UNVEILING ENZYMATIC MECHANISMS WITH MALONYL-THIOESTER ISOSTERES

2019-12-05T19:34:38Z (GMT) by Lee M Stunkard
Malonyl-thioesters are reactive at the thioester carbonyl and the carboxylate moieties, as seen in acyl transfer or hydrolysis and decarboxylation. Enzymes use these reactive centers to perform different enzyme chemistry throughout metabolism. This enzyme chemistry coupled with the inherent reactivity of malonyl-thioesters makes structure-function studies difficult. When malonyl-thioesters are used for structure-function studies, it usually results in a hydrolyzed or decarboxylated product. There are examples, however, where this is overcome, many of which are discussed throughout this thesis. To overcome the inherent reactivity of malonyl-thioesters and enzymes, analogs have been synthesized to perform structure-function studies. Initial studies focused on altering the thioester carbonyl to limit hydrolysis and decarboxylation; however, these studies revealed the importance of retaining the thioester carbonyl to be positioned in the oxyanion hole. My thesis work focused on the synthesis, characterization, and use in structure-function studies of malonyl-thioester analogs that either preserve the thioester carbonyl or alter it to an ester or amide, and alter the carboxylate to a sulfonate or nitro group. After synthesizing the methylmalonyl-CoA analogs, we performed structure-function studies with methylmalonyl-CoA decarboxylase. This case study revealed the potential of these analogs to both inhibit decarboxylase activity and their use in structure-function studies to gain mechanistic insights. This successful study prompted us to continue these structure-function studies in enzymes with different chemistries such as an epimerase or bi-functional acyltransferase/decarboxylase. The widespread use of these methylmalonyl-CoA analogs also motivated us to add more malonyl-thioester analogs to our toolbox. I have preliminary data that these malonyl-thioester analogs inhibit β-keto-acyl-synthase III, an enzyme involved in fatty acid production in E. coli. This inhibition gives us confidence that these analogs will be useful in structure-function studies that will reveal answers to long standing mechanism and protein-protein interaction questions in the polyketide and fatty acid synthase field.