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Pantothenate kinase
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Pantothenate kinase
Pantothenate kinase (EC 2.7.1.33, PanK; CoaA) is the first enzyme in the Coenzyme A (CoA) biosynthetic pathway. It phosphorylates pantothenate (vitamin B5) to form 4'-phosphopantothenate at the expense of a molecule of adenosine triphosphate (ATP). It is the rate-limiting step in the biosynthesis of CoA.
CoA is a necessary cofactor in all living organisms. It acts as the major acyl group carrier in many important cellular processes, such as the citric acid cycle (tricarboxylic acid cycle) and fatty acid metabolism. Consequently, pantothenate kinase is a key regulatory enzyme in the CoA biosynthetic pathway.
Three distinct types of PanK has been identified - PanK-I (found in bacteria), PanK-II (mainly found in eukaryotes, but also in the Staphylococci) and PanK-III, also known as CoaX (found in bacteria). Eukaryotic PanK-II enzymes often occur as different isoforms, such as PanK1, PanK2, PanK3 and PanK4. In humans, multiple PanK isoforms are expressed by four genes. PANK1 gene encodes the PanK1α and PanK1β forms, and PANK2 and PANK3 encode PanK2 and PanK3, respectively. The four major isoforms found in mammals have different subcellular localizations. PanK1α is nuclear, while PanK1β and PanK3 are cytosolic. In mice, PanK2 is also cytosolic, while in humans, this enzyme is mitochondrial and nuclear. The tissue distribution of these isoforms also varies. In mouse models, PanK1 is the predominant species in the heart, liver and brown adipose tissue, along with the kidneys. PanK2 and PanK3 are more prominent in the brain and skeletal muscle, and PanK3 is particularly high in the intestines and white adipose tissue.
PanK-II contains two protein domains, as illustrated in Figure 1. The A domain and A' domain each has a glycine-rich loop (sequence GXXXXGKS; P loop) that is characteristic of nucleotide-binding sites; this is where ATP is assumed to bind. located between residues 95 and 102 on the A domain
The two ATP binding sites display cooperative behavior. The dimerization interface consists of two long helices, one from each monomer, that interact with each other. The C-terminal ends of the helices are held together by van der Waals interactions between valine and methionine residues of each monomer. The middle of the helices is attached by hydrogen bonds between asparagine residues. At the N-terminal end, each helix widens and forms a four-helix bundle with two shorter helices. This bundle consists of a hydrophobic core formed by non-polar residues that utilize van der Waals forces to further stabilize the dimer.
In the active site, pantothenate is oriented by hydrogen bonds between pantothenate and the side chains of aspartate, tyrosine, histidine, tyrosine, and asparagine residues. Asparagine, histidine, and arginine residues are involved in catalysis.
Human PanK-II isoforms PanK1α, PanK1β, PanK2, and PanK3 have a common, highly homologous catalytic core of approximately 355 residues. PanK1α and PanK1β are both encoded by the PANK1 gene and have the same catalytic domain of 363 amino acids, encoded by exons 2 through 7. The PanK1α transcript starts with exon 1α that encodes a 184-residue regulatory domain at the N-terminus. This region allows for feedback inhibition by free CoA and acyl-CoA and regulation by acetyl-CoA and malonyl-CoA. On the other hand, the PanK1β transcript starts with exon 1β, which produces a 10-residue N-terminus that does not include a feedback regulatory domain.
PanK-III also contains two protein domains, and the key catalytic residues of PanK-II are conserved. The monomer units of PanK-II and PanK-III are virtually identical, but they have distinctly different dimer assemblies. A study between the structures of Staphylococcus aureus type II and the Pseudomonas aeruginosa type III demonstrate that the PanK-II monomer has a loop region that is absent from the PanK-III monomer, and the PanK-III monomer has a loop region that is absent from the PanK-II monomer. This minor variation has a crucial difference on the dimerization interface in which the helices of the PanK-II dimer coil around one another and the helices of the PanK-III dimer interact at a 70° angle (Figure 2).
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Pantothenate kinase
Pantothenate kinase (EC 2.7.1.33, PanK; CoaA) is the first enzyme in the Coenzyme A (CoA) biosynthetic pathway. It phosphorylates pantothenate (vitamin B5) to form 4'-phosphopantothenate at the expense of a molecule of adenosine triphosphate (ATP). It is the rate-limiting step in the biosynthesis of CoA.
CoA is a necessary cofactor in all living organisms. It acts as the major acyl group carrier in many important cellular processes, such as the citric acid cycle (tricarboxylic acid cycle) and fatty acid metabolism. Consequently, pantothenate kinase is a key regulatory enzyme in the CoA biosynthetic pathway.
Three distinct types of PanK has been identified - PanK-I (found in bacteria), PanK-II (mainly found in eukaryotes, but also in the Staphylococci) and PanK-III, also known as CoaX (found in bacteria). Eukaryotic PanK-II enzymes often occur as different isoforms, such as PanK1, PanK2, PanK3 and PanK4. In humans, multiple PanK isoforms are expressed by four genes. PANK1 gene encodes the PanK1α and PanK1β forms, and PANK2 and PANK3 encode PanK2 and PanK3, respectively. The four major isoforms found in mammals have different subcellular localizations. PanK1α is nuclear, while PanK1β and PanK3 are cytosolic. In mice, PanK2 is also cytosolic, while in humans, this enzyme is mitochondrial and nuclear. The tissue distribution of these isoforms also varies. In mouse models, PanK1 is the predominant species in the heart, liver and brown adipose tissue, along with the kidneys. PanK2 and PanK3 are more prominent in the brain and skeletal muscle, and PanK3 is particularly high in the intestines and white adipose tissue.
PanK-II contains two protein domains, as illustrated in Figure 1. The A domain and A' domain each has a glycine-rich loop (sequence GXXXXGKS; P loop) that is characteristic of nucleotide-binding sites; this is where ATP is assumed to bind. located between residues 95 and 102 on the A domain
The two ATP binding sites display cooperative behavior. The dimerization interface consists of two long helices, one from each monomer, that interact with each other. The C-terminal ends of the helices are held together by van der Waals interactions between valine and methionine residues of each monomer. The middle of the helices is attached by hydrogen bonds between asparagine residues. At the N-terminal end, each helix widens and forms a four-helix bundle with two shorter helices. This bundle consists of a hydrophobic core formed by non-polar residues that utilize van der Waals forces to further stabilize the dimer.
In the active site, pantothenate is oriented by hydrogen bonds between pantothenate and the side chains of aspartate, tyrosine, histidine, tyrosine, and asparagine residues. Asparagine, histidine, and arginine residues are involved in catalysis.
Human PanK-II isoforms PanK1α, PanK1β, PanK2, and PanK3 have a common, highly homologous catalytic core of approximately 355 residues. PanK1α and PanK1β are both encoded by the PANK1 gene and have the same catalytic domain of 363 amino acids, encoded by exons 2 through 7. The PanK1α transcript starts with exon 1α that encodes a 184-residue regulatory domain at the N-terminus. This region allows for feedback inhibition by free CoA and acyl-CoA and regulation by acetyl-CoA and malonyl-CoA. On the other hand, the PanK1β transcript starts with exon 1β, which produces a 10-residue N-terminus that does not include a feedback regulatory domain.
PanK-III also contains two protein domains, and the key catalytic residues of PanK-II are conserved. The monomer units of PanK-II and PanK-III are virtually identical, but they have distinctly different dimer assemblies. A study between the structures of Staphylococcus aureus type II and the Pseudomonas aeruginosa type III demonstrate that the PanK-II monomer has a loop region that is absent from the PanK-III monomer, and the PanK-III monomer has a loop region that is absent from the PanK-II monomer. This minor variation has a crucial difference on the dimerization interface in which the helices of the PanK-II dimer coil around one another and the helices of the PanK-III dimer interact at a 70° angle (Figure 2).