A marriage full of surprises: forty-five years living with glutamate dehydrogenase

Detailed kinetic studies of bovine glutamate dehydrogenase [GDH] from the 1960s revealed complexities that remain to be fully explained. In the absence of heterotropic nucleotide regulators the enzyme follows a random pathway of substrate addition but saturation with ADP enforces a compulsory-order mechanism in which glutamate is the leading substrate. The rate dependence on NAD(P)(+) concentration is complex and is probably only partly explained by negative binding cooperativity. Bovine GDH eluded successful analysis by crystallographers for 30 years but the final structural solution presented in this symposium at last provides a comprehensible framework for much of the heterotropic regulation, focussing attention on an antenna region in the C-terminal tail, a structure that is missing in the slightly smaller hexameric GDHs of lower organisms. Nonetheless, our studies with one such smaller (clostridial) GDH reveal that even without the antenna the underlying core structure still mediates homotropic cooperativity, and the ability to generate a variety of mutants has made it possible to start to dissect this machinery. In addition, this short personal review discusses a number of unresolved issues such as the significance of phospholipid inhibition and of specific interaction with mRNA, and above all the question of why it is necessary to regulate an enzyme reputedly maintaining its reactants at equilibrium and whether this might be in some way related to its coexistence with an energy-linked transhydrogenase.