar from simple precursors such as (e.g., , aspartate, and ), CO₂, and ribose-5-phosphate derived from the ; and salvage pathways, which recycle free bases or nucleosides from dietary sources or cellular degradation using enzymes like . The de novo pathway for purines begins with the formation of phosphoribosylamine and builds the and rings stepwise, while pyrimidines are synthesized as from and aspartate. Beyond their role as precursors for polymerization into DNA and RNA, nucleotides function in coenzymes such as ATP, which stores through its phosphoanhydride bonds, and NAD⁺, involved in reactions. The hydrolysis of ATP to ADP and inorganic releases approximately -30.5 kJ/mol of free energy under standard biochemical conditions (ΔG°'), driven by the instability of the phosphoanhydride linkages due to electrostatic repulsion and resonance stabilization of products. This energy release facilitates endergonic processes like and transport.

DNA and RNA Structures

Deoxyribonucleic acid (DNA) is a polymer composed of nucleotide monomers linked by phosphodiester bonds, forming a right-handed double helix known as the B-form, which measures approximately 20 Å in diameter and 34 Å per helical turn. This structure consists of two antiparallel strands, where the 5' end of one strand runs parallel but opposite to the 3' end of the other, stabilized by Watson-Crick base pairing: adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds. The sugar-phosphate backbone forms the outer rails of the helix, with the bases stacking inside, creating major and minor grooves that allow access for proteins to recognize specific sequences. Ribonucleic acid (RNA), in contrast, is typically single-stranded and adopts complex secondary structures through intramolecular base pairing, including hairpins (stem-loops formed by complementary sequences), bulges, and internal loops, which contribute to its functional diversity. These structures arise from the same A-U and G-C pairing rules as DNA (with uracil substituting for thymine), enabling RNA to fold into functional motifs essential for processes like catalysis and regulation. Major RNA types include messenger RNA (mRNA), which is largely unstructured but can form local hairpins; transfer RNA (tRNA), which folds into a characteristic cloverleaf secondary structure with three hairpin loops and an acceptor stem; and ribosomal RNA (rRNA), which assembles into intricate multidomain structures with multiple hairpins and junctions. Key structural differences between DNA and RNA include the sugar moiety—deoxyribose in DNA lacks the 2'-hydroxyl (OH) group present in RNA's ribose—and the bases, with DNA using thymine instead of uracil. The absence of the 2'-OH in DNA enhances its chemical stability by preventing base-catalyzed hydrolysis that forms a 2',3'-cyclic phosphate intermediate in RNA, making DNA more resistant to degradation and suitable for long-term genetic storage. In vivo, DNA undergoes supercoiling, where the double helix twists beyond its relaxed state, introducing positive or negative writhe to compact the genome or facilitate processes like replication; enzymes called topoisomerases, such as type I and type II, relieve torsional stress by nicking and religating strands. Further packaging occurs in eukaryotes via chromatin, where DNA wraps around histone octamers (two each of H2A, H2B, H3, and H4) to form nucleosomes, the basic repeating unit consisting of approximately 147 base pairs of DNA wrapped around the histone octamer and typically 20–60 base pairs of linker DNA, enabling higher-order folding into chromosomes. RNA molecules, particularly eukaryotic mRNA, undergo post-transcriptional modifications for stability and export: a 5' cap (7-methylguanosine linked via a 5'-5' triphosphate bridge) protects against exonucleases and aids translation initiation, while a 3' poly-A tail (typically 200-250 adenines) enhances stability and facilitates nuclear export.90128-8) A critical physical property of DNA is its melting temperature (Tm), the point at which half dissociates into single strands, which depends on length and due to the stronger G-C pairs. For short in standard buffer, an approximate equation is: Tm≈69.3+0.41×(%GC)−650LT_m \approx 69.3 + 0.41 \times (\%GC) - \frac{650}{L} where %GC is the percentage of guanine-cytosine bases and L is the length in base pairs; higher raises Tm by up to 40°C compared to AT-rich sequences.

Carbohydrates

Saccharides

Saccharides, also known as carbohydrates, are organic biomolecules composed primarily of carbon, , and oxygen, typically in a approximating \ce(CH2O)n\ce{(CH2O)_n}, serving as fundamental sources and structural elements in living organisms. They are classified based on the number of sugar units: monosaccharides (single units), disaccharides (two units), oligosaccharides (3-10 units), and (many units linked by glycosidic bonds). This classification reflects their increasing complexity and roles, from simple energy providers to complex storage and structural polymers. Monosaccharides represent the simplest saccharides, consisting of polyhydroxy aldehydes (aldoses) or ketones (ketoses) with 3 to 7 carbon atoms. They are categorized by chain length, such as trioses (3 carbons, e.g., glyceraldehyde), tetroses (4 carbons), pentoses (5 carbons, e.g., ribose), and hexoses (6 carbons, e.g., glucose and fructose). Glucose, an aldohexose with the formula \ceC6H12O6\ce{C6H12O6}, exemplifies the open-chain form featuring an aldehyde group at C1 and hydroxyl groups on the other carbons, while fructose, a ketohexose, has a ketone at C2. In aqueous solutions, monosaccharides predominantly exist in cyclic forms via intramolecular hemiacetal reactions, forming five-membered furanose or six-membered pyranose rings, represented in Haworth projections as flat rings with substituents above or below the plane. The anomeric carbon, typically C1 in aldoses or C2 in ketoses, arises from this cyclization and gives rise to α and β anomers differing in configuration at that chiral center. Disaccharides form through condensation reactions between two monosaccharides, eliminating water to create a glycosidic bond, which links the anomeric carbon of one sugar to a hydroxyl group of another. For instance, sucrose comprises glucose and fructose joined by an α-1,2-glycosidic bond, resulting in the formula \ceC12H22O11\ce{C12H22O11} and rendering it non-reducing due to the involvement of both anomeric carbons. Other examples include maltose (two glucose units via α-1,4 bond) and lactose (galactose and glucose via β-1,4 bond). Hydrolysis of these bonds, catalyzed by acids or enzymes like sucrase, reverses the process, yielding the constituent monosaccharides and water. Oligosaccharides consist of 3 to 10 units linked by glycosidic bonds, often serving as recognition signals, while are long s of s providing or . , a storage , includes (linear chains of glucose linked by α-1,4 bonds) and (branched with α-1,6 branches every 25-30 residues). , the animal counterpart, is a highly branched glucose with α-1,4 main chains and α-1,6 branches every 8-12 residues, enabling rapid mobilization. , a structural in , features linear β-1,4-linked glucose units forming rigid fibers with the repeating unit \ce(C6H10O5)n\ce{(C6H10O5)_n}, where n ranges from 500 to 5000. Saccharides exhibit distinct chemical properties arising from their functional groups. Reducing sugars, such as glucose and , possess a free anomeric carbon that can open to an or ketone, allowing reduction of agents like in the presence of the form. Non-reducing sugars like lack this free group due to full glycosidic linkage. Most saccharides display optical activity from their chiral carbons; for example, D-glucose rotates plane-polarized light due to four asymmetric centers. In , converts glucose anaerobically to and CO₂ via and , regenerating NAD⁺: \ceC6H12O6−>2C2H5OH+2CO2\ce{C6H12O6 -> 2C2H5OH + 2CO2}. Biosynthesis of saccharides involves metabolic pathways integrating simple sugars into larger forms, with providing key intermediates for both breakdown and synthesis. Glucose enters as \ceC6H12O6\ce{C6H12O6} and is phosphorylated to glucose-6-phosphate by , an early intermediate that interconverts with fructose-6-phosphate and feeds into for net glucose production from non-carbohydrate precursors like lactate. Further intermediates include fructose-1,6-bisphosphate (split into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate) and proceed to phosphoenolpyruvate, linking to glycogen synthesis via glucose-1-phosphate. In , generates glucose as a primary product, which polymerizes into .

Lipids

Structure and Classification

Lipids constitute a heterogeneous group of primarily hydrophobic or amphipathic small molecules that are insoluble in but soluble in organic solvents such as or . This solubility profile arises from their predominantly nonpolar components, which contrasts with the polar nature of . are classified into three principal categories based on their and products: simple lipids, compound lipids, and derived lipids. Simple lipids include fats (triglycerides) and waxes, which are esters of s with alcohols like or long-chain alcohols, yielding at most two types of products upon . For instance, triglycerides consist of a molecule esterified to three chains. Compound lipids, such as and glycolipids, yield three or more products upon and incorporate additional groups beyond and alcohols; , a common , features a backbone esterified at the sn-1 and sn-2 positions to two , with the sn-3 position linked to a group esterified to choline. Derived lipids are obtained from the of simple or compound and include substances like and steroids, with serving as a prototypical example. The structural foundation of many rests on s, which are long, unbranched carboxylic acids typically containing an even number of carbon atoms ranging from 4 to 28. Saturated s, lacking carbon-carbon s, exhibit straight-chain configurations, as exemplified by (CH3(CH2)14COOHCH_3(CH_2)_{14}COOH), a 16-carbon . In contrast, unsaturated s contain one or more cis s, introducing kinks in the chain; , for example, is an 18-carbon monounsaturated with a between carbons 9 and 10. The refers to the number of these s, influencing the fluidity and packing of lipid assemblies. Amphipathic like phospholipids can spontaneously form micelles (spherical aggregates with hydrophobic tails inward) or bilayers (sheet-like structures with tails sequestered between hydrophilic heads) in aqueous media, driven by hydrophobic interactions. Steroids possess a rigid core of four fused rings (three six-membered and one five-membered), as seen in , which also includes a hydroxyl group at carbon 3 and an eight-carbon at carbon 17. Fatty acid nomenclature follows International Union of Pure and Applied Chemistry (IUPAC) conventions, designating the systematic name based on the longest chain length, unsaturation sites, and configurations; thus, is named cis-9-octadecenoic acid, where "octadec" indicates 18 carbons, "enoic" denotes one , and "cis-9" specifies its position and . The shorthand notation, such as 18:1Δ9cis for , further simplifies this by listing total carbons:double bonds followed by the double bond position. Biosynthesis of fatty acids commences with , which is carboxylated to and then iteratively elongated by two-carbon units via the multifunctional complex, primarily in the of eukaryotic cells. This process yields palmitate as the primary product in animals, serving as a precursor for longer or modified chains.

Biological Functions

Lipids play a central role in within the body, primarily through triglycerides stored in , which serve as the main reservoir for long-term energy needs. When energy demands arise, such as during or exercise, triglycerides are hydrolyzed into free s and , with the fatty acids undergoing β-oxidation in mitochondria to produce ATP. For instance, the complete β-oxidation of one of palmitate (a 16-carbon fatty acid) yields approximately 106 ATP molecules, compared to about 36 ATP from one of glucose, highlighting the higher of . In cellular membranes, lipids are essential structural components that form the phospholipid bilayer, providing compartmentalization and maintaining cellular integrity. , with their hydrophilic heads and hydrophobic tails derived from chains, self-assemble into bilayers that separate intracellular compartments from the extracellular environment. , embedded within these bilayers, modulates by interacting with phospholipid tails; at low temperatures, it prevents tight packing and gel-phase formation, while at higher temperatures, it restricts excessive motion, thereby influencing the temperature and overall membrane dynamics. Lipids also function as key signaling molecules, enabling communication in physiological processes. Eicosanoids, such as prostaglandins derived from the oxidation of —a polyunsaturated released from membrane phospholipids—act as local hormones that mediate , pain, and fever responses. Similarly, steroid hormones, including , are biosynthesized from in endocrine glands like the , regulating stress responses, metabolism, and immune function through binding. Beyond these primary roles, lipids contribute to various other physiological functions, including emulsification, insulation, and nutrient absorption. Bile salts, amphipathic derivatives of produced in the liver, emulsify dietary fats in the intestine by forming micelles that facilitate lipid and absorption. In the , lipids in the sheath provide electrical insulation around axons, accelerating impulse conduction and protecting against signal leakage. Additionally, lipids are crucial for the absorption of fat-soluble vitamins A, D, E, and K, as these vitamins require incorporation into mixed micelles for efficient uptake in the . However, dysregulated lipid accumulation can lead to pathological conditions; in , oxidized low-density lipoproteins infiltrate arterial walls, forming lipid-rich plaques that promote , , and increased risk of cardiovascular events.

Amino Acids and Proteins

Amino Acids

Amino acids are the fundamental building blocks of proteins, consisting of a central α-carbon atom bonded to a , an amino group (-NH₂), a carboxyl group (-COOH), and a variable denoted as . This general structure, represented as H₂N-CH()-COOH, allows for diverse chemical properties determined by the R group. There are 20 standard proteinogenic encoded by the and incorporated into proteins during . These are classified based on the polarity and charge of their side chains, which influence their interactions in biological environments. Non-polar amino acids, such as (where R = H) and (with a branched R group), have hydrophobic side chains that typically reside in protein interiors. Polar uncharged amino acids, like serine (R = -CH₂OH), feature side chains capable of hydrogen bonding. Acidic amino acids, including (R = -CH₂COOH), possess negatively charged side chains at physiological , while basic amino acids, such as (R = -(CH₂)₄NH₂), have positively charged side chains. Additionally, are categorized as essential or non-essential based on human dietary needs; nine are essential—, , , , , , , , and —and must be obtained from the diet, as exemplified by , which cannot be synthesized endogenously. At physiological (around 7.4), predominantly exist in their zwitterionic form, where the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO⁻), resulting in a net neutral charge but with separated charges. The (pI) is the pH at which the has no net charge, varying by properties (e.g., ranging from about 2.8 for acidic residues to 10.8 for basic ones). Nearly all proteinogenic are chiral, with the L-enantiomer (L-form) being overwhelmingly predominant in biological systems due to the specificity of ribosomal synthesis. Biosynthesis of occurs through pathways deriving from metabolic intermediates, often involving reactions where an amino group from glutamate is transferred to a carbon . For instance, is synthesized via of pyruvate, a key glycolytic intermediate, catalyzed by . Other examples include aromatic like , derived from the , and sulfur-containing , which forms bonds (-S-S-) between side chains to stabilize protein structures. These monomers polymerize via bonds to form polypeptide chains in proteins.

Protein Structures

Proteins exhibit a that dictates their three-dimensional shape and stability, comprising primary, secondary, tertiary, and structures. This organization arises from the chemical properties of side chains, which influence the folding process by providing diverse interactions such as hydrophobic effects and hydrogen bonding. The primary structure forms the foundational linear of , while higher levels build upon it through spatially organized interactions, ultimately enabling the protein's functional conformation. The primary structure of a protein is the linear sequence of covalently linked by bonds, which are linkages formed between the carboxyl group of one and the amino group of the next (-CO-NH-). This sequence, determined by the , uniquely identifies each protein and serves as the template for all higher-order structures. Any alteration in this sequence, such as a single substitution, can disrupt folding and stability. Secondary structure refers to local conformations stabilized primarily by hydrogen bonds between the backbone atoms of the polypeptide chain. The most common elements include the α-helix, a right-handed coil with 3.6 residues per turn and a pitch of 5.4 Å, where hydrogen bonds form between the carbonyl oxygen of residue n and the amide hydrogen of residue n+4. Another key motif is the β-sheet, composed of β-strands arranged in parallel or antiparallel orientations, with hydrogen bonds linking adjacent strands to form a pleated sheet-like structure. These elements, along with turns and loops that connect them, provide the initial folding scaffolds. The α-helix and β-sheet were first proposed by and Robert Corey in based on model-building constrained by known bond lengths and angles. Tertiary structure describes the overall three-dimensional folding of a single polypeptide chain, resulting from interactions among side chains that position distant regions in space. Key stabilizing forces include hydrophobic interactions, which bury nonpolar residues in the protein core; hydrogen bonds between polar groups; ionic bonds or salt bridges between oppositely charged residues; and disulfide bridges, covalent linkages between sulfhydryl groups. This folding often organizes into structural motifs and domains, compact units that function semi-independently within the protein. The native tertiary structure is thermodynamically favored, as demonstrated by Christian Anfinsen's experiments on ribonuclease A, showing that the amino acid sequence encodes the information necessary for correct folding in vitro.56522-X/fulltext) Quaternary structure arises in proteins composed of multiple polypeptide subunits, which associate non-covalently to form a functional complex. Subunit interfaces are stabilized by the same interactions as in tertiary structure, including hydrophobic contacts and hydrogen bonds. A classic example is tetrameric protein consisting of two α and two β subunits (α₂β₂), which enables cooperative oxygen binding through conformational changes upon subunit interactions. Proteins can undergo denaturation, the disruption of higher-order structures leading to loss of native conformation, triggered by factors such as elevated temperature, extreme , or chemical denaturants like , which weaken non-covalent interactions. Denaturation is often reversible through renaturation, where the protein refolds to its native state under appropriate conditions, underscoring the sequence-directed nature of folding. , molecular chaperones like assist in folding and prevent aggregation by binding exposed hydrophobic regions of nascent or misfolded polypeptides, using to cycle between substrate-bound and release states.

Specialized Protein Forms

Specialized protein forms in enzymes arise from modifications in that enable catalytic activity, , and tissue-specific functions. These forms often involve the association or dissociation of non-protein components and variations in subunit composition, allowing proteins to adapt to diverse physiological roles. Such adaptations are grounded in the inherent flexibility of protein tertiary and structures, which facilitate binding events critical for function. Apoenzymes represent the inactive protein portion of enzymes that require cofactors for activity, consisting solely of the polypeptide chain without bound prosthetic groups or coenzymes. These structures are catalytically inert until activated by the binding of necessary non-protein components, such as metal ions or organic molecules. For instance, is the protein shell of ferritin devoid of its iron core, which serves as a storage mechanism; iron loading transforms it into the functional holoferritin. occurs through specific interactions where cofactors occupy designated sites, restoring the enzyme's three-dimensional conformation essential for . In contrast, holoenzymes denote the complete, catalytically active form of an , comprising the apoenzyme bound to its cofactor. This assembly ensures the enzyme can perform its biological reaction efficiently, as the cofactor often participates directly in substrate binding or . Holoenzymes are prevalent in metabolic pathways, where tight cofactor integration prevents dissociation under physiological conditions. The distinction between apo- and holoenzymes underscores the modular nature of enzyme function, allowing cells to regulate activity by controlling cofactor availability. Isoenzymes, also known as isozymes, are multiple forms of an that catalyze the same reaction but differ in , subunit composition, and tissue distribution due to expression from distinct genes. These variants exhibit subtle kinetic or stability differences suited to specific cellular environments. A prominent example is (LDH), which exists as five isozymes (LDH1 through LDH5) formed by combinations of heart-type (H) and muscle-type (M) subunits. LDH1, composed of four H subunits (H4), predominates in heart tissue, while LDH5, with four M subunits (M4), is abundant in ; intermediate forms like LDH2 (H3M) and LDH3 (H2M2) bridge these distributions. This isoform diversity enables tissue-specific metabolic adaptations, such as favoring lactate production in anaerobic muscle conditions versus pyruvate oxidation in aerobic heart cells. Allosteric regulation provides a dynamic mechanism for modulating activity through conformational changes induced by effector binding at sites distinct from the active center. Effectors, which can be activators or inhibitors, bind to allosteric sites and trigger shifts between relaxed () and tense (T) states, altering substrate affinity or catalytic rate without competing directly with the substrate. This , first conceptualized in the Monod-Wyman-Changeux model, allows for cooperative interactions in multimeric enzymes, enabling rapid responses to metabolic signals. For example, in aspartate transcarbamoylase, CTP binding stabilizes the T state to inhibit activity, while ATP promotes the R state for , illustrating feedback control in . Isoenzymes like those of (CK) exemplify tissue-specific diagnostic utility in clinical settings. CK exists primarily as CK-MM in and CK-MB (a hybrid of M and B subunits) in cardiac tissue, with elevated serum CK-MB levels indicating due to its release from damaged heart cells. This isoform pattern allows for precise localization of injury, as CK-MM elevations signal damage while CK-MB specificity aids in confirming cardiac events within hours of onset. Such applications highlight how structural variations in isozymes enhance both physiological specialization and medical diagnostics.

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