Mutagenesis and heterologous expression in yeast of a plant Delta(6)-fattyacid desaturase

Membrane-bound microsomal fatty acid desaturases are known to have three co nserved histidine boxes, comprising a total of up to eight histidine residu es. Recently, a number of deviations from this consensus have been reported , with the substitution of a glutamine for the first histidine residue of t he third histidine box being present in the so called 'front end' desaturas es. These enzymes are also characterized by the presence of a cytochrome b( 5) domain at the protein N-terminus. Site-directed mutagenesis has been use d to probe the functional importance of a number of amino acid residues whi ch comprise the third histidine box of a 'front end' desaturase, the borage Delta (6)-fatty acid desaturase. This showed that the variant glutamine in the third histidine box is essential for enzyme activity and that histidin e is not able to substitute for this residue.