The hybridization of glyceraldehyde 3-phosphate dehydrogenases from rabbit muscle and yeast. Kinetics and thermodynamics of the reaction and isolation of the hybrid

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Abstract
A kinetic and thermodynamic study was made of the formation of the hybrid (R2Y2) glyceraldehyde 3-phosphate dehydrogenase from the yeast (Y4) and rabbit (R4) enzymes. The values of the thermodynamic parameters for the equilibrium between R4, Y4 and R2Y2 suggest that the R2–R2 and Y2–Y2 interactions are similar. However, the failure to observe the RY3 and R3Y hybrids is interpreted in terms of differences at the interfaces of the R–R and Y–Y interactions (the glyceraldehyde 3-phosphate dehydrogenase molecule being regarded as a dimer of dimers). The kinetics of formation of the R2Y2 hybrid were studied and a model was proposed to account for the results. Best-fit values for the rate constants of the individual steps were evaluated by computer simulation, and the rate-limiting steps were identified as the dissociation of tetramers to dimers. It is proposed that the cleavage plane for dissociation of the tetramers corresponds to the region of low electron density through the centre of the molecule in the X-ray-crystallographic structure for human glyceraldehyde 3-phosphate dehydrogenase (Watson et al., 1972), which is probably the plane containing the Q and R axes in the lobster enzyme (Buehner et al., 1974). The R2Y2 hybrid was isolated in milligram amounts by ion-exchange chromatography and its rate of reversion to the native enzyme was shown to be consistent with the kinetic model proposed from the hybrid-formation experiments.