Steroids and steroidases. IX. Activation parameters and the mechanism of base- and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione. The nature of the active center of the Δ5 → Δ4-3-ketosteroid isomerase of Pseudomonas testosteroni

Determination of the activation parameters for the acid-, base-, and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione has revealed that the facility of the enzymic process is mainly due to an extremely low enthalpy of activation of 5.0 kcal mole−1. Further circumstantial evidence regarding the nature of the reacting groups at the active center has also been obtained, and a mechanism of enzyme action is proposed employing tyrosine and histidine as the principal amino acids responsible for catalyzing the isomerization.