Mechanism of negative cooperativity in glyceraldehyde-3-phosphate dehydrogenase deduced from ligand competition experiments.

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Abstract
The modulation in the negative cooperativity of ligand binding by another, competing ligand that binds noncooperatively is accounted for exclusively by the ligand-induced sequential model. Whenever such a phenomenon is observed it may argue strongly in favor of the sequential model. The advantages and limitations of this approach are evaluated. The binding of the coenzymes NAD+ and nicotinamide-1-N6-ethenoadenine dinucleotide to rabbit muscle apo-glyceraldehyde-3-phosphate dehydrogenase exhibits strong negative cooperativity, whereas acetylpyridine adenine dinucleotide, ATP and ADP-ribose bind noncooperatively to the NAD+ sites. The strong negative cooperativity in coenzyme binding was abolished in the presence of acetylpyridine adenine dinucleotide and strongly weakened by ATP, ADP and AMP, but was not affected by addition of ADP-ribose. The negative cooperativity in coenzyme binding to this enzyme apparently results from sequential conformational changes and exclude the pre-existent asymmetry model as a possible explanation. The structure of the pyridine moiety of the coenzyme analogs may play a role in orienting the adenine moiety at the adenine subsite, therefore affecting the cooperativity in the binding of the coenzyme analog which is mediated through the adenine subsites.