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Pyruvate kinase



Pyruvate Kinase 1

Enzyme Pyruvate Kinase
PDB Code PDB 1A3W
Organism Yeast
Complexed molecules FBP, PG, Mn2+ and K+
Symbol(s): PKLR
Genetic data
Locus: Chr. 1 q21
Database Links
Codes: EntrezGene 5313, RefSeq NM_000298, UniProt P30613, Online 'Mendelian Inheritance in Man' (OMIM) 266200 EC 2.7.1.40
Pyruvate kinase 2
Symbol(s): PKM2
Genetic data
Locus: Chr. 15 [1]
Database Links
Codes: EntrezGene 5315, RefSeq NM_182470, UniProt P14618, Online 'Mendelian Inheritance in Man' (OMIM) 179050 EC 2.7.1.40

Pyruvate kinase is an enzyme involved in glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding one molecule of pyruvate and one molecule of ATP.

Additional recommended knowledge

Contents

Reaction

The reaction with pyruvate kinase:

        pyruvate 
         kinase
   PEP ----------> pyruvate 
         /     \
       ADP     ATP 
                                    

This process also requires a manganese ion. The enzyme is a hydrolase under the international classification of enzymes.

This step is the final one in the glycolytic pathway, which produces pyruvate molecules. The pyruvate may next be used to regenerate NAD+ via fermentation, or can be converted (as acetyl CoA) to ATP.

Regulation

This reaction has a large negative free energy change, one of three in glycolysis. All three such steps regulate the overall activity of the pathway, and are generally irreversible under physiological conditions.

Pyruvate kinase activity is regulated by:

  • Its own substrate PEP and fructose 1,6-bisphosphate, an intermediate in glycolysis; which both enhance enzymatic activity. Thus, glycolysis is driven to operate faster when more substrate is present.
  • Citrate and ATP, which allosterically inhibit it. This accounts for parallel regulation with PFK 1.
  • Alanine, a negative allosteric modulator

Pyruvate kinase is also regulated indirectly by insulin and glucagon, which control a protein kinase. This protein kinase phosphorylates pyruvate kinase to inactivate it and dephosphorylates the enzyme to activate it. Glucagon signals fasting (no glucose available), and insulin signals the opposite. These two signaling molecules--in conjunction with the protein kinase--prevent pyruvate kinase from being active at the same time as the enzymes which catalyze the reverse reaction (pyruvate carboxylase and phosphoenolpyruvate carboxykinase), preventing a futile cycle.

In fact, to say that the forward reaction and reverse reaction are not both active simultaneously may not be entirely accurate. Futile cycles, also known as substrate cycles, are known to fine-tune flux through metabolic pathways.

Deficiency

Genetic defects of this enzyme cause the disease known as pyruvate kinase deficiency. In this condition, a lack of pyruvate kinase slows down the process of glycolysis. This effect is especially devastating in cells that lack mitochondria, because these cells must use anaerobic glycolysis as their sole source of energy because the TCA cycle is not available.

One example is red blood cells, which in a state of pyruvate kinase deficiency rapidly become deficient in ATP and can undergo hemolysis. Therefore, pyruvate kinase deficiency can cause hemolytic anemia.

Role in gluconeogenesis

Pyruvate kinase also serves as a regulatory enzyme for gluconeogenesis, a biochemical pathway in which the liver generates glucose from pyruvate and other substrates. When pyruvate kinase is inhibited by phosphorylation (which occurs in the fasting state, via glucagon), phosphoenolpyruvate is prevented from conversion to pyruvate. Instead, it is converted to glucose in a series of gluconeogenesis reactions that are mostly (but not exactly) the reverse sequence of glycolysis.

The glucose thus produced is expelled from the liver, providing energy for vital tissues in the fasting state.

 v  d  e 
Glycolysis Metabolic Pathway
Glucose Hexokinase Glucose-6-phosphate Phosphoglucoisomerase Fructose 6-phosphate Phosphofructokinase Fructose 1,6-bisphosphate Fructose bisphosphate aldolase Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate Triosephosphate isomerase Glyceraldehyde 3-phosphate Glyceraldehyde phosphate dehydrogenase
ATP ADP ATP ADP NAD+ + Pi NADH + H+
+ 2
NAD+ + Pi NADH + H+
1,3-Bisphosphoglycerate Phosphoglycerate kinase 3-Phosphoglycerate Phosphoglycerate mutase 2-Phosphoglycerate Enolase Phosphoenolpyruvate Pyruvate kinase Pyruvate Pyruvate dehydrogenase Acetyl-CoA
ADP ATP H2O ADP ATP CoA + NAD+ NADH + H+ + CO2
2 2 2 2 2 2
ADP ATP H2O
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pyruvate_kinase". A list of authors is available in Wikipedia.
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