PKM and myocardial infarction: For example, pyruvate kinase muscle isoforms (PKM) switch during postnatal cardiac maturation; while PKM2 is the main isoform expressed in neonatal hearts and is highly regulated by effectors and allosteric interactions, PKM1 is constitutively active and is the main adult isoform.38 Over-expression of PKM2 after myocardial infarction in mice increases cardiomyocyte proliferation and improves left ventricular systolic function, indicative of functional heart regeneration.