Substrate attachment in enzymes. The interaction of pyridoxal phosphate with amino acids

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Abstract
The changes which occur in the absorption spectrum of pyridoxal phosphate on addition of graded concentrations of glycine, alanine, valine, leucine, glutamic acid, arginine, ornithine, serine and isobutylamine are reported. Measurements were made over the pH range 4-8. From these results the affinity constants for the formation of Schiff bases between pyridoxal phosphate and the amino acids have been calculated. Comparison is made between these affinity constants and the classical Michaelis constants of pyridoxal phosphate enzymes reacting with the amino acids under investigation. The variation in Michaelis constant with pH had been investigated in the leucine decarboxylase of Proteus vulgaris; the values run parallel with those obtained for the leucine-pyridoxal phosphate affinity constant. Maximum rate of enzyme action at substrate saturation remains constant from pH 5 to 7, the apparent pH optimum at pH 6.5-7 being due to the fall in substrate affinity at lower pH values.