Affinity Labeling of Glyceraldehyde‐3‐phosphate Dehydrogenase from Sturgeon and Bacillus stearothermophilus by 3‐Chloroacetylpyridine–adenine Dinucleotide

Abstract

The inactivation of glyceraldehyde‐3‐phosphate dehydrogenase from Bacillus stearothermophilus and sturgeon by 3‐chloroacetylpyridine–adenine dinucleotide, which is not active as a coenzyme, obeys pseudo‐first‐order kinetics. The mechanism of inactivation corresponds to an affinity label. Four moles of inactivator are incorporated per mole of tetramer for both enzymes. All the kinetic results strongly support the idea that neither enzyme exhibits negative cooperativity for the non‐covalent binding of 3‐chloroacetylpyridine–adenine dinucleotide. NAD⁺ strongly protects both glyceraldehyde‐3‐phosphate dehydrogenases from inactivation. In the presence of NAD⁺, the inactivation process seems also to exhibit pseudo‐first‐order kinetics. The fact that each kinetic experiment can be described by only one pseudo‐first‐order rate constant is unexpected. The results of thiol titration are in favor of the modification of the essential Cys‐149, although another thiol group per monomer, probably Cys‐153, becomes titratable on both native alkylated enzymes. This latter result suggests that the alkylation of the Cys‐149 by the 3‐chloroacetylpyridine–adenine dinucleotide should affect the tertiary structure of these enzymes.