Protection against inhibition of creatine kinase by iodoacetamide is measured by the decrease in the rate constant for the inhibition reaction; A mixture of purified substrates at equilibrium protects quite strongly when all the components of the mixture are nearly saturating. The protection by substrates ‘working’ in the forward direction only (from creatine and MgATP) was measured by carrying out the experiment rapidly at low concentrations of the enzyme; by varying the concentration of substrate it was found that the amount of protection when the substrates of the forward reaction are saturating is about 80% (100% protection would imply a value of zero for the rate constant of the inhibition reaction). The effects of Ca2+ and Mg2+ are compared. It is already known that the complex creatine-NO3−-MgADP, which is considered to be either a transition-state analogue or an analogue of an intermediate in the reaction pathway, protects fully against iodoacetamide, whereas creatine and MgADP alone, or together without NO3−, do not protect. This suggests that the degree of protection by the working enzyme represents the proportion of enzyme molecules that have a conformation complementary to a creatine-PO3-MgADP intermediate.

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