Pyruvate carboxylase

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Pyruvate carboxylase
ExPASy
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KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
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PMCarticles
PubMedarticles
NCBIproteins
Pyruvate carboxyltransferase
Identifiers
SymbolPYR_CT
PfamPF00682
InterProIPR000891
PROSITEPDOC50991
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
Pyruvate carboxylase
Identifiers
SymbolPC
Chr. 11 q11-q13.1
Search for
StructuresSwiss-model
DomainsInterPro
PC
Available structures
Gene ontology
Molecular function
Cellular component
Biological process
Sources:Amigo / QuickGO
Ensembl
UniProt
RefSeq (mRNA)

NM_000920
NM_001040716
NM_022172

NM_001162946
NM_008797

RefSeq (protein)

NP_000911
NP_001035806
NP_071504

n/a

Location (UCSC)Chr 11: 66.85 – 66.96 MbChr 19: 4.56 – 4.67 Mb
PubMed search[4][5]
Wikidata
View/Edit HumanView/Edit Mouse

Pyruvate carboxylase (PC) encoded by the gene PC is an

oxaloacetate
(OAA).

The reaction it catalyzes is:

pyruvate + HCO
3
+ ATP → oxaloacetate + ADP + P

It is an important anaplerotic reaction that creates oxaloacetate from pyruvate. The enzyme is a mitochondrial protein containing a biotin prosthetic group,[1] requiring magnesium or manganese and acetyl-CoA.

Pyruvate carboxylase was first discovered in 1959 at Case Western Reserve University by M. F. Utter and D. B. Keech.[6][7] Since then it has been found in a wide variety of prokaryotes and eukaryotes including fungi, bacteria, plants, and animals.[8] In mammals, PC plays a crucial role in gluconeogenesis and lipogenesis, in the biosynthesis of neurotransmitters, and in glucose-induced insulin secretion by pancreatic islets. Oxaloacetate produced by PC is an important intermediate, which is used in these biosynthetic pathways.[9] In mammals, PC is expressed in a tissue-specific manner, with its activity found to be highest in the liver and kidney (gluconeogenic tissues), in adipose tissue and lactating mammary gland (lipogenic tissues), and in pancreatic islets. Activity is moderate in brain, heart and adrenal gland, and least in white blood cells and skin fibroblasts.[10]

Structure

Structural studies of PC have been conducted by

electron microscopy, by limited proteolysis, and by cloning and gasa sequencing of genes and cDNA encoding the enzyme. Most well characterized forms of active PC consist of four identical subunits arranged in a tetrahedron-like structure. Each subunit contains a single biotin moiety acting as a swinging arm to transport carbon dioxide to the catalytic site that is formed at the interface between adjacent monomers. Each subunit of the functional tetramer contains four domains: the biotin carboxylation (BC) domain, the transcarboxylation (CT) domain, the biotin carboxyl carrier (BCCP) domain and the recently termed PC tetramerization (PT) domain.[11][12] From the two most complete crystal structures available, an asymmetric and symmetric form of the protein have been visualized.[13] The Staphylococcus aureus tetramer in complex with the activator coenzyme A is highly symmetric, possessing 222 symmetry, and has been confirmed by cryo-EM studies.[12] In contrast the Rhizobium etli, tetramer in complex with ethyl-CoA, a non-hydrolyzable analog of acetyl-CoA, possesses only one line of symmetry.[13]

Pyruvate carboxylase uses a covalently attached biotin cofactor which is used to catalyze the ATP– dependent carboxylation of pyruvate to oxaloacetate in two steps. Biotin is initially carboxylated at the BC active site by ATP and bicarbonate. The carboxyl group is subsequently transferred by carboxybiotin to a second active site in the CT domain, where pyruvate is carboxylated to generate oxaloacetate. The BCCP domain transfers the tethered cofactor between the two remote active sites. The allosteric binding site in PC offers a target for modifiers of activity that may be useful in the treatment of obesity or type II diabetes, and the mechanistic insights gained from the complete structural description of RePC (R. etli) permit detailed investigations into the individual catalytic and regulatory sites of the enzyme.[13]

Reaction mechanism

black and white schematic diagram depicting the mechanism of pyruvate carboxylase
Proposed mechanism of pyruvate carboxylase:
(A) ATP dependent carboxylation of biotin (BC domain);
(B) Transcarboxylation of pyruvate (CT domain).

The reaction mechanism can be subdivided into two partial reactions (see figure to the right). In the first reaction,

oxaloacetate is released. The biotin molecule is protonated by the aforementioned active site residue and released from the active site of the CT domain to be recarboxylated.[12][13] The major regulator of enzyme activity, acetyl-CoA, stimulates the cleavage of ATP in the first partial reaction and also it has been shown to induce a conformational change in the tetrameric structure of the enzyme.[9]

Function

During

mitochondria to produce PEP. Under ordinary gluconeogenic conditions, OAA is converted into PEP by mitochondrial PEPCK; the resultant PEP is then transported out of the mitochondrial matrix by an anion transporter carrier system,[14] and converted into glucose by cytosolic gluconeogenic enzymes. However, during starvation when cytosolic NADH concentration is low and mitochrondrial NADH levels are high oxaloacetate can be used as a shuttle of reducing equivalents. As such OAA is converted into malate by mitochondrial malate dehydrogenase (MDH). After export into the cytosol, malate is converted back into OAA, with concomitant reduction of NAD+; OAA is subsequently converted to PEP which is available for gluconeogenesis in the cytosol along with the transported reducing equivalent NADH.[1]

Very high levels of PC activity, together with high activities of other gluconeogenic enzymes including

glucose-6-phosphatase in liver and kidney cortex, suggest that a primary role of PC is to participate in gluconeogenesis in these organs. During fasting or starvation when endogenous glucose is required for certain tissues (brain, white blood cells and kidney medulla), expression of PC and other gluconeogenic enzymes is elevated.[15] In rats and mice, alteration of nutrition status has been shown to affect hepatic PC activity.[16] Fasting promotes hepatic glucose production sustained by an increased pyruvate flux, and increases in PC activity and protein concentration; diabetes similarly increases gluconeogenesis through enhanced uptake of substrate and increased flux through liver PC in mice and rats.[17][18] Similarly to other gluconeogenic enzymes, PC is positively regulated by glucagon and glucocorticoids while negatively regulated by insulin.[8] Further supporting the key role of PC in gluconeogenesis, in dairy cattle, which have hexose absorption ability at adequate nutrition levels, PC and the associated gluconeogenic enzyme PEPCK are markedly elevated during the transition to lactation in proposed support of lactose synthesis for milk production.[19]

Aside from the role of PC in gluconeogenesis, PC serves an anaplerotic role (an enzyme catalyzed reaction that can replenish the supply of intermediates in the citric acid cycle) for the tricarboxylic acid cycle (essential to provide oxaloacetate), when intermediates are removed for different biosynthetic purposes.

Click on genes, proteins and metabolites below to link to respective articles.[§ 1]

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GlycolysisGluconeogenesis_WP534go to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to WikiPathwaysgo to articlego to Entrezgo to article
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Glycolysis and Gluconeogenesis edit
  1. ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534".

Regulation

Pyruvate carboxylase is allosterically regulated by acetyl-CoA, Mg-ATP, and pyruvate.[20]

Clinical significance

As a crossroad between carbohydrate and lipid metabolism, pyruvate carboxylase expression in gluconeogenic tissues, adipose tissues and pancreatic islets must be coordinated. In conditions of over nutrition, PC levels are increased in pancreatic β-cells to increase pyruvate cycling in response to chronically elevated levels of glucose.[21] In contrast, PC enzyme levels in the liver are decreased by insulin;[22] during periods of overnutrition adipocyte tissue is expanded with extreme expression of PC and other lipogenic enzymes.[10][23] Hepatic control of glucose levels is still regulated in an over nutrition situation, but in obesity induced type 2 diabetes the regulation of peripheral glucose levels is no longer under regulation of insulin. In

β-cell
phenotype in decompensated diabetes.

A

blood sugar, deficiency of pyruvate carboxylase can also lead to hypoglycemia
.

See also

References

External links