Histidine-41 of the Cytochrome b(5) Domain of the Borage Δ(6) Fatty Acid Desaturase Is Essential for Enzyme Activity

Unlike most other plant microsomal desaturases, the Δ(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 Δ(6)-fatty acid desaturase, mutagenesis and functional analysis by expression in transgenic Arabidopsis was carried out. Although expression of the wild-type borage Δ(6)-fatty acid desaturase resulted in the synthesis and accumulation of Δ(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 Δ(6)-desaturase in transgenic plants failed to produce Δ(6)-unsaturated fatty acids. These data indicate that the Cyt b(5) domain of the borage Δ(6)-fatty acid desaturase is essential for enzymatic activity.