What structural features characterize different isoforms of pyruvate kinase?

Pyruvate kinase (PK) has four unique isoforms: PKM1, PKM2, PKL, and PKR, each exhibiting distinct regulatory mechanisms and tissue distributions. The active form of PKR is a homotetramer composed of ~60-kDa subunits, with each subunit containing four domains that facilitate domain–domain interactions necessary for its catalytic activity.