Kinetic analysis of two purified forms of arginine kinase: Absence of cooperativity in substrate binding of dimeric phosphagen kinase

Arginine kinase from sea urchin eggs and sea cucumber muscle are dimeric enzymes, unlike the more widely distributed monomeric enzyme found in other invertebrates. Both purified enzymes exhibited features characteristic of the monomeric arginine kinases including pH optima, formation of a catalytic dead-end complex (enzyme–MgADP–arginine) and stabilization of this complex by monovalent anions. A complete analysis of initial velocity data, in both directions for each substrate, indicated that substrate binding cooperativity was either minimal or non-existent. Unlike many other multi-subunit enzymes, the significance of the dimeric state of the phosphagen kinases remains unclear. These present results would suggest that (a) cooperativity, or so-called synergism in substrate binding is not a characteristic of the dimeric state of the protein and (b) the functional significance of the dimeric state is not related to the ability of some of these enzymes to undergo cooperativity in substrate binding. The significance of the dimeric state for the creatine kinases and arginine kinases remains to be established.