Biosynthetic Studies of ω-Cycloheptyl Fatty Acids in Alicyclobacillus cycloheptanicus. Formation of Cycloheptanecarboxylic Acid from Phenylacetic Acid

Abstract

The formation of the structurally novel, mono-substituted cycloheptane ring in omega-cycloheptyl fatty acids in Alicyclobacillus cycloheptanicus (formerly Bacillus cycloheptanicus) has been examined. Feeding experiments with (13)C- and (2)H-labeled intermediates demonstrated that cycloheptanecarboxylic acid (3), probably as its CoA thioester, is the starter unit for omega-cycloheptyl fatty acid biosynthesis. Analysis of the resultant labeling pattern from a feeding experiment with [U-(13)C(6)]glucose suggested a shikimate pathway origin of 3 via aromatic amino acids. [1,2-(13)C(2)]Phenylacetic acid (6) was efficiently metabolized into the 3-derived moiety in a manner reminiscent of the seven-membered ring Pseudomonas metabolite thiotropocin. The fates of the aromatic and benzylic hydrogens of 6 were determined; these dictated various boundary conditions for the biosynthetic pathway from 6 to 3. Taken together with the results from feeding experiments with postulated cycloheptenylcarboxylate biosynthetic intermediates, the data lead us to propose a pathway which involves an oxidative ring-expansion of 6 to a hydroxynorcaradiene intermediate followed by a series of double bond reductions and dehydrations to the saturated 3.

Studies on the Mechanism of Ring Hydrolysis in Phenylacetate Degradation

“…Astonishingly, the derivative (VII) resembles parts of the unusual predominant eponymous -cycloheptyl fatty acids (XI) from Alicyclobacillus cycloheptanicus (46). The terminal cycloheptane ring is derived from phenylacetate and formed through a so far unknown ring expansion mechanism similar to the production of tropodithietic acid/thiotropocin (46). Possibly all tropone/ tropolone-related compounds found in bacteria may be formed this way either from proffered phenylalanine/phenylacetate (catabolic) or from a C 9 Shikimate pathway intermediate (anabolic).…”

“…Moreover, a very similar sulfur-bridged dicyclic tropolone with broad antibacterial activity spectrum was isolated from Burkholderia cenocepacia (43,44), which can grow on phenylacetate (45). Astonishingly, the derivative (VII) resembles parts of the unusual predominant eponymous -cycloheptyl fatty acids (XI) from Alicyclobacillus cycloheptanicus (46). The terminal cycloheptane ring is derived from phenylacetate and formed through a so far unknown ring expansion mechanism similar to the production of tropodithietic acid/thiotropocin (46).…”

Oxidative Cyclization in Natural Product Biosynthesis

“…A different mechanism for tropolone ring formation via an epoxide intermediate was confirmed in the degradation of phenylacetyl-CoA, 586 and was proposed in the biosynthesis of omega-cycloheptyl fatty acids. 587 …”

The biosynthesis of shikimate metabolites

“…The biosynthesis of the cycloheptane ring in 173 (the most abundant fatty acid) has recently been the subject of some very elegant and detailed studies. 66 ]phenylacetic acid) defined the fate of the aromatic hydrogens of phenylacetic acid in the biosynthesis of the cycloheptyl moiety. Incorporation patterns detected by combined use of 2 H NMR and mass spectroscopy following the feeding experiment with l-[ 2 H 5 ]phenylalanine indicated that one quarter of the maximum theoretical deuterium enrichment at C14/17 of 173 is lost during the course of the biosynthesis and moreover, the biosynthetic pathway must proceed through a C 2 symmetrical intermediate in order to account for the observed equal distribution of deuterium between the axial and equatorial positions at the ring methylene carbons.…”