Optimized expression and catalytic properties of a wheat obtusifoliol 14 alpha-demethylase (CYP51) expressed in yeast - Complementation of erg11 Delta yeast mutants by plant CYP51

CYP51s form the only family of P450 proteins conserved in evolution from pr okaryotes to fungi, plants and mammals. In all eukaryotes, CYP51s catalyse 14 alpha-demethylation of sterols. We have recently isolated two CYP51 cDNA s from sorghum [Bak, S., Kahn, R.A., Olsen, C.E. Br Halkier, B.A. (1997) Pl ant J. 11, 191-201] and wheat [Cabello-Hurtado, F., Zimmerlin, A., Rahier, A., Taton, M., DeRose, R., Nedelkina, S., Batard, Y., Durst, F. Pallett, K. E. & Werck-Reichhart, D. (1997) Biophys. Biochem. Res. Commun. 230, 381-385 ]. Wheat and sorghum CYP51 proteins show a high identity (92%) compared wit h their identity with their fungal and mammalian orthologues (32-39%). Data obtained with plant microsomes have previously suggested that differences in primary sequences reflect differences in sterol pathways and CYP51 subst rate specificities between animals, fungi and plants. To investigate more t horoughly the properties of the plant CYP51, the wheat enzyme was expressed in yeast strains overexpressing different P450 reductases as a fusion with either yeast or plant (sorghum) membrane targeting sequences. The endogeno us sterol demethylase gene (ERG11) was then disrupted. A sorghum-wheat fusi on protein expressed with the Arabidopsis thaliana reductase ATR1 showed th e highest level of expression and activity. The expression induced a marked proliferation of microsomal membranes so as to obtain 70 nmol P450.(L cult ure)(-1) with CYP51 representing 1.5% of microsomal protein. Without disrup tion of the ERG11 gene, the expression level was fivefold reduced. CYP51 fr om wheat complemented the ERG11 disruption, as the modified yeasts did not need supplementation with exogenous ergosterol and grew normally under aero bic conditions. The fusion plant enzyme catalysed 14 alpha-demethylation of obtusifoliol very actively (K-m,K-app = 197 mu M, k(cat) = 1.2 min(-1)) an d with very strict substrate specificity. No metabolism of lanosterol and e buricol, the substrates of the fungal and mammalian CYP51s, nor metabolism of herbicides and fatty acids was detected in the recombinant yeast microso mes. Surprisingly lanosterol (K-s = 2.2 mu M) and eburicol (K-s = 2.5 mu M) were found to bind the active site of the plant enzyme with affinities hig her than that for obtusifoliol (K-s = 289 mu M), giving typical type-I spec tra. The amplitudes of these spectra, however, suggested that lanosterol an d eburicol were less favourably positioned to be metabolized than obtusifol iol. The recombinant enzyme was also used to test the relative binding cons tants of two azole compounds, LAB 170250F and gamma-ketotriazole, which wer e previously reported to be potent inhibitors of the plant enzyme. The K-s of plant CYP51 for LAB 170250F (0.29 mu M) and gamma-ketotriazole (0.40 mu M) calculated from the type-II sp(2) nitrogen-binding spectra were in bette r agreement with their reported effects as plant CYP51 inhibitors than valu es previously determined with plant microsomes. This optimized expression s ystem thus provides an excellent tool for detailed enzymological and mechan istic studies, and for improving the selectivity of inhibitory molecules.