13/16-Specific Allene Oxide Synthase BnAOS (CYP74A) of Rapeseed (Brassica napus L.): Identification and Biochemical Properties.

Lipoxygenase cascade is the source of key plant bioregulators, oxylipins, which are oxidized derivatives of polyunsaturated fatty acids that play a crucial role in growth and development, as well as stress response. The key enzymes of the lipoxygenase cascade are allene oxide synthases (AOS) and hydroperoxide lyases, which are present in all known angiosperms. Although these enzymes play a central role in defining the pattern of the final metabolites of the cascade, significant gaps still exist in understanding their catalytic properties and positional specificity. This paper describes the allene oxide synthase BnAOS from rapeseed (Brassica napus L.), an important agricultural crop and source of valuable rapeseed oil. The BnAOS enzyme belongs to the CYP74A subfamily, which traditionally includes 13-specific allene oxide synthases. However, some members of this group are universal and capable of converting both 13- and 9-hydroperoxides of linoleic and α-linolenic acids. Thus, they direct the biosynthesis of oxylipins towards formation of jasmonates (in the case of conversion of (13S)-hydroperoxide of α-linolenic acid) or other metabolites when using other hydroperoxides as substrates. Our studies have shown that the BnAOS also exhibits dual positional specificity, but, in addition to 13-hydroperoxides, it efficiently converts 16-hydroperoxide of α-linolenic acid ((16S)-HPOT) to form the α-ketol we recently described, 15-oxo-16-hydroxy-9,12-octadecadienoic acid. Thus, BnAOS is a 13/16-specific AOS. Prior to this study, the ability to efficiently convert (16S)-HPOT was described only in the cucumber CsAOS (CYP74A1) and tomato LeAOS3 (CYP74C3).