Structure, Function and Applications of Metal-Requiring Enzymes: Carbonic Anhydrase and Epi-Isozizaene Synthase
Cryptophane Biosensors for Targeting Human Carbonic Anhydrase Cryptophanes represent an exciting class of xenon-encapsulating molecules that can be exploited as probes for nuclear magnetic resonance imaging. A series of carbonic anhydrase-targeting, xenon-binding cryptophane biosensors were designed and synthesized. Isothermal titration calorimery and surface plasmon resonance measurements confirmed nanomolar affinity between human carbonic anhydrase II and the cryptophane biosensors. A 1.70 Å resolution crystal structure of a cryptophane-derivatized benezenesulfonamide human carbonic anhydrase II complex was determined, and shows how an encapsulated xenon atom can be directed to a specific biological target. Furthermore, this work illustrates the utility and promise of developing xenon biosensors to diagnose human diseases characterized by the upregulation of specific carbonic anhydrase biomarkers, specifically human carbonic anhydrase IX and XII. Structural Studies of epi-Isozizaene Synthase from Streptomyces coelicolor The X-ray crystal structure of recombinant epi-isozizaene synthase (EIZS), a sesquiterpene cyclase from Streptomyces coelicolor A3(2), has been determined at 1.60 Å resolution. Specifically, the structure of wild-type EIZS is that of its closed conformation in complex with three Mg2+ ions, inorganic pyrophosphate (PPi), and the benzyltriethylammonium cation (BTAC). Additionally, the structure of D99N EIZS has been determined in an open, ligand-free conformation at 1.90 Å resolution. Comparison of these two structures provides the first view of conformational changes required for substrate binding and catalysis in a bacterial terpenoid cyclase, and enables a comparison of substrate recognition amongst terpenoid synthases from different domains of life. Mutagenesis of aromatic residues in the enzyme active site alters the cyclization template and results in the production of alternative sesquiterpene products. The structure and activity of several active site mutants have been explored. The 1.64 Å resolution crystal structure of F198A EIZS in a complex with three Mg2+ ions, PPi, and BTAC reveals an alternative binding orientation of BTAC, whereas the crystal structures of L72V, A236G and V329A EIZS reveal an unchanged BTAC binding orientation. Alternative binding orientations of a carbocation intermediate could lead to the formation of alternative products.
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