On the specificity of allene oxide cyclase.

Jasmonic acid is a carbocyclic fatty acid that is biosynthesized from alpha-linolenic acid in several steps. The formation of the ring structure of jasmonic acid is catalyzed by the enzyme allene oxide cyclase (EC 5.3.99.6) and involves the cyclization of an unstable allene oxide into the cyclopentenone 12-oxo-10,15(Z)-phytodienoic acid. In this study, a number of allene oxides were generated, and their enzymatic and nonenzymatic cyclization into cyclopentenones was investigated. Nonenzymatic cyclization was observed with allene oxides having one pair of conjugated double bonds and an additional isolated double bond in the beta,gamma position relative to the epoxide group, i.e., the partial structure 4,5-epoxy-1,3,7-octatriene. Enzymatic cyclization took place provided that this structural element was inserted in the fatty acid chain with its epoxide group in the n-6,7 position and the isolated double bond in the n-3 position. A number of oxygenated fatty acids having structural features in common with the natural allene oxides were tested as inhibitors of allene oxide cyclase. Fatty acids having an allene oxide structure in the n-6,7 position but lacking the double bond in the n-3 position, as well as fatty acids having a saturated epoxide group in the n-6,7 position, served as competitive inhibitors of the enzyme. Data on the substrate specificity of allene oxide synthase (EC 4.2.1.92) from corn seeds indicated that fatty acid hydroperoxides with a double bond at n-3 and with the hydroperoxide function at n-6 exhibit the highest affinity but the slowest reaction velocity.