Emeritus Professor of Biochemistry and Biophysics
(PhD – University of Southern California)
Allosteric Enzymes: Size, Regulation and Function
For most enzymes, their subunits associate to form dimers, tetramers, or larger oligomers. Often this is a mechanism for change in conformation and activity. Where physiologically appropriate ligands (substrates, activators, inhibitors) stabilize only one of these conformations, this becomes a mechanism for allosteric regulation.
To make a correct assignment about the intrinsic catalytic activity of some oligomer or subunit, rapid kinetic studies are done as the enzyme goes through the conformational transition following the addition of the appropriate regulatory ligand. In each case one substrate is also an activator. Comparing the enzyme’s affinity for the substrate as an activator (change in con-formation as measured by physical studies) to the enzyme’s affinity for the substrate as a reagent (affinity at catalytic site, or Km), and modeling such ligand binding leads to a general pattern for the evolution of a class of regulatory enzymes. To explore this model we use mutagenesis to produce variant proteins that are altered in catalysis or in response to regulation. This helped to demonstrate that CTP inhibits uridine-cytidine kinase as a bisubstrate analog binding at the catalytic site.
Another feature of our research is based on the hypothesis that protein subunit size increases as proteins have more complex functions. Current work is focused on the bifunctional UMP synthase, and its two separate catalytic domains.