David W. Christianson
Protein Structure and Function
X-ray crystallography is a powerful technique which can be used to visualize the three-dimensional structures of biologically-important macromolecules such as proteins. We use X-ray crystallography as a tool to probe and engineer the structure and function of novel proteins.
Protein Engineering of Transition Metal Binding Sites
We use the zinc metalloenzyme human carbonic anhydrase II as a paradigm for developing a rationale for the redesign of existing protein-metal binding sites as well as the design of de novo sites. We have probed the components of the zinc coordination polyhedron of carbonic anhydrase II which are important for metal affinity and function, and we have determined the three-dimensional structures of relevant variants with altered zinc binding sites. Our ultimate goal in protein design is to confer new catalytic activities upon the protein scaffolding of carbonic anhydrase II; for example, can we convert this metalloenzyme into a zinc protease or into a blue copper protein?
Structural and Mechanistic Studies of Hydrolytic Enzymes
We are interested in structural aspects of the mechanisms of hydrolytic metalloenzymes such as carbonic anhydrase II and Mn2+2-arginase. In genetic-structural studies of the carbonic anhydrase II active site, we have determined the minimum size and shape of the hydrophobic pocket required for substrate association, and we have determined the importance of hydrogen bond networks with zinc-bound hydroxide for its chemical reactivity. The structures of enzyme-inhibitor complexes yield additional insight on functionally-important residues. Upon completing the three-dimensional structure determinations of other hydrolytic enzymes such as epoxide hydrolase and juvenile hormone esterase, we will similarly probe the functional importance of active site residues through structural studies of enzyme variants and enzyme-inhibitor complexes.
Protein Inhibitors of Serine Proteases
Serpins (serine protease inhibitors) comprise a superfamily of more than forty proteins which are implicated in various regulatory cascades in vivo such as blood coagulation, fibrinolysis, complement activation, matrix proteolysis, and inflammation. Since these 45 kD proteins undergo a massive beta-sheet rearrangement during the course of their biological function, serpins are intriguing from the perspectives of protein folding as well as medical relevance. Antichymotrypsin has been engineered to be an inhibitor of human neutrophil elastase in vitro, and the engineered serpin exhibits antiinflammatory properties in vivo. We have determined the structure of this serpin, and current work focuses on a structure-assisted approach toward engineering the specificity and stability of the active serpin. Additionally, we will determine the structure of the serpin in its complex with a target serine protease, and we will also determine the structures of other members of the serpin superfamily, e.g., those involved in the pathogenesis of certain pox viruses.