Δ⁵-3-Ketosteroid Isomerase

This chapter discusses the molecular and catalytic properties of ∆⁵-3-ketosteroid isomerase. The ∆⁵-3-ketosteroid isomerase promotes the conversion of a variety of ∆⁵⁽⁶⁾- and ∆⁵⁽¹⁰⁾-3-ketosteroids to the corresponding ∆⁴-3-ketosteroids. Although the transformation of a variety of ∆⁵-3-hydroxysteroids to ∆⁴-3-ketosteroids had been described in a number of crude enzymic systems of animal, vegetable, and bacterial origin, it had not been appreciated that these conversions involved two enzymic steps, including a freely reversible nicotinamide-adenine nucleotide-dependent oxidation of the hydroxyl group to the ketone, followed by a largely irreversible transposition of the double bond into a position of conjugation. The ultraviolet absorption spectrum of crystalline isomerase in dilute sodium phosphate buffer at pH 7.0 shows a principal absorption peak at 277 nm and a well-defined shoulder at 282–284 nm, both of which are characteristic of tyrosine, although slightly displaced toward longer wavelengths than in the case of a free amino acid. In addition, the intact protein displays clearly defined maxima at 253, 259, 266, and 269 nm, which are diagnostic of phenylalanine absorptions but are also displaced toward longer wavelengths.