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Comparative structures and evolution of sterol-C5-desaturase (SC5D) genes and proteins: A major role in cholesterol biosynthesis.

Roger S Holmes

Sterol-C5-desaturase (SC5D) (EC 1.14.21.6) catalyses a reaction in the cholesterol biosynthetic pathway, converting lathosterol into 7-dehydrocholesterol. The enzyme is broadly distributed in human tissues, with highest levels in liver and brain. Bioinformatic methods were used to predict the amino acid sequences, transmembrane and gene locations for SC5D genes and encoded proteins using data from several vertebrate and invertebrate genome projects. Multiple histidine rich motifs (HR[G/F]LHH, HKPHH and HHTDHH) were conserved and four transmembrane regions were usually observed for the vertebrate SC5D protein sequences examined. Vertebrate SC5D genes predominantly contained 4 coding exons transcribed on either the positive or negative DNA strands. Evidence is presented for chromosomally duplicated SC5D genes for the Xenopus laevis genome. Vertebrate SC5D protein subunits shared 66-100% sequence identities and exhibited sequence alignments and identities for amino acid residues as well as extensive conservation of predicted transmembrane structures. Phylogenetic analyses demonstrated the relationships and evolutionary origins of the vertebrate SC5D genes which were related to an invertebrate SC5D gene observed in nematode and fruit fly genomes.