Synthesis of a Bicyclobutane Fatty Acid Identified from the Cyanobacterium Anabaena PCC 7120

Abstract

Fatty acids and their metabolites play critical roles in mammalian and nonmammalian cell biology as signaling molecules, components of membranes, and storage lipids. [1] Enzymatic oxidation of polyunsaturated fatty acids leads to structurally diverse metabolites including unstable products such as allene oxides, divinyl ethers, and endoperoxides. [2,3] For example, in plants 12-oxophytodienoic acid IV is produced by an initial oxidation of linolenic acid by 13-lipoxygenase, followed by a transformation to an unstable allene oxide III (Scheme 1). [4] In 1988 Brash and co-workers showed that brief exposure of 13S-hydroperoxide II to a preparation of an allene oxide synthase followed by rapid organic extraction and treatment with diazomethane resulted in the isolation and characterization of the delicate allene oxide III. [5] The advent of whole genome sequencing of microbes has further expanded the discovery of novel enzymatic transformations and characterization of unstable products. [6,7] More recently, Brash and co-workers [8] identified, by genomic analysis, a dual-function protein encoded in the cyanobacterium Anabaena PCC 7120; this protein consists of a lipoxygenase domain fused to a catalase-related domain. After in vitro reconstitution of this protein, linolenic acid (I) was examined as an enzyme substrate and found to yield bicyclobutane fatty acid 1, the structure of which was confirmed by the characterization of methyl ester 2. The novel two-step transformation was proposed to occur by lipoxygenase-** We thank the National Institutes of Health (5RO1CA059515) for support of this research. S.M.D. acknowledges the support of the Vanderbilt Chemical Biology Interface (CBI) training program (T32 GM065086). We gratefully acknowledge Prof. Alan Brash for valuable discussion.