Evidence for a Catalytically and Kinetically Competent Enzyme鈥揝ubstrate Cross-Linked Intermediate in Catalysis by Lipoyl Synthase

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• 作者：Nicholas D. Lanz ; Maria-Eirini Pandelia ; Elizabeth S. Kakar ; Kyung-Hoon Lee ; Carsten Krebs ; Squire J. Booker
• 刊名：Biochemistry
• 出版年：2014
• 出版时间：July 22, 2014
• 年：2014
• 卷：53
• 期：28
• 页码：4557-4572
• 全文大小：573K
• ISSN：1520-4995

文摘

Lipoyl synthase (LS) catalyzes the final step in lipoyl cofactor biosynthesis: the insertion of two sulfur atoms at C6 and C8 of an (N⁶-octanoyl)-lysyl residue on a lipoyl carrier protein (LCP). LS is a member of the radical SAM superfamily, enzymes that use a [4Fe鈥?S] cluster to effect the reductive cleavage of S-adenosyl-l-methionine (SAM) to l-methionine and a 5鈥?deoxyadenosyl 5鈥?radical (5鈥?dA^{鈥?/sup>). In the LS reaction, two equivalents of 5鈥?dA鈥?/sup> are generated sequentially to abstract hydrogen atoms from C6 and C8 of the appended octanoyl group, initiating sulfur insertion at these positions. The second [4Fe鈥?S] cluster on LS, termed the auxiliary cluster, is proposed to be the source of the inserted sulfur atoms. Herein, we provide evidence for the formation of a covalent cross-link between LS and an LCP or synthetic peptide substrate in reactions in which insertion of the second sulfur atom is slowed significantly by deuterium substitution at C8 or by inclusion of limiting concentrations of SAM. The observation that the proteins elute simultaneously by anion-exchange chromatography but are separated by aerobic SDS-PAGE is consistent with their linkage through the auxiliary cluster that is sacrificed during turnover. Generation of the cross-linked species with a small, unlabeled (N6-octanoyl)-lysyl-containing peptide substrate allowed demonstration of both its chemical and kinetic competence, providing strong evidence that it is an intermediate in the LS reaction. M枚ssbauer spectroscopy of the cross-linked intermediate reveals that one of the [4Fe鈥?S] clusters, presumably the auxiliary cluster, is partially disassembled to a 3Fe-cluster with spectroscopic properties similar to those of reduced [3Fe鈥?S]0 clusters.}