Unique tryptic peptide of pyruvate kinase muscle 1 (PKM1). PKM1 is a splice variant (containing exon 9) of the pyruvate kinase (PK) gene. PK catalyses the final step in the glycolytic pathway, and is responsible for net energy production within that pathway. There are four isozymes of PK in mammals, PKM1 (pyruvate kinase, muscle) is the main isoform found in the muscle and brain and is expressed in differentiated cells. PKM1 is essential for ATP generation by promoting oxidative phosphorylation. During neuronal differentiation, pyruvate kinase gene splicing is switched from PKM2 to PKM1, resulting in a switch from aerobic glycolysis to oxidative phosphorylation, this ensures the survival of neurons after differentiation.

This peptide represents a tryptic peptide of PKM1. The leucine residue at position 5 of this peptide is isotopically labelled with carbon-13 (6) and nitrogen-15 (1), giving this peptide a mass increase of 7 compared to the unlabelled peptide.