Question

What happens to the [ADP] / [ATP] and [NAD+] / [NADH] ratios in red blood cells with a pyruvate kinase deficiency?

Answer 1

The pyruvate kinase reaction

RegulationThis 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

This protein may use the morpheein model of allosteric regulation. ^[1]

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.