Distinct Mechanistic Behaviour of Tomato CYP74C3 and Maize CYP74A19 Allene Oxide Synthases: Insights from Trapping Experiments and Allene Oxide Isolation

The product specificity and mechanistic peculiarities of two allene oxide synthases, tomato LeAOS3 (CYP74C3) and maize ZmAOS (CYP74A19), were studied. Enzymes were vortexed with linoleic acid 9-hydroperoxide in a hexane–water biphasic system (20–60 s, 0 °C). Synthesized allene oxide (9,10-epoxy-10,12-octadecadienoic acid; 9,10-EOD) was trapped with ethanol. Incubations with ZmAOS produced predominantly 9,10-EOD, which was converted into an ethanolysis product, (12Z)-9-ethoxy-10-oxo-12-octadecenoic acid. LeAOS3 produced the same trapping product and 9(R)–α–ketol at nearly equimolar yields. Thus, both α–ketol and 9,10-EOD appeared to be kinetically controlled LeAOS3 products. NMR data for 9,10-EOD (Me) preparations revealed that ZmAOS specifically synthesized 10(E)-9,10-EOD, whereas LeAOS3 produced a roughly 4:1 mixture of 10(E) and 10(Z) isomers. The cyclopentenone cis-10-oxo-11-phytoenoic acid (10-oxo-PEA) and the Favorskii-type product yields were appreciable with LeAOS3, but dramatically lower with ZmAOS. The 9,10-EOD (free acid) kept in hexane transformed into macrolactones but did not cyclize. LeAOS3 catalysis is supposed to produce a higher proportion of oxyallyl diradical (a valence tautomer of allene oxide), which is a direct precursor of both cyclopentenone and cyclopropanone. This may explain the substantial yields of cis-10-oxo-PEA and the Favorskii-type product (via cyclopropanone) with LeAOS3. Furthermore, 10(Z)-9,10-EOD may be produced via the reverse formation of allene oxide from oxyallyl diradical.