Isotope-exchange evidence for an ordered mechanism for rat-liver glucokinase, a monomeric cooperative enzyme

Abstract
The order of addition of substrates and release of products in the reaction catalyzed by rat-liver glucokinase was studied by measurements of isotope showed some degree of randomness but steady-state experiments showed a predominantly ordered process with glucose binding first and G-6-P released last. Experiments to trap binary complexes in the steady-state demonstrated the existence of complexes of the enzyme with glucose and with G-6-P but gave no evidence for the occurrence of corresponding complexes with ATP or ADP. Flux ratios measured in the forward and reverse reactions provided a more rigorous and quantitative confirmation of these characteristics of the reaction. These observations support the interpretation of glucokinase cooperativity in terms of a mnemonical mechanism and conflict with an alternative interpretation in terms of a random addition of substrates.