Thesis advisor: Steven D. Bruner / The enediyne antitumor antibiotics are produced by complex biosynthetic machinery in acetomycetes. This dissertation will focus on the study of three enzymes involved in key steps in the biosynthesis of two enediynes, neocarzinostatin and C-1027. Neocarzinostatin is biosynthesized by a number of enzymes that synthesize and decorate the enediyne core and the peripheral moieties. NcsB1 is one enzyme involved in functionalizing the naphthoic acid portion of neocarzinostatin, a key group involved in binding to target DNA duplexes. The enzyme has been shown to be a promiscuous (<italic>S</italic>)-adenosylmethionine-dependent <italic>O</italic>-methyltransferase responsible for methylating a variety of hydroxynaphthoic acids. Multiple crystal structures of NcsB1 cocomplexed to substrate and/or cofactor have been solved. These structures revealed a displacement of the C-terminal domain when not bound to substrate, a movement that likely opens up the active site for naphthoate binding. Additionally, the ternary complex structure of 1,4-dihydroxynaphthoic acid, (<italic>S</italic>)-adenosylhomocysteine, and NcsB1 was solved and showed a rotation of this alternate substrate in the binding pocket, allowing for methylation. These results led us to probe NcsB1 activity using active site mutants, demonstrating altered substrate specificity and revealing key residues in substrate binding. The final step of neocarzinostatin biosynthesis involves multiple enzymes that convergently assemble the multiple biosynthetic intermediates to form the chromophore. NcsB2, originally proposed to catalyze the attachment of the naphthoic acid moiety to the enediyne core, has been characterized <italic>in vitro</italic>. Studies into its substrate specificity as an adenylation domain led to a revised biosynthetic pathway of 2-hydroxy-7-methoxy-5-methyl naphthoic acid. Instead of catalyzing the attachment of an enzyme bound naphthoic acid to the enediyne core, NcsB2 was found to act as a CoA-ligase, activating a variety of naphthoic acids and forming a naphthoyl-CoA intermediate. The results of these studies present an outstanding opportunity to produce novel analogs of neocarzinostatin by manipulating its biosynthesis. C-1027 is an architecturally similar enediyne that is also biosynthesized in a convergent route. C-1027 is a member of a class of enediynes that contains a functionalized β-tyrosine derived from L-tyrosine. The first catalytic step towards this beta-tyrosine moiety is achieved by <italic>Sg</italic>TAM, a tyrosine aminomutase that catalyzes a 2,3-amino shift on L-tyrosine to form (<italic>S</italic>)-β-tyrosine. The first X-ray crystal structure of <italic>Sg</italic>TAM was recently solved by our group, revealing structural homology to ammonia lyases. Through site-directed mutagenesis, X-ray crystallography, and biochemical analysis, residues that influence the mechanism by which <italic>Sg</italic>TAM catalyzes this difficult transformation were explored. From these studies, the enzymatic base and other pertinent residues involved in catalysis have been identified. In addition, residues that close the tunnel leading to the active site, thought to play a key role in mutase activity, were probed. Further study of rational mutants of <italic>Sg</italic>TAM will allow us to engineer its activity to alter its substrate specificity and the type of product it produces. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.
| Identifer | oai:union.ndltd.org:BOSTON/oai:dlib.bc.edu:bc-ir_101545 |
| Date | January 2009 |
| Creators | Cooke, Heather A. |
| Publisher | Boston College |
| Source Sets | Boston College |
| Language | English |
| Detected Language | English |
| Type | Text, thesis |
| Format | electronic, application/pdf |
| Rights | Copyright is held by the author, with all rights reserved, unless otherwise noted. |
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