Bioorganic, medicinal, and enzyme chemistry

The research in my group can be summarized as investigations of the molecular mechanisms of action, rational design, and syntheses of potential medicinal agents. Numerous drugs are known to function as specific inhibitors of particular enzymes. When little is known about the enzyme's molecular mechanism of action, chemical model studies are designed to determine reasonable nonenzymatic pathways applicable to the enzyme. Based on the proposed mechanism of enzyme action, inhibitors are designed and synthesized. Organic synthesis is of primary importance in this work. The enzymes are isolated from either mammalian tissue or from overexpressed cells containing recombinant enzyme. Active site labeling studies utilize MALDI TOF and electrospray ionization mass spectrometry as well as radiolabeled inactivators and peptide mapping.

My group is using these approaches to study enzymes involved in the treatment of several neurodegenerative diseases. We have designed and synthesized several new inhibitors and inactivators of brain aminobutyric acid (GABA) aminotransferase and have investigated their mechanisms. Compounds that inhibit this enzyme exhibit anticonvulsant activity and are important in the treatment of addiction. In collaboration with a crystallography group in Basel, we have obtained high resolution crystal structures of several of our inactivators bound to GABA aminotransferase and are doing structure based design of new inhibitors.

Another enzyme in which we are interested is nitric oxide synthase, the enzyme that generates the important second messenger nitric oxide. This enzyme exists in three isozymic forms, one in brain (nNOS), in macrophage (iNOS, the inducible form), and in endothelial cells (eNOS). Inhibitors of the brain isoform may be important in the treatment of neurotoxicity and stroke, but only if selective inhibition of this isoform can be accomplished to avoid blockage of NO production where it is needed. We have synthesized several new classes of compounds that are highly selective for nNOS. In collaboration with a crystallographer at UC Irvine, we have several high resolution crystal structures (see the figure below for one of our inhibitors bound to nNOS) of all of the isozymes with some of our inhibitors bound and are using these structures for the design of new classes of inhibitors.

Recently, the Chinese herbal medicine containing celastrol was determined in my collaborator's group to induce a heat shock response in neuronal cells. Heat shock transcription factor is involved in maintaining the structural integrity of brain proteins, thereby preventing neurodegeneration. My group is synthesizing analogs of celastrol and is studying the mechanism for its heat shock activation in collaboration with Prof. Richard Morimoto in our Department of Biochemistry, Molecular Biology, and Cell Biology.

The group also is interested in the synthesis of cyclic peptides and cyclic depsipeptides as potential inhibitors for enzymes that are important to tumor cell growth. Currently, our interests are directed at marine natural products that have been found to have potent anticancer activity. We also are designing inhibitors of enzymes in the mevalonate pathway of S. pneumoniae as selective antimicrobial agents.

Zhu, Y.; Nikolic, D.; Van Breemen, R. B.; Silverman, R. B.   Mechanism of Inactivation of Inducible Nitric Oxide Synthase by Amidines.   Irreversible Enzyme Inactivation without Inactivator Modification.  J. Am. Chem. Soc. 2005, 127, 858-868.

Liu, S.; Gu, W.; Lo, D.; Ding, X.-Z.; Ujiki, M.; Adrian, T. E.; Soff, G. A.; Silverman, R. B. N -Methylsansalvamide A Peptide Analogues. Potent New Antitumor Agents. J. Med. Chem. 2005, 48, 3630-3638.

Li, H.; Flinspach, M. L.; Igarashi, J.; Jamal, J.; Yang, W.; Gómez-Vidal, J. A.; Litzinger, E. A.; Huang, H.; Erdal, E. P.; Silverman, R. B.; Poulos, T. P.   Exploring the Binding Conformations of Bulkier Dipeptide Amide Inhibitors in Constitutive Nitric Oxide Synthases. Biochemistry 2005, 44, 15222-15229.

Yuan, H.; Silverman, R. B. New Substrates and Inhibitors of g -Aminobutyric Acid Aminotransferase Containing Bioisosteres of the Carboxylic Acid Group: Design, Synthesis, and Biological Activity. Bioorg. Med. Chem. 2006, 14, 1331-1338.

Ujiki, M. B.; Milam, B.; Ding, X.-Z.; Roginsky, A. B.; Salabat, M. R.; Talamonti, M. S.; Bell, R. H.; Gu, W.; Silverman, R. B.; Adrian, T. E. A Novel Peptide Sansalvamide   Analogue Inhibits Pancreatic Cancer Cell Growth Through G0/G1 Cell Cycle Arrest. Biochem. Biophys. Res. Commun. 2006, 340, 1224-1228.

Gu, W.; Nusinzon, I.; Smith, R. D. Jr.; Horvath, C. M.; Silverman, R. B. Carbonyl- and Sulfur-Containing Analogs of S uberoylanilide H ydroxamic A cid. Potent   Inhibition of Histone Deacetylases. Bioorg. Med. Chem. 2006, 14, 3320-3329 .

Lu, H.; Silverman, R. B. Fluorinated Conformationally-Restricted g -Aminobutyric Acid Aminotransferase Inhibitors. J. Med. Chem., 2006, 49, 7404-7412.

Wang, Z.; Yuan, H.; Silverman, R. B. (±)-(1S,2R,5S)-5-Amino-2-fluorocyclohex-3-ene Carboxylic Acid, A Potent GABA Aminotransferase Inactivator that Irreversibly Inhibits through an Elimination-Aromatization Pathway. Biochemistry, 2006, 45, 14513-14522.

Zhu, Y.; Silverman, R. B. Electronic Effects of Peripheral Substituents at Porphyrin Meso-Positions. J. Org. Chem., 2007, 72, 233-239.

Yuan, H.; Silverman, R. B. Structural Modifications of (1 S ,3 S )-3-Amino-4-Difluoromethylenecyclopentanecarboxylic Acid, a Potent Irreversible Inhibitor of GABA Aminotransferase. Bioorg. Med. Chem. Lett. 2007, 17, 1651-1654.

View all publications by publications by Richard B. Silverman listed in the National Library of Medicine (PubMed).