Lizbeth K. Hedstrom
My laboratory is investigating the mechanism of enzyme action with two general aims:
- to understand the relationship between protein structure and function; and
- to use this knowledge to design novel enzymes and specific enzyme inhibitors.
This work involves enzyme kinetics, protein purification, site directed mutagenesis, organic synthesis and various spectroscopic methods of monitoring protein structure. Our studies focus on two enzyme systems: inosine monophosphate dehydrogenase (IMPDH) and trypsin.
One of our projects centers on the mechanism of drug action and rational approaches to drug design. We have chosen IMPDH as our target enzyme. IMPDH catalyzes the rate limiting step in guanine nucleotide biosynthesis, and is therefore a target for the development of cancer, viral, and immunosuppressive chemotherapy. We have characterized the mechanism of IMPDH inhibition by the antiviral drug EICAR, demonstrating that this compound forms a covalent adduct with the enzyme. This covalent adduct mimics an intermediate in the IMPDH reaction. We are also investigating the mechanism of mycophenolic acid inhibition of IMPDH. Mycophenolic acid is a promising immunosuppressive drug and a potent uncompetitive inhibitor of mammalian IMPDH's, although a poor inhibitor of bacterial and parasite IMPDH's. We are interested in the mechanism of species specificity of mycophenolic acid in order to design inhibitors specific for bacterial and parasite enzymes. We are also delineating the mechanism of the IMPDH reaction by identifying the enzyme residues involved in catalysis and inhibition.
Another project seeks to investigate the relationship between protein structure and function by analyzing the roles of specific amino acid residues in the conversion of the inactive precursor trypsinogen to its active counterpart trypsin. Our work probes the forces governing protein stability and the mechanism of protein conformational change by studying the transition between two well characterized protein conformations. Moreover, the activation of trypsinogen is a model for the activation of homologous serine proteases involved in blood clotting, fibrinolysis and complement activation. Therefore these experiments will define the structural features which are responsible for the high activity of the precursors of such physiologically important serine proteases as tissue plasminogen activator.
A third project is to define the structural components of substrate specificity in the trypsin family of serine proteases. This family is chosen because its members display a great variety of different substrate specificities, the enzymatic mechanism is well characterized and a large amount of structural information is available. In previous work, this laboratory constructed trypsin mutants with chymotrypsin-like specificity. These mutant enzymes have been extensively characterized: the kinetics of substrate hydrolysis, inhibition and inactivation have been analyzed, and the x-ray crystal structures have been solved. This work revealed:
- Surface loops, which do not contact the substrate, determine specificity in the S1 site.
- These mutants process substrates at rates comparable to chymotrypsin, but are defective in substrate binding.
- The specificity of the S1 site is linked to the S1' and S2-S4 binding sites.
- The structure of the S1 site is compromised in these mutants.
We are currently attempting to identify additional structural determinants of substrate specificity and determined if the structural motifs that we have already identified are general determinants of specificity in the trypsin family. In addition, we are trying to design enzymes with novel substrate specificities.