Amino acid effector binding to rabbit muscle pyruvate kinase

l-Phenylalanine, an allosteric inhibitor of rabbit muscle pyruvate kinase, is shown to bind to the tetrameric enzyme in a ratio of 4 moles effector per mole of tetramer. This binding is slightly cooperative in the absence of divalent cation activators, but the cooperativity is strongly increased when measured in the presence of 2.5 mm Mg²⁺ or Mn²⁺. The effector affinity is somewhat decreased under these conditions. l-Alanine was known to antagonize all measured phenylalanine effects and is shown here to also bind to 4 sites on the protein. The binding is noncooperative and little affected by the presence of the divalent activating cations. Competition experiments with phenylalanine and alanine suggest competition for the same site. Substrate kinetic measurements at P-enolpyruvate and Mg²⁺ concentrations under 100 μm show considerable inhibition of the enzyme at phenylalanine concentrations around 100 μm, near the serum levels of the free amino acid. The approach to the phenylalanine-inhibited velocity occurs with half-times less than 1 sec.