An activating interaction between the unphosphorylated n-terminus of human liver pyruvate kinase and the main body of the protein is interrupted by phosphorylation.

The initial 26 amino acids of human liver pyruvate kinase (L-PYK) are not present/observed in the crystal structure. This region includes Ser12, the site of hormone-dependent phosphorylation. Truncating the N-terminus of L-PYK mimics the effects of phosphorylation by causing a decrease in apparent phosphoenolpyruvate (PEP) affinity. An N-terminus truncation series was used to map the minimum number of residues that could be removed to result in the decrease in apparent PEP affinity. Results are consistent with a mechanism by which phosphorylation at Ser12 interrupts an activating interaction of N-terminal residues (including those at positions 7-10) with the main body of the protein, as a means of inhibiting substrate affinity.