File:Pyruvate kinase protein domains.png|
Pyruvate kinase with three domains presented in different colors.
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
The pyruvate kinase reaction:
This process also requires a magnesium ion. The enzyme is a 'transferase' under the international classification of enzymes.
This step is the final one in the glycolytic pathway, which produces pyruvate molecules, the final product of aerobic glycolysis. However, in anaerobic glycolysis, lactate dehydrogenase will utilize the NADH produced by glyceraldehyde phosphate dehydrogenase to reduce pyruvate to lactate. In humans, there are two pyruvate kinase isozymes: type M (muscle, SwissProt P14618) and type L,R (liver and erythrocyte, SwissProt P30613). The isozymes differ in primary structure and 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, in general, irreversible under wild-type conditions.
Pyruvate kinase activity is regulated by
- Its own substrate PEP and fructose 1,6-bisphosphate, an intermediate in glycolysis, both of which enhance enzymatic activity. Thus, glycolysis is driven to operate faster when more substrate is present.
- ATP is a negative allosteric inhibitor. This accounts for parallel regulation with PFK 1.
- It is not known whether citrate plays a role in negative allosteric inhibition, however it is believed that acetyl-CoA does.
- Alanine, a negative allosteric modulator
Like PFK, pyruvate kinase is regulated both by allosteric effectors and by covalent modification (phosphorylation). Pyruvate kinase is activated by F-1,6-BP in the liver, a second example of feedforward stimulation. ATP and alanine (a biosynthetic product of pyruvate) act as allosteric inhibitors of pyruvate kinase.
Liver pyruvate kinase is also regulated indirectly by epinephrine and glucagon, through protein kinase A. This protein kinase phosphorylates liver pyruvate kinase to deactivate it. Muscle pyruvate kinase is not inhibited by epinephrine activation of protein kinase A. Glucagon signals fasting (no glucose available). Thus, glycolysis is inhibited in the liver but unaffected in muscle when fasting. An increase in blood sugar leads to secretion of insulin, which activates phosphoprotein phosphatase I, leading to dephosphorylation and activation of pyruvate kinase. These controls prevent pyruvate kinase from being active at the same time as the enzymes that 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.
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 being converted 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.
A reversible enzyme with a similar function, Pyruvate phosphate dikinase (PPDK), is found in some bacteria and has been transferred to a number of anaerobic eukaryote groups (for example, Streblomastix, Giardia, Entamoeba, and Trichomonas), it seems via horizontal gene transfer on two or more occasions. In some cases, the same organism will have both Pyruvate kinase and PPDK.
- T. Selwood and E. K. Jaffe. (2011). "Dynamic dissociating homo-oligomers and the control of protein function.". Arch. Biochem. Biophys. 519 (2): 131–43. PMC 3298769. PMID 22182754. doi:10.1016/j.abb.2011.11.020.
- Liapounova, Na; Hampl, V; Gordon, Pm; Sensen, Cw; Gedamu, L; Dacks, Jb (Dec 2006), "Reconstructing the mosaic glycolytic pathway of the anaerobic eukaryote Monocercomonoides" (FREE FULL TEXT), Eukaryotic Cell 5 (12): 2138–46, ISSN 1535-9778, PMC 1694820, PMID 17071828, doi:10.1128/EC.00258-06