β-Hydroxydecanoyl Thioester Dehydrase

Molecular Basis for Olefin Rearrangement in the Gephyronic Acid Polyketide Synthase

“…When exposed to an ( E )-2-enoyl NAC substrate, the E. coli FAS DH FabA produced an equilibrium mixture of 68% 3-hydroxy, 29% ( E )-2-enoyl, and 3% ( Z )-3-enoyl products . In an analogous manner, we used 6 and rac - 7 to assess the mixture of dehydration and isomerization products of GphF DH1 (Figure S5, Table ).…”

“…The DH domain generates a 2-enoyl thioester intermediate via syn -elimination of a 3-hydroxy group, resulting in the production of alkenes in even-to-odd positions in the final polyketide structure. − DHs are notable for a strict stereospecificity of the 3-hydroxy substrate, which has been correlated with the geometry of the product double bond. − , Despite high-resolution crystal structures of several type-I PKS DHs, the structural basis for substrate specificity and product selectivity remains unclear. ,− While “A” and “B” sequence motifs within KR domains predict the stereochemical configuration of the 3-hydroxy group, no such motifs have been identified in PKS DHs. , To date, only DHs that act on A-type KR products have been shown to produce cis double bonds, whereas B-type KR reduction followed by DH dehydration leads to trans double bonds in most cases, ,,, with notable exceptions. ,,, …”

“…However, FabA also catalyzes a subsequent double-bond migration to produce an odd-to-even olefin in the cis configuration (( Z )-3-decenoyl, Scheme S1). The final products of type II FAS with FabA include fatty acids with a cis , odd-to-even alkene situated six carbons from the terminal methyl group. An analogous system from Enterococcus faecalis has been characterized in which FabN, a dehydratase more similar to FabZ than FabA, dehydrates and isomerizes a 3-hydroxydecanoyl-ACP substrate to an odd-to-even ( Z )-3-decenoyl product .…”

Structure of a dehydratase–isomerase from the bacterial pathway for biosynthesis of unsaturated fatty acids: two catalytic activities in one active site

“…The fatty acyl chain and part of this arm-like prosthetic group bind inside the enzyme, and approximately half of the prosthetic group is assumed to protrude from the activesite tunnel at the dimer interface. Although the requirement of dehydrase for ACP-thiol ester substrates is not as stringent as for other enzymes of fatty acid biosynthesis in E. coli [1], the K m of dehydrase is more than 100-fold lower [30,31], and the turnover rate is six times greater with the ACP substrate than with the NAC thiol ester substrate [5]. ACP substrates also show a significantly different product distribution than that shown by NAC substrates [31].…”

“…The substrate specificity of dehydrase is notable [1,5,6]. It is highly specific for unbranched 10-carbon substrates, and this accounts for the fact that E. coli contains a very limited constellation of unsaturated fatty acids, all with one cis double bond six carbons from the terminal methyl group.…”