Half-of-the-sites and all-of-the-sites reactivity in rabbit muscle glyceraldehyde 3-phosphate dehydrogenase

The behavior of rabbit muscle apoglyceraldehyde 3-phosphate dehydrogenase towards alkylating agents depends on the chemical nature of the modifying reagent. Two classes of reagents were found, those yielding half-of-the-sites reactivity and those resulting in all-of-the-sites reactivity. On the basis of results obtained in this study and our previous investigations on the nature of enzyme-coenzyme interactions, a molecular explanation for this phenomenon is offered. It is suggested that interactions of the alkyl group of the modifying reagent at the nicotinamide subsite of the NAD⁺ site or the glyceraldehyde 3-phosphate site are not transmitted to other subunits and all-of-the-sites reactivity results. The reagents that contain alkyl groups which interact with the adenine subsite region of the NAD⁺ site induce a conformational change that is transmitted to a vacant subunit across one of the three sets of isologous intersubunit binding domains within the tetramer, thus inducing half-of-the-sites reactivity. The pattern of half-of-the-sites reactivity indicates that the enzyme tetramer functions as a dimer of dimers. It is suggested that the interaction that leads to half-of-the-sites reactivity occurs between the hydrophobic alkyl group and the hydrophobic trimethylene aide chain of Lys 183 originating from the neighboring subunit across the intersubunit domain, or with other amino acid residues.