Mutagenesis and heterologous expression in yeast of a plant Δ6‐fatty acid desaturase

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Abstract
Membrane‐bound microsomal fatty acid desaturases are known to have three conserved histidine boxes, comprising a total of up to eight histidine residues. Recently, a number of deviations from this consensus have been reported, with the substitution of a glutamine for the first histidine residue of the third histidine box being present in the so called ‘front end’ desaturases. These enzymes are also characterized by the presence of a cytochrome b5 domain at the protein N‐terminus. Site‐directed mutagenesis has been used to probe the functional importance of a number of amino acid residues which comprise the third histidine box of a ‘front end’ desaturase, the borage Δ6‐fatty acid desaturase. This showed that the variant glutamine in the third histidine box is essential for enzyme activity and that histidine is not able to substitute for this residue.