Cytochrome P450scc (CYP11A1): effects of glycerol and identification of the membrane binding domain

The first step in the synthesis of steroid hormones occurs in the mitochondria where cholesterol is converted to pregnenolone by cytochrome P450scc (CYP11A1). Cholesterol is insoluble in water and is supplied to the CYP11A1 directly from the inner mitochondrial membrane to which the enzyme is bound. The aim of this study was to characterise the interaction of bovine CYP11A1 with the phospholipid membrane. The effect of osmotic stress provided by glycerol on the spin-state, activity and degree of hydration of CYP11A1 was also investigated. Multiple sequence alignment of mitochondrial P450s revealed that there are 46 absolutely conserved residues, with the highest conservation in the heme-binding region at the C-terminal. The greatest variablility between subfamilies is in the regions believed to be involved in substrate binding (SRSs), as defined for the CYP2B family. The secondary structure prediction for CYP11A1 suggests that there is strong similarity in secondary structure to P450s of known structure. A model structure of CYP11A1 was built from primary sequence alignment to template P450 structures using the SwissModel automated server. From the model and other bioinformatic analyses, the distal face of the P450 which includes the A’ helix, F-G loop and beta sheet 1 regions, were predicted to interact with the membrane. Tryptic digests of CYP11A1 were performed with the aim of identifying membrane bound peptides that may be protected from protease activity. HPLC tryptic maps were similar in profile between soluble and vesicle-bound P450 which suggests that there is not a large region of CYP11A1 protected from protease digestion when the enzyme is attached to a membrane. Mass spectrometric analysis of peptides resulting from tryptic digestion revealed a number of peptides in the soluble digest that were not present in the digest of vesicle-bound P450.