Properties of a CH3-blocked creatine kinase with altered catalytic activity. Kinetic consequences of the presence of the blocking group.

Steady state kinetic parameters for rabbit muscle creatine kinase (EC 2.7.3.2) and this enzyme stoichiometrically blocked at the iodoacetamide-sensitive cysteinyl residue with a CH3S-group have been measured at 30+/-0.1 degrees, pH 9.00, using Mg(II) as the required metal ion cofactor. The double reciprocal plots for the CH3S-blocked enzyme with MgATP as the variable substrate are biphasic, each curve showing a break at approximately 1.9 mM MgATP, and suggest the possibility of negative cooperativity in metal-nucleotide binding. Furthermore, extrapolated lines at high MgATP concentrations intersect on the abscissa, indicating loss of synergism in binding of substrates. In contrast, observed Michaelis constants for creatine are, within experimental error, the same for both native and blocked enzymes. The maximal velocity of the CH3S-blocked enzyme is found to be 28.1% of the value of the native enzyme. Product inhibition patterns for both native and blocked enzyme are also compared. Again, these patterns indicate that the CH3S-blocking group alters the nucleotide binding site more than the guanidino substrate binding site. Calculations using the methods of Chou and Fasman (1970) Biochemistry 13, 211-222) lead to the prediction that the active cysteinyl residue occurs at the beginning of a beta turn which separates two portions of beta sheet structure of the enzyme, and so may be in a position to mediate conformational changes in the protein.