Mechanism of the Aldolase Reaction.

Upon treatment with carboxypeptidase, muscle aldolase (ketose-l-phosphate aldehyde-lyase, EC 4.1.2.7) loses 3 COOHterminal tyrosine residues (3, 4) and exhibits a 20-fold fall in the rate of fructose diphosphate cleavage (3) and a 500-fold fall in the rate of exchange detritiation of dihydroxyacetone phosphate (5). The cleavage of fructose-l-P is relatively unchanged (3). The present investigation shows that it is not necessary to invoke an unusual explanation (6) for these differential effects but that they follow from the fact that different steps are rate-determining in the three processes examined and hence these would not be expected to change concomitantly when only a single reaction step has been modified by the carboxypeptidase treatment, as will be shown is the case. By use of isotope exchange at equilibrium, comparing native with carboxypeptidase modified enzyme, information is obtained about the order of substrate reaction with the enzyme in the condensation reaction. Further exchange experiments are done in an attempt to observe isomerization steps in the mechanism.