Histidine-41 of the cytochrome b5 domain of the borage Δ6 fatty acid desaturase is essential for enzyme activity

Unlike most other plant microsomal desaturases, the Delta(6)-fatty acid desaturase from borage (Borago officinalis) contains an N-terminal extension that shows homology to the small hemoprotein cytochrome (Cyt) b(5). To determine if this domain serves as a functional electron donor for the Delta(6)-fatty acid desaturase, mutagenesis and functional analysis by expression in transgenic Arabidopsis was carried out. Although expression of the wild-type borage Delta(6)-fatty acid desaturase resulted in the synthesis and accumulation of Delta(6)-unsaturated fatty acids, this was not observed in plants transformed with N-terminally deleted forms of the desaturase. Site-directed mutagenesis was used to disrupt one of the axial heme-binding residues (histidine-41) of the Cyt b(5) domain; expression of this mutant form of the Delta(6)-desaturase in transgenic plants failed to produce Delta(6)-unsaturated fatty acids. These data indicate that the Cyt b(5) domain of the borage Delta(6)-fatty acid desaturase is essential for enzymatic activity.