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Hexose
Hexose
Hexose
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D-Glucose.
D-Fructose.
Two important hexoses, in the Fischer projection.

In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms.[1][2] The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol.[3]

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.[4] The open-chain form of a hexose, which usually is favored in solutions, has the general structure H−(CHOH)n−1−C(=O)−(CHOH)6−n−H, where n is 1, 2, 3, 4, 5. Namely, five of the carbons have one hydroxyl functional group (−OH) each, connected by a single bond, and one has an oxo group (=O), forming a carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by seven hydrogen atoms. The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl.

Hexoses are extremely important in biochemistry, both as isolated molecules (such as glucose and fructose) and as building blocks of other compounds such as starch, cellulose, and glycosides. Hexoses can form dihexose (like sucrose) by a condensation reaction that makes 1,6-glycosidic bond.

When the carbonyl is in position 1, forming a formyl group (−CH=O), the sugar is called an aldohexose, a special case of aldose. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a ketone, and is called a ketohexose, a special case of ketose; specifically, an n-ketohexose.[1][2] However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize;[5] so the term "ketohexose" usually means 2-ketohexose.

In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, stereoisomers that differ in the spatial position of the hydroxyl groups. These species occur in pairs of optical isomers. Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "D-" or "L-", depending on whether the hydroxyl in position 5, in the Fischer projection of the molecule, is to the right or to the left of the axis, respectively. These labels are independent of the optical activity of the isomers. In general, only one of the two enantiomers occurs naturally (for example, D-glucose) and can be metabolized by animals or fermented by yeasts.

The term "hexose" sometimes is assumed to include deoxyhexoses, such as fucose and rhamnose: compounds with general formula C6H12O6−y that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.

Classification

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Aldohexoses

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The aldohexoses are a subclass of the hexoses which, in the linear form, have the carbonyl at carbon 1, forming an aldehyde derivative with structure H−C(=O)−(CHOH)5−H.[1][2] The most important example is glucose.

In linear form, an aldohexose has four chiral centres, which give 16 possible aldohexose stereoisomers (24), comprising 8 pairs of enantiomers. The linear forms of the eight D-aldohexoses, in the Fischer projection, are

Of these D-isomers, all except D-altrose occur in living organisms, but only three are common: D-glucose, D-galactose, and D-mannose. The L-isomers are generally absent in living organisms; however, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.[6]

When drawn in this order, the Fischer projections of the D-aldohexoses can be identified with the 3-digit binary numbers from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111. The three bits, from left to right, indicate the position of the hydroxyls on carbons 4, 3, and 2, respectively: to the right if the bit value is 0, and to the left if the value is 1.

The chemist Emil Fischer is said[citation needed] to have devised the following mnemonic device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings:

All altruists gladly make gum in gallon tanks.

referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose.

The Fischer diagrams of the eight L-aldohexoses are the mirror images of the corresponding D-isomers; with all hydroxyls reversed, including the one on carbon 5.

Ketohexoses

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A ketohexose is a ketone-containing hexose.[1][2][7] The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose is fructose.

Besides the 2-ketoses, there are only the 3-Ketoses, and they do not exist in nature, although at least one 3-ketohexose has been synthesized, with great difficulty.

In the linear form, the 2-ketohexoses have three chiral centers and therefore eight possible stereoisomers (23), comprising four pairs of enantiomers. The four D-isomers are:

The corresponding L forms have the hydroxyls on carbons 3, 4, and 5 reversed. Below are depiction of the eight isomers in an alternative style:

3-Ketohexoses

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In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namely H−(CHOH)2−C(=O)−(CHOH)3−H. However, these compounds are not known to occur in nature, and are difficult to synthesize.[5]

In 1897, an unfermentable product obtained by treatment of fructose with bases, in particular lead(II) hydroxide, was given the name glutose, a portmanteau of glucose and fructose, and was claimed to be a 3-ketohexose.[12][13] However, subsequent studies showed that the substance was a mixture of various other compounds.[13][14]

The unequivocal synthesis and isolation of a 3-ketohexose, xylo-3-hexulose, through a rather complex route, was first reported in 1961 by George U. Yuen and James M. Sugihara.[5]

Cyclic forms

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Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by an internal rearrangement between the carbonyl group and one of the hydroxyl groups.

The reaction turns the =O group into a hydroxyl, and the hydroxyl into an ether bridge (−O−) between the two carbon atoms, thus creating a ring with one oxygen atom and four or five carbons.

If the cycle has five carbon atoms (six atoms in total), the closed form is called a pyranose, after the cyclic ether tetrahydropyran, that has the same ring. If the cycle has four carbon atoms (five in total), the form is called furanose after the compound tetrahydrofuran.[4] The conventional numbering of the carbons in the closed form is the same as in the open-chain form.

If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a hemiacetal with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a hemiketal with a five-membered ring.

The closure turns the carboxyl carbon into a chiral center, which may have either of two configurations, depending on the position of the new hydroxyl. Therefore, each hexose in linear form can produce two distinct closed forms, identified by prefixes "α" and "β".

α-D-Glucopyranose.
β-D-Glucopyranose.
α-D-Fructofuranose.
β-D-Fructofuranose.
Closed forms of D-glucose and D-fructose, in the Haworth projection.

It has been known since 1926 that hexoses in the crystalline solid state assume the cyclic form. The "α" and "β" forms, which are not enantiomers, will usually crystallize separately as distinct species. For example, D-glucose forms an α crystal that has specific rotation of +112° and melting point of 146 °C, as well as a β crystal that has specific rotation of +19° and melting point of 150 °C.[4]

The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms.[4] Nevertheless, it plays an essential role as the intermediate stage between those closed forms.

In particular, the "α" and "β" forms can convert to into each other by returning to the open-chain form and then closing in the opposite configuration. This process is called mutarotation.

Chemical properties

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Although all hexoses have similar structures and share some general properties, each enantiomer pair has its own chemistry. Fructose is soluble in water, alcohol, and ether.[9] The two enantiomers of each pair generally have vastly different biological properties.

2-Ketohexoses are stable over a wide pH range, and with a primary pKa of 10.28, will only deprotonate at high pH, so are marginally less stable than aldohexoses in solution.

Natural occurrence and uses

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The aldohexose that is most important in biochemistry is D-glucose, which is the main "fuel" for metabolism in many living organisms.

The 2-ketohexoses psicose, fructose and tagatose occur naturally as the D-isomers, whereas sorbose occurs naturally as the L-isomer.

D-Sorbose is commonly used in the commercial synthesis of ascorbic acid.[10] D-Tagatose is a rare natural ketohexose that is found in small quantities in food.[11] D-Fructose is responsible for the sweet taste of many fruits, and is a building block of sucrose, the common sugar.

Deoxyhexoses

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See also: Deoxy sugar

The term "hexose" may sometimes be used to include the deoxyhexoses, which have one or more hydroxyls (−OH) replaced by hydrogen atoms (−H). It is named as the parent hexose, with the prefix "x-deoxy-", the x indicating the carbon with the affected hydroxyl. Some examples of biological interest are

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hexose

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from Grokipedia
A hexose is a , or simple sugar, consisting of six carbon atoms and having the molecular formula C₆H₁₂O₆. With a molecular weight of 180.16 g/mol, hexoses typically exist in cyclic forms such as or rings in biological systems, and they serve as fundamental units in and energy production across living organisms. Hexoses are classified into aldohexoses, which feature an group at carbon 1, and ketohexoses, which have a group at carbon 2. Aldohexoses possess 16 possible stereoisomers due to four chiral centers, while ketohexoses have 8, with the D-enantiomers being predominant in nature. These stereoisomers differ in their configuration at the chiral carbons, influencing their biological functions and interactions. Among the most abundant and biologically significant hexoses are D-glucose, D-galactose, and D-fructose, all sharing the formula C₆H₁₂O₆ but varying in structure and sweetness. D-glucose, an aldohexose also known as blood sugar or dextrose, is the primary energy source for cells, derived from dietary carbohydrates and , and maintained in human at 70–99 mg/dL (). D-galactose, another aldohexose, is obtained from the breakdown of in and plays a key role in the synthesis of glycolipids, particularly in the myelin sheath of and cells, earning it the nickname "brain sugar." D-fructose, a ketohexose found in high concentrations in fruits (up to 40% in ) and known as levulose due to its levorotatory of -92.4°, is the sweetest natural sugar, approximately 1.7 times sweeter than , and serves as an alternative energy source in metabolism. In biochemistry, hexoses are central to processes like , the , and glycoconjugate formation, where glucose is stored as in animals or in for energy reserves. Their transport and metabolism are tightly regulated, as seen in microbial systems like , where β-D-glucose acts as a key metabolite. Disruptions in hexose processing, such as in , highlight their critical role in human health.

Definition and Nomenclature

Definition

Hexoses are a class of monosaccharides consisting of six carbon atoms and having the molecular formula C6H12O6; they can exist in either open-chain or cyclic configurations. These simple sugars are distinguished from other monosaccharides by the number of carbon atoms in their backbone, such as with five carbons or with seven, and hexoses represent the most abundant form encountered in nature. In biological systems, hexoses function primarily as sources, readily metabolized to provide for cellular processes, and as fundamental building blocks for synthesizing and other complex carbohydrates. They occur as structural isomers including aldoses, which possess an group, and ketoses, which contain a group. The term "hexose" emerged in the late 19th century amid the pioneering work of , who systematically classified carbohydrates based on chain length and functional groups during his studies of sugar .

Nomenclature

Hexoses, as six-carbon monosaccharides, are systematically named using the stem "hexose" derived from the corresponding "" by replacing the final "-ane" with "-ose". Aldohexoses, which possess an group, are identified by the prefix "aldo-" attached to the stem name, with the carbonyl positioned at carbon 1 by convention; for instance, aldohexose denotes a hexose with the aldehyde at C-1. Ketohexoses, featuring a group, use the prefix "keto-" followed by the position of the carbonyl, commonly at C-2, as in 2-ketohexose. The of hexoses is designated as D- or L- based on the convention, where the series is determined by the orientation of the hydroxyl group at the highest-numbered (the penultimate carbon) relative to D- or L-glyceraldehyde; in the D-series, this hydroxyl is on the right, and in the L-series, on the left. In the open-chain form, carbon atoms are numbered starting from the carbonyl carbon as C-1 and proceeding to the terminal CH₂OH group as C-6 for hexoses, ensuring consistent structural reference across representations. According to IUPAC recommendations, the full systematic name for the open-chain form of an aldohexose incorporates the R/S descriptors for each chiral center along with the nomenclature; for example, D-glucose is named (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal. This approach prioritizes the aldehydic function as the parent chain, with hydroxy substituents at the specified positions.

Structural Features

Open-Chain Form

The open-chain form of a hexose represents its linear, acyclic structure, typically depicted using the convention, which arranges the carbon chain vertically with the most oxidized carbon () at the top and the hydroxyl groups attached to chiral carbons shown as horizontal lines. In this projection, the vertical bonds extend away from the viewer, while horizontal bonds project toward the viewer, providing a standardized 2D representation of the 3D configuration. For aldohexoses, the open-chain structure features an group at carbon 1 (C1), hydroxyl groups on carbons 2 through 5 (C2–C5), and a (CH₂OH) at carbon 6 (C6), resulting in the general formula: \ceH−C=O∣(\ceCHOH)4∣\ceCH2OH\begin{align*} &\ce{H-C=O}\\ &|\quad\quad\\ &(\ce{CHOH})_4\\ &|\quad\quad\\ &\ce{CH2OH} \end{align*} This configuration yields four chiral centers at C2–C5, allowing for 24=162^4 = 16 possible stereoisomers. In contrast, ketohexoses possess a group typically at C2, flanked by hydroxymethyl groups at both C1 and C6, with hydroxyl groups on C3–C5, creating three chiral centers at C3–C5 and thus 23=82^3 = 8 stereoisomers. In , cyclic structures predominate and equilibrate with the open-chain form.

Cyclic Forms

In aqueous solution, hexoses predominantly exist in cyclic forms resulting from intramolecular nucleophilic addition reactions that form hemiacetals or hemiketals. For aldohexoses, the aldehyde carbonyl at C1 reacts with the hydroxyl group at C5 to generate a six-membered ring, while for ketohexoses, the ketone carbonyl at C2 reacts with the hydroxyl at C5 (yielding a five-membered ring) or at C6 (yielding a six-membered ring). These cyclizations create a new stereogenic center at the anomeric carbon—C1 in aldoses and C2 in ketoses—producing two diastereomers known as anomers. In projections, which depict the cyclic forms as planar rings, the α-anomer has the anomeric hydroxyl group oriented below the ring plane, whereas the β-anomer has it above the plane. rings adopt a conformation in three dimensions, which is more stable than the boat form due to minimized steric interactions and angle strain. Stability is further enhanced when bulky substituents, such as hydroxyl groups, occupy equatorial positions rather than axial ones, as seen in the all-equatorial β-D-glucopyranose form. The α- and β-anomers, along with the open-chain form as a minor equilibrium component, interconvert through ring opening and reclosure in a process termed , which is observable as a change in . For example, freshly dissolved α-D-glucopyranose exhibits a specific rotation of +112°, which decreases to an equilibrium value of +52.7° as the mixture of forms establishes. forms are less prevalent in free hexoses compared to but appear in certain derivatives, such as the fructose moiety in .

Classification

Aldohexoses

Aldohexoses are a subclass of hexoses characterized by an group at carbon 1 and four chiral centers at carbons 2 through 5, resulting in 2^4 = 16 possible stereoisomers: 8 in the D-series and 8 in the L-series. The D- and L-designations refer to the at the penultimate carbon (C5 in aldohexoses), determined using Cahn-Ingold-Prelog (CIP) priority rules, where the D-series corresponds to the (R) configuration at C5, analogous to D-glyceraldehyde. D-Glucose serves as the reference aldohexose, predominant in nature, and exists primarily in cyclic forms; in its β-D-glucopyranose conformation, all hydroxyl substituents are equatorial, conferring exceptional stability due to minimized steric interactions. Other common D-aldohexoses include D-mannose, an of D-glucose differing in configuration at C2, and D-galactose, an epimer at C4; less abundant examples are D-allose (differing from glucose at C3), D-altrose, D-gulose, D-idose, and D-talose. Epimers such as these arise from inversion at a single chiral center, altering their physical and biochemical properties while maintaining the aldohexose framework. Aldohexoses are synthesized via the Kiliani-Fischer synthesis, which extends aldopentoses by adding a carbon atom to the group through formation followed by , yielding two epimeric aldohexoses that differ at the new C2 chiral center. Optical activity distinguishes aldohexoses, with specific rotations measured in aqueous equilibrium mixtures accounting for between α- and β-anomers. For instance, D-glucose exhibits an equilibrium of +52.7°, D-mannose +14.2°, and D-galactose +80.2°, reflecting their distinct stereochemical configurations and anomeric equilibria.

Ketohexoses

Ketohexoses are hexoses featuring a group at the C2 position, distinguishing them from aldohexoses by the absence of an at C1 and the presence of three chiral centers at C3, C4, and C5. This configuration yields four stereoisomers in the D-series and four in the L-series, for a total of eight possible ketohexoses. Unlike aldohexoses, which predominantly adopt six-membered rings, ketohexoses like show a notable tendency toward five-membered rings due to the enhanced stability of these structures in configurations. The most prominent example is D-fructose, commonly occurring in its β-D-fructofuranose form, where the ring forms between C2 and C5, with hydroxymethyl groups (-CH₂OH from C1 at C2 and from C6 at C5). Other key D-ketohexoses include D-psicose (the C3-epimer of D-fructose), D-sorbose (the C5-epimer), and D-tagatose (the C4-epimer), each exhibiting distinct configurations at the chiral centers that influence their biological roles and reactivity. D-fructose stands out for its sensory properties, possessing a relative 1.2 to 1.8 times that of on a molar basis, attributed to its interaction with sweet taste receptors, and it is levorotatory, with an equilibrium of approximately -92° in . Ketohexoses are typically synthesized through the Lobry de Bruyn–van Ekenstein transformation, an base-catalyzed that proceeds via a common enediol intermediate, enabling interconversion between aldoses like D-glucose and D-mannose and the ketohexose D- under mild alkaline conditions. This reversible reaction, first described in the late , is widely used in to produce fructose from glucose and highlights the dynamic equilibrium among these monosaccharides in solution.

Modified Hexoses

Keto Variants

Keto variants of hexoses encompass those with the positioned at carbons other than the conventional C2, rendering them rare and typically transient due to heightened reactivity. Among these, 3-ketohexoses, bearing the at C3, exhibit reduced stability compared to 2-ketohexoses, primarily owing to their propensity for enolization at the adjacent C2-C3 bond, which facilitates reversion to more stable 2-keto or forms via common enediol intermediates. This tautomerism is exacerbated in aqueous environments, where 3-keto-D-glucose, for instance, equilibrates among multiple isomeric forms, including enols, limiting its persistence. Representative examples of 3-ketohexoses include synthetic 3-keto-D-glucose and its , prepared through selective oxidation, as well as 3-keto-levoglucosan, a cyclic generated enzymatically from levoglucosan. In bacterial , 3-keto-levoglucosan functions as a key intermediate in the of levoglucosan—a pyrolysis product of —catalyzed by levoglucosan , which oxidizes the substrate at C3 before subsequent hydration and reduction steps yield glucose. Such intermediates underscore the role of 3-ketohexoses in microbial adaptation to environmental carbohydrates, though their isolation remains challenging due to instability. Synthesis of 3-ketohexoses poses significant hurdles, often relying on enzymatic oxidation of the C3 hydroxyl group using oxidases like , which achieves high yields (up to 80%) under controlled conditions such as low temperature and neutral . Chemical methods, involving protection of other hydroxyls followed by selective oxidation, are similarly employed but complicated by the compounds' lability; for example, 3-keto-levoglucosan decomposes via β-elimination at the C1-C5 positions, with a of approximately 16 hours at pH 7 and 30°C, though it remains stable in acidic media. Ketohexoses with the carbonyl at other positions, such as C4 or C5, are exceedingly rare in free form owing to even greater instability and are predominantly encountered as enzyme-bound biosynthetic intermediates. For instance, 4-ketohexoses like UDP-4-keto-glucose arise transiently during the conversion of UDP-glucose to UDP-galactose by UDP-glucose 4-epimerase, involving NAD+-dependent oxidation at C4 followed by reduction. Unlike standard ketohexoses like , these variants rarely accumulate, emphasizing their ephemeral nature in biological contexts.

Deoxyhexoses

Deoxyhexoses constitute a subclass of hexoses characterized by the replacement of one or more hydroxyl groups with atoms, yielding a typical molecular formula of C₆H₁₂O₅ for monodeoxy variants and altering their chemical properties compared to standard hexoses. These modifications most commonly occur at the C-6 position in natural deoxyhexoses, converting the to a , though deoxygenations at C-2, C-3, or C-4 are documented in specialized compounds. The stereochemistry at unmodified chiral centers is preserved from the parent hexose, maintaining configurational similarity to aldohexoses like or . Prominent natural examples include L-fucose, known chemically as 6-deoxy-L-galactose, which serves as a terminal in N- and O-linked glycans on mammalian cell surfaces and in glycoproteins. Another widespread deoxyhexose is L-rhamnose, or 6-deoxy-L-mannose, integral to pectins such as rhamnogalacturonan I and II, where it links galacturonic acid residues and influences wall rigidity. In microbial contexts, daunosamine—a 3-amino-3,6-dideoxy-D-mannose—occurs as a component in antibiotics like , produced by Streptomyces peucetius. Synthetic analogs, such as , mimic D-glucose but lack the 2-hydroxyl, enabling targeted biochemical studies. Biosynthesis of deoxyhexoses generally proceeds through nucleotide diphosphate (NDP)-sugar pathways, starting from common hexose precursors like glucose or . For L-fucose, the pathway begins with GDP-D-, which undergoes 4,6- by GDP- 4,6-dehydratase to form GDP-4-keto-6-deoxy-D-, followed by epimerization and reduction via GDP-fucose to yield GDP-L-fucose. L-rhamnose synthesis mirrors this, involving GDP-4,6-dehydratase-mediated of GDP-D- to GDP-4-keto-6-deoxy-D-, then NADPH-dependent reduction by a 4-keto reductase to GDP-L-rhamnose. In , daunosamine is assembled from dTDP-D-glucose through sequential at C-3 and C-6, at C-3 by a , and methyltransferase activity, culminating in dTDP-L-daunosamine. These sugars find applications in therapeutics and research due to their modified reactivity. 2-Deoxy-D-glucose functions as a glycolysis inhibitor by competitively binding hexokinase, forming 2-deoxy-D-glucose-6-phosphate that accumulates and blocks phosphoglucose isomerase, thereby depleting cellular ATP in glucose-dependent cells like tumors. Daunosamine, when glycosidically linked to the aglycone in daunorubicin, enhances the drug's intercalation into DNA and improves its membrane permeability, underpinning its efficacy as an anticancer agent.

Chemical Properties

Reactivity

Hexoses, as monosaccharides containing six carbon atoms, exhibit reactivity primarily driven by their carbonyl ( or ) and multiple hydroxyl functional groups. In aldohexoses, the group at C1 undergoes reactions more readily than the group at C2 in ketohexoses, owing to greater electrophilicity and reduced steric hindrance around the carbonyl carbon. These carbonyls are also susceptible to oxidation; for instance, aldohexoses like glucose are oxidized to aldonic acids such as using , which selectively targets the in the open-chain form present in equilibrium with the cyclic structure./22%3A_The_Organic_Chemistry_of_Carbohydrates/22.06%3A_The_Oxidation-Reduction_Reactions_of_Monosaccharides) Ketohexoses, while less reactive to mild oxidants, can isomerize under basic conditions to aldoses, enabling similar oxidation./22%3A_The_Organic_Chemistry_of_Carbohydrates/22.06%3A_The_Oxidation-Reduction_Reactions_of_Monosaccharides) The hydroxyl groups in hexoses confer weak acidity, with pKa values typically ranging from 12 to 14, reflecting their alcoholic nature and enabling reactions such as esterification with carboxylic acids or formation under appropriate conditions. The anomeric hydroxyl at C1 in cyclic forms shows slightly higher acidity (pKa around 12.3 for glucose) due to its position adjacent to the ring oxygen, influencing reactivity at this site. This acidity allows in strongly basic media, facilitating further transformations. At the anomeric carbon, reactivity is pronounced, particularly in the open-chain form where the carbonyl is free, but persists in cyclic hemiacetals through equilibrium, leading to glycoside formation via nucleophilic attack by alcohols on the protonated anomeric carbon. The cyclic vs. open-chain equilibrium modulates overall reactivity, with the small proportion of open-chain species (less than 1% for glucose) sufficient to drive many responses. Hexoses are sensitive to heat and acid; upon heating above 150°C, they undergo caramelization, a non-enzymatic browning involving dehydration, fragmentation, and polymerization to form colored, flavorful compounds like hydroxymethylfurfural from glucose. In acidic conditions, the cyclic rings hydrolyze to the open-chain form, increasing carbonyl availability and reactivity. All hexoses act as reducing sugars because the equilibrium with their open-chain tautomers provides a free carbonyl group capable of reducing Cu²⁺ in Fehling's or Benedict's solutions to Cu₂O, producing a red precipitate./09%3A_Lab_9-_Tests_for_Carbohydrates) This property holds for both aldo- and ketohexoses, as ketohexoses isomerize to aldoses under the test's alkaline conditions.

Common Reactions

Hexoses undergo glycosylation reactions, where the anomeric carbon of one hexose molecule forms a glycosidic bond with a hydroxyl group of another molecule or aglycone, leading to disaccharides or more complex carbohydrates. For instance, two glucose molecules can condense to form maltose through an α-1,4-glycosidic linkage, eliminating water in a dehydration process. Oxidation of hexoses can be complete or selective, depending on the reagents used. Complete oxidation with concentrated oxidizes both the aldehyde group (in aldoses) and the primary alcohol at C6 to carboxylic acids, yielding saccharic acids such as glucaric acid from glucose. The reaction proceeds as follows: \ceC6H12O6(glucose)+3HNO3−>HOOC−(CHOH)4−COOH(glucaricacid)+3NO2+3H2O\ce{C6H12O6 (glucose) + 3 HNO3 -> HOOC-(CHOH)4-COOH (glucaric acid) + 3 NO2 + 3 H2O} Selective oxidation targets only the group to form aldonic acids or the C6 hydroxyl to produce uronic acids, such as from glucose, often using milder agents like for the former or enzymatic/templated methods for the latter. Reduction of hexoses involves the being converted to a hydroxyl, producing alditols. For example, glucose is reduced to using (NaBH₄) as a mild , which donates to the carbonyl carbon. The Kiliani-Fischer synthesis lengthens the carbon of an by one unit, converting an aldopentose to two epimeric aldohexoses. This involves addition of (HCN) to the aldehyde group, forming cyanohydrins, followed by to aldonic acids and reduction to the aldoses. In contrast, the shortens the chain of an by one carbon. The process oxidizes the aldose to an aldonic acid with , forms the calcium salt, and then oxidizes with and ferric to decarboxylate, yielding the lower aldose.

Biological Significance

Natural Occurrence

Hexoses are ubiquitous in nature, serving as fundamental building blocks of life across diverse organisms. They constitute a significant portion of Earth's , primarily in polymerized forms such as , , and , with plant-derived carbohydrates forming the majority of global biomass ( account for ≈82% of total biomass, largely as hexose polymers). , the most abundant hexose, is the primary product of via the Calvin-Benson cycle, where is fixed into glyceraldehyde-3-phosphate and subsequently converted into glucose as the key output in . In animals, glucose circulates as the principal blood sugar, maintaining , and forms the monomeric unit of storage like in liver and muscle tissues. Additionally, glucose polymers and provide energy reserves in and structural support in plant cell walls, respectively, underscoring its role in terrestrial ecosystems. Fructose, a ketohexose, is prevalent in plant-derived sources, particularly fruits and floral nectars, where it contributes to and . In , fructose comprises up to 50% of the total sugar content, often exceeding glucose in concentration and aiding in the preservation of this through its hygroscopic properties. Fructose also combines with glucose to form , the predominant in many , facilitating efficient and storage of carbohydrates in sources like and beets. Galactose, an aldohexose, is notably found in , the primary carbohydrate in mammalian , where it pairs with glucose to provide essential for neonatal development. In plants, galactose integrates into galactolipids, which form the majority of membrane in chloroplasts and thylakoids, supporting photosynthetic processes and cellular integrity. Other hexoses, such as , occur in specialized natural matrices; is a key component of gums like and , where it contributes to viscous exudates for protection against environmental stress. It also features prominently in bacterial , such as those in biofilms and cell walls, enhancing microbial adhesion and structural diversity in ecosystems. From an evolutionary perspective, hexoses have been central to life since its origins, with prebiotic synthesis pathways like the enabling the abiotic formation of sugars from under conditions, potentially seeding the pool for primordial metabolic networks.

Metabolism and Biosynthesis

In , glucose, a primary aldohexose, is biosynthesized from and through , a light-driven process that captures to fix CO₂ into organic compounds via the Calvin-Benson cycle, ultimately yielding glucose as a key product. In animals, glucose synthesis occurs via , a pathway that generates glucose from non-carbohydrate precursors such as lactate, , and glucogenic like , primarily in the liver and kidneys to maintain blood glucose levels during fasting. This process reverses key steps of but employs distinct enzymes to bypass irreversible reactions, ensuring efficient net production of glucose. Hexoses like are catabolized primarily through , a cytosolic pathway that begins with the phosphorylation of to glucose-6-phosphate (G6P) by , consuming one ATP molecule per glucose. Subsequent steps convert G6P to two molecules of pyruvate, generating four ATP and two NADH, resulting in a net yield of two ATP per glucose molecule under anaerobic conditions. , a ketohexose, follows a distinct route in the liver, where fructokinase phosphorylates it to fructose-1-phosphate (F1P) using ATP, and then cleaves F1P into (DHAP) and , with DHAP directly entering . An alternative metabolic route for hexoses is the , which branches from G6P and serves dual purposes: the oxidative phase generates NADPH for reductive biosynthesis and antioxidant defense by oxidizing G6P to ribulose-5-phosphate, producing two NADPH molecules per glucose equivalent. The non-oxidative phase interconverts sugars, enabling the production of ribose-5-phosphate for nucleotide synthesis from glucose-derived intermediates without net NADPH generation. Industrially, glucose syrup is produced by enzymatic hydrolysis of starch from sources like corn, involving liquefaction with alpha-amylase followed by saccharification with glucoamylase to yield high-glucose content syrups used in food processing. High-fructose corn syrup is derived similarly, with additional glucose isomerization to fructose using xylose isomerase, achieving up to 55% fructose for sweetened beverages. In medicine, 2-deoxyglucose, a glucose analog and deoxyhexose, acts as an adjunct in cancer therapy by inhibiting glycolysis in tumor cells, enhancing the efficacy of radiotherapy and chemotherapy through glycolytic blockade and oxidative stress induction. Biosynthesis of deoxyhexoses, such as L-fucose, proceeds from GDP-mannose in a de novo pathway: GDP-mannose is first converted to GDP-4-keto-6-deoxy-D-mannose by GDP-mannose 4,6-dehydratase, followed by action of a bifunctional GDP-4-keto-6-deoxy-D-mannose epimerase/reductase, which epimerizes at C5 and reduces the keto group using NADPH to yield GDP-L-fucose for and assembly. This pathway is conserved across eukaryotes and essential for fucosylation in cellular recognition processes.

References

  1. https://pubchem.ncbi.nlm.nih.gov/compound/Hexose
  2. https://www.biologyonline.com/dictionary/hexose
  3. https://chem.libretexts.org/Courses/American_River_College/CHEM_309%3A_Applied_Chemistry_for_the_Health_Sciences/07%3A_Carbohydrates_-_An_Introduction/7.02%3A_Important_Pentoses_and_Hexoses
  4. https://diabetes.org/about-diabetes/diagnosis
  5. https://pubmed.ncbi.nlm.nih.gov/37702899/
  6. https://www.uspto.gov/web/patents/classification/cpc/html/defC13B.html
  7. https://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/carbo5.html
  8. https://people.umass.edu/~mcclemen/581Carbohydrates.html
  9. https://www.ncbi.nlm.nih.gov/books/NBK579972/
  10. https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/carbhyd.htm
  11. https://www.ncbi.nlm.nih.gov/books/NBK579927/
  12. https://iupac.qmul.ac.uk/2carb/02.html
  13. https://media.iupac.org/publications/pac/1996/pdf/6810x1919.pdf
  14. https://iupac.qmul.ac.uk/2carb/03n04.html
  15. https://pubchem.ncbi.nlm.nih.gov/compound/aldehydo-D-glucose
  16. https://chem.libretexts.org/Courses/Brevard_College/CHE_301_Biochemistry/02%3A_Carbohydrates/2.03%3A_Fischer_Projections
  17. https://www.vanderbilt.edu/AnS/Chemistry/Rizzo/Chem220b/Ch25.pdf
  18. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/25%3A_Biomolecules-_Carbohydrates/25.04%3A_Configurations_of_Aldoses
  19. https://www.sciencedirect.com/topics/neuroscience/aldohexose
  20. https://guweb2.gonzaga.edu/faculty/cronk/CHEM245pub/carbohydrates.html
  21. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28Morsch_et_al.%29/25%253A_Biomolecules-_Carbohydrates/25.04%253A_Configurations_of_Aldoses
  22. https://openstax.org/books/organic-chemistry/pages/25-4-configurations-of-the-aldoses
  23. https://s10.lite.msu.edu/res/msu/botonl/b_online/library/newton/Chy251_253/Lectures/CarbohydratesI/CarbohydratesI.html
  24. https://openstax.org/books/organic-chemistry/pages/25-5-cyclic-structures-of-monosaccharides-anomers
  25. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28Morsch_et_al.%29/25%253A_Biomolecules-_Carbohydrates/25.06%253A_Reactions_of_Monosaccharides
  26. https://flexbooks.ck12.org/cbook/ck-12-cbse-chemistry-class-12/section/10.3/primary/lesson/cyclic-structure-of-glucose/
  27. https://www.pearson.com/channels/organic-chemistry/asset/728cd77a/the-specific-rotation-of-a-d-galactose-is-1507-and-that-of-b-d-galactose-is-528-
  28. https://www.sciencedirect.com/topics/chemistry/ketohexose
  29. https://pmc.ncbi.nlm.nih.gov/articles/PMC5413837/
  30. https://pubchem.ncbi.nlm.nih.gov/compound/439709
  31. https://pubmed.ncbi.nlm.nih.gov/2758952/
  32. https://pubchem.ncbi.nlm.nih.gov/compound/D-Fructose
  33. https://www.sciencedirect.com/science/article/abs/pii/S0008621522001276
  34. https://doi.org/10.5458/jag.jag.JAG-2017_013
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC389495/
  36. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/deoxy-sugar
  37. https://www.sciencedirect.com/topics/chemistry/deoxy-sugar
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC5458543/
  39. https://pubs.acs.org/doi/10.1021/acschembio.1c00646
  40. https://academic.oup.com/glycob/article/13/7/41R/612936
  41. https://portlandpress.com/biochemj/article/478/4/685/227880/NDP-rhamnose-biosynthesis-and
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC9998157/
  43. https://www.sciencedirect.com/topics/medicine-and-dentistry/daunosamine
  44. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/19%3A_Aldehydes_and_Ketones-_Nucleophilic_Addition_Reactions/19.04%3A_Nucleophilic_Addition_Reactions_of_Aldehydes_and_Ketones
  45. https://documents.thermofisher.com/TFS-Assets/CMD/Technical-Notes/tn-20-hpae-pad-carbohydrates-tn70671-en.pdf
  46. https://pubmed.ncbi.nlm.nih.gov/23706015/
  47. https://www.chemistrysteps.com/glycosides/
  48. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/25%3A_Biomolecules-_Carbohydrates/25.05%3A_Cyclic_Structures_of_Monosaccharides_-_Anomers
  49. https://asbe.org/article/caramelization/
  50. https://foodcrumbles.com/caramelization-honeycomb-making/
  51. https://www.masterorganicchemistry.com/2017/09/12/reducing-sugars/
  52. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%253A_Organic_Chemistry_%28Wade%29_Complete_and_Semesters_I_and_II/Map%253A_Organic_Chemistry_%28Wade%29/24%253A_Carbohydrates/24.08%253A_Disaccharides_and_Glycosidic_Bonds
  53. https://pubs.acs.org/doi/10.1021/jf60012a005
  54. https://www.sciencedirect.com/topics/chemistry/uronic-acid
  55. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%253A_Organic_Chemistry_%28Vollhardt_and_Schore%29/24%253A_Carbohydrates%253A_Polyfunctional_Compounds_in_Nature/24.06%253A_Reduction__of_Monosaccharides__to_Alditols
  56. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%253A_Organic_Chemistry_%28Bruice%29/22%253A_The_Organic_Chemistry_of_Carbohydrates/22.08%253A_Lengthening_the_Chain-_The_Kiliani-Fischer_Synthesis
  57. https://kpu.pressbooks.pub/organicchemistry2/chapter/9-5-degradation-and-synthesis-of-monosaccharides/
  58. https://www.pnas.org/doi/10.1073/pnas.1711842115
  59. https://pmc.ncbi.nlm.nih.gov/articles/PMC5264509/
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC7778149/
  61. https://open.oregonstate.education/animalnutrition/chapter/chapter-3/
  62. https://pmc.ncbi.nlm.nih.gov/articles/PMC5817209/
  63. https://www.alimentarium.org/en/fact-sheet/fructose
  64. https://www.sciencedirect.com/science/article/pii/S0304416521000568
  65. https://www.sciencedirect.com/topics/neuroscience/mannose
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC6617062/
  67. https://pmc.ncbi.nlm.nih.gov/articles/PMC11016555/
  68. https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_%28Biological_Chemistry%29/Metabolism/Anabolism/Gluconeogenesis
  69. https://pmc.ncbi.nlm.nih.gov/articles/PMC7573258/
  70. https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/a/glycolysis
  71. https://chem.libretexts.org/Courses/American_River_College/CHEM_309%253A_Applied_Chemistry_for_the_Health_Sciences/07%253A_Carbohydrates_-_An_Introduction/7.07%253A_Glycolysis
  72. https://pmc.ncbi.nlm.nih.gov/articles/PMC5785258/
  73. https://pmc.ncbi.nlm.nih.gov/articles/PMC11251397/
  74. https://bio.libretexts.org/Bookshelves/Biochemistry/Fundamentals_of_Biochemistry_%28Jakubowski_and_Flatt%29/02%253A_Unit_II-_Bioenergetics_and_Metabolism/13%253A_Glycolysis_Gluconeogenesis_and_the_Pentose_Phosphate_Pathway/13.04%253A_Pentose_Phosphate_Pathway_of_Glucose_Oxidation
  75. https://www.myandegroup.com/blog/glucose-syrup-production-from-starch
  76. https://www.cargill.com/food-beverage/emea/production-process
  77. https://pubmed.ncbi.nlm.nih.gov/9817848/
  78. https://www.uniprot.org/uniprotkb/Q13630/entry
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