Biochemistry and site-directed mutational analysis of Delta7-sterol-C5(6)-desaturase

This report describes recent work on the process of desaturation at C5(6) of sterol precursors in plants. Biochemical characterization of the plant Delta(7)-sterol C5(6)-desaturase (5-DES) indicates that the enzyme system involved shows important similarities to the soluble and membrane-bound non-haem iron desaturases found in eukaryotes, including cyanide and hydrophobic chelators sensitivity, CO resistance and a requirement for exogenous reductant and molecular oxygen. Site-directed mutational analysis of highly conserved residues in 5-DES indicated that eight histidine residues from three histidine-rich motifs were essential for the catalysis, possibly by providing the ligands for a putative Fe centre. This mutational analysis also revealed the catalytic role of the functionally conserved Thr-114.