[Possible nature of negative cooperation in D-glyceraldehyde-3-phosphate dehydrogenase]

Some considerations concerning the detailed mechanism of negative cooperativity in GPD are proposed. The hypothesis represents a modification of the sequential model (Koshland et al.) taking into account last experimental data about the binding of NAD analogs and fragments. Two main facts have been used as a basis for the model: 1. Neither ADP-ribose nor nicotinamide mononucleotide (NMN) fragments of NAD show negative cooperative binding to GPD. 2. Neither modifications of adenine and nicotinamide part of NAD (epsilon-NAD, hypoxantine-NAD, oxidized and reduced-NAD) nor enzyme modifications by various reagents acting in the catalytic site affect considerably the cooperativity of coenzyme binding although the affinity between enzyme and coenzyme (analogs) substantially changes depending on the nature of modification. Probably the structural integrity of a coenzyme molecule is necessary for the cooperative binding to GPD. On the other hand, numerous modification studies can be interpreted as proving the absence of direct participation of adenine and nicotinamide rings in the mechanism of negative interactions between NAD-binding sites. It appears reasonable to assume that direct or indirect interactions of riboseAD and pyrophosphate groups of NAD with the "loop" of adjacent subunit might be necessary for the tight coenzyme binding to the first active site of the r-dimer(s) symmetric across the R-axis. After the tight binding of the first NAD molecule on r-dimer with the "loop" participation, the symmetrical movement of second "loop" might be highly restricted. It was postulated that only asymmetric conformational transition is possible in contact areas between subunits across the R-axis. Such asymmetric rearrangement can explain the nonequivalent binding of NAD to a prior symmetric dimmer(s).