Pyruvate kinase

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 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 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.
• PFK 1.
• Alanine, a negative allosteric modulator

Pyruvate kinase is also regulated indirectly by 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 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 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.