Thesis (MSc (Biochemistry))--University of Stellenbosch, 2005. / This study describes: 1. The cloning of baboon cytochrome P450 side-chain cleavage (CYP11A1) cDNA by in vitro site-directed mutagenesis. 2. The identification and sequencing of three baboon CYP11A1 mutants: CYP11A1a, CYP11A1b and CYP11A1c. 3. The expression and characterisation of baboon and human CYP11A1 cDNA, CYP11A1a, CYP11A1b and CYP11A1c in nonsteroidogenic COS-1 cells. The Km and V-values for the metabolism of 25-hydroxycholesterol were determined. 4. The construction of the first homology model of CYP11A1, using both mammalian (CYP2C5) and bacterial (CYP102) cytochromes P450 crystal structures as templates. 5. A structure/function study into the role of the amino acid residues Ile98, Lys103 and Thr291 in substrate binding and enzymatic activity. 6. The proposal of a topological model of the CYP11A1 active pocket as determined by substrate docking studies.