Anatomy of the beta-branching enzyme of polyketide biosynthesis and its interaction with an acyl-ACP substrate

TitleAnatomy of the beta-branching enzyme of polyketide biosynthesis and its interaction with an acyl-ACP substrate
Publication TypeJournal Article
Year of Publication2016
AuthorsMaloney F.P, Gerwick L, Gerwick WH, Sherman D.H, Smith J.L
JournalProceedings of the National Academy of Sciences of the United States of America
Date Published2016/09
Type of ArticleArticle
ISBN Number0027-8424
Accession NumberWOS:000383092000040
Keywords3-hydroxy-3-methylglutaryl-coa synthase; Acyl; bacillus-subtilis; carrier protein; colchicine site; crystal-structures; curacin; escherichia-coli; HMG synthase; molecular-interactions; natural products; polyketide synthase; structural basis; synthase docking domains

Alkyl branching at the beta position of a polyketide intermediate is an important variation on canonical polyketide natural product biosynthesis. The branching enzyme, 3-hydroxy-3-methylglutaryl synthase (HMGS), catalyzes the aldol addition of an acyl donor to a beta-keto-polyketide intermediate acceptor. HMGS is highly selective for two specialized acyl carrier proteins (ACPs) that deliver the donor and acceptor substrates. The HMGS from the curacin A biosynthetic pathway (CurD) was examined to establish the basis for ACP selectivity. The donor ACP (CurB) had high affinity for the enzyme (K-d = 0.5 mu M) and could not be substituted by the acceptor ACP. High-resolution crystal structures of HMGS alone and in complex with its donor ACP reveal a tight interaction that depends on exquisite surface shape and charge complementarity between the proteins. Selectivity is explained by HMGS binding to an unusual surface cleft on the donor ACP, in a manner that would exclude the acceptor ACP. Within the active site, HMGS discriminates between pre-and postreaction states of the donor ACP. The free phosphopantetheine (Ppant) cofactor of ACP occupies a conserved pocket that excludes the acetyl-Ppant substrate. In comparison with HMG-CoA (CoA) synthase, the homologous enzyme from primary metabolism, HMGS has several differences at the active site entrance, including a flexible-loop insertion, which may account for the specificity of one enzyme for substrates delivered by ACP and the other by CoA.

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