Pyruvate kinase

The anaerobic conversion of glucose to lactate by the glycolytic pathway provides the red blood cell with energy in the form of adenosine triphosphate (ATP). Pyruvate kinase (PK) is a key regulatory enzyme of glycolysis. It catalyzes the irreversible phosphorylgroup transfer from phosphoenolpyruvate to ADP, yielding pyruvate and ATP. PK is allosterically activated by phosphoenolpyruvate and fructose-1,6-bisphosphate (FBP), and negatively regulated by its product ATP. Furthermore, the enzyme has an absolute requirement for cations, normally Mg²⁺ and K⁺.

PK is a tetrameric enzyme and four different isozymes are expressed in mammals. The red blood cell-specific isozyme PK-R is transcribed from the PKLR gene, located on chromosome 1q21. This gene also encodes the PK-L isozyme, which is predominately expressed in the liver. PKLR consists of 12 exons and spans 9.5 kb. Exon 1 is erythroid-specific whereas exon 2 is liver specific. The red blood cell-specific mRNA is 2 kb in length and codes for a PK-R subunit of 574 amino acids. The two other mammalian isozymes PK-M1 and PK-M2 are produced from the PKM gene. The PK-M2 isozyme is expressed in early fetal tissues, but also in most adult tissues. In basophilic erythroblasts, both PK-R and PK-M2 are expressed. During further erythroid differentiation and maturation, a switch in isozymes occurs whereby progressively increased PK-R expression gradually replaces PK-M2.

Each PK-R subunit can be divided into four domains: N, A, B, and C. The active site lies in a cleft between the A domain and the flexible B domain. The C domain contains the binding site for FBP.