EFFECT OF SUBSTRATE AND PHOSPHATE IONS ON THE QUATERNARY STRUCTURE SYMMETRY OF GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASES OF MUNG BEANS AND RABBIT MUSCLE

Effects of glyceraldehyde-3-phosphate (G-3-P) and phosphate ions on th ermal inactivation of glyceraldehyde-3-phosphate dehydrogenases (GPDHs ) of mung beans and rabbit muscle have been studied at different pH. I n the absence of any ligand, the two enzymes show a striking similarit y in the pH-dependence of the kinetics of thermal activation. At lower pH values both the enzymes show biphasic kinetics with each phase acc ounting for about half of the starting activity (a C2 symmetry of the homo-tetrameric enzyme molecule). The kinetics change to a single expo nential decay at higher pH values, a D2 symmetry Malhotra & Srinivasa n (1985) Arch. Biochem. Biophys. 236, 775-781; Malhotra & Tikoo (1991) Indian. J. Biochem. Biophys. 28, 16-21!. With each enzyme, phosphate ions are found to have no effect on the kinetic pattern at lower pH, b ut G-3-P brings about a change from biphasic to a single exponential d ecay. At higher pH values, G-3-P has no effect. on the single exponent ial decay kinetic pattern, but phosphate ions change the same to a bip hasic loss of activity with each phase accounting for about half of th e starting activity. It has been concluded that with both the enzymes, G-3-P and phosphate ions have higher affinity and stabilise the D2- a nd C2-symmetry conformation, respectively. Binding isotherms of the tw o substrates for these enzymes have been described based on the ligand concentration-dependence of the changes in the rate constants and kin etic pattern of thermal inactivation. A weak or strong positive cooper ativity is observed only where the ligand binding brings about a confo rmational change in the enzyme protein. With each enzyme, both the lig ands are found to bind more strongly at lower pH than at higher pH val ues.