Dimers generated from tetrameric phosphorylating glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus are inactive but exhibit cooperativity in NAD binding.

Tetrameric phosphorylating glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from Bacillus stearothermophilus has been described as a "dimer of dimers" with three nonequivalent interfaces, P-axis (between subunits O and P and between subunits Q and R), Q-axis (between subunits O and Q and between subunits P and R), and R-axis interface (between subunits O and R and between subunits P and Q). O-P dimers, the most stable and the easiest to generate, have been created by selective disruption of hydrogen bonds across the R- and Q-axis interfaces by site-directed mutagenesis. Asp-186 and Ser-48, and Glu-276 and Tyr-46, which are hydrogen bond partners across the R- and Q-axis interfaces, respectively, have been replaced with glycine residues. All mutated residues are highly conserved among GAPDHs from different species and are located in loops. Both double mutants D186G/E276G and Y46G/S48G were dimeric, while all single mutants remained tetrameric. As previously described [Clermont, S., Corbier, C., Mely, Y., Gerard, D., Wonacott, A., and Branlant, G. (1993) Biochemistry 32, 10178-10184], NAD binding to wild type GAPDH (wtGAPDH) was interpreted according to the induced-fit model and exhibited negative cooperativity. However, NAD binding to wtGAPDH can be adequately described in terms of two independent dimers with two interacting binding sites in each dimer. Single mutants D186G, E276G, and Y46G exhibited behavior in NAD binding similar to that of the wild type, while both dimeric mutants D186G/E276G and Y46G/S48G exhibited positive cooperativity in binding the coenzyme NAD. The fact that O-P dimer mutants retained cooperative behavior shows that (1) the P-axis interface is important in transmitting the information induced upon NAD binding inside the O-P dimer from one subunit to the other and (2) the S-loop of the R-axis-related subunit is not directly involved in cooperative binding of NAD in the O-P dimer. In both O-P dimer mutants, the absorption band of the binary enzyme-NAD complex had a highly decreased intensity compared to that of the wild type and, in addition, totally disappeared in the presence of G3P or 1,3-dPG. However, no enzymatic activity was detected, indicating that the formed ternary enzyme-NAD-G3P or -1, 3-dPG complex was not catalytically efficient. In the O-P dimers, the interaction with the S-loop of the R-axis-related subunit is disrupted, and therefore, the S-loop should be less structured. This resulted in increased accessibility of the active site to the solvent, particularly for the adenosine-binding site of NAD. Thus, together, this is likely to explain both the lowered affinity of the dimeric enzyme for NAD and the absence of activity.