NANOSECOND FLUORESCENCE OF TRYPTOPHANS IN CYTOCHROME-P-450SCC (CYP11A1) - EFFECT OF SUBSTRATE BINDING

Fluorescence of eight tryptophan residues in cytochrome P-450scc with bound endogenous cholesterol could be fitted with a two component model: a single exponential and a "top-hat" distribution of lifetimes as the second component. The short-lived component (τ1 about 700 ps) does not change significantly upon binding of substrate (22R-hydroxycholesterol). The parameters of the long-lived component (central lifetime τm about 3.4 ns) change upon binding of carbon monoxide and substrate. 22R-hydroxycholesterol binding broadens the distribution of the long-lived component; that is the heterogeneity of the Trp environment is increased when this substrate displaces the endogenous cholesterol. © 1991.