'Half-of-the-sites' reactivity of steroid isomerase

On the number of steroid-binding sites of delta 5-3-oxosteroid isomerase

The number of steroid-binding sites in delta 5-3-oxosteroid isomerase of Pseudomonas testosteroni (EC 5.3.3.1) has been determined from measurements of the red shift of the ultraviolet chromophore of 19-nortestosterone upon binding to the enzyme. The experiments include spectroscopic measurements when limiting concentrations of either 19-nortestosterone or isomerase are titrated with varying concentrations of the complementary ligand. Analysis of the results indicates one binding site per subunit (Mr 13 394). Scatchard plots indicate a single family of equivalent binding sites. 5, 10-Secoestr-5-yne-3, 10, 17-trione is a suicide substrate of isomerase [Batzold & Robinson (1975) J. Am. Chem. Soc. 97, 2576]. The time course for inactivation of isomerase with an excess of 5, 10-seco[7-3H]estr-5-yne-3, 10, 17-trione was parallel to the covalent incorporation of steroid and gave a final stoichiometry of nearly one steroid molecule per subunit of enzyme. Alkylation of [14C] isomerase with excess of this 3H-labeled steroid followed by gel-filtration and dialysis gave an inactivated enzyme with a 3H/14C ratio that corresponds to one molecule of steroid bound per subunit; this stoichiometry was constant over a wide range of protein concentrations (0.1--10 mg/ml). Diffusion of [3H]progesterone into hexagonal crystals of isomerase showed that at saturation one steroid molecule was bound per protomer. Taken together these findings strongly support the conclusion that one molecule of steroid is bound per subunit of isomerase both in solution and in the crystal state.

Influence of the position of the double bond in steroid substrates on the efficiency of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo-steroid delta 4-delta 5-isomerase

Studies of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo steroid Delta(4)-Delta(5)-isomerase with Delta(5(6))- and Delta(5(10))-steroid substrates demonstrate the importance of the position of the double bond for the efficiency of the isomerization process. Thus 3-oxo-Delta(5(6))-substrates have markedly high k(cat.) values, whereas those of 3-oxo-Delta(5(10))-substrates are very low and their apparent K(m) values approach equilibrium dissociation constants. The first step in the isomerization process is: [Formula: see text] which is governed by the k(-1)/k(+1) ratio and is shown to be very similar for the two classes of substrates (3-oxo-Delta(5(6))- and -Delta(5(10))-steroids). They therefore differ in the steps distal to the initial formation of the Michaelis-Menten complex. The use of the deuterated androst-5(6)-ene-3,17-dione substrate enabled us to calculate individual rate constants k(+1) and k(-1) as well as to determine the apparent rate-limiting step in the isomerization process. With the deuterated oestr-5(10)-ene-3,17-dione substrate, no significant isotope effect was observed suggesting that a different rate-limiting step may be operative in this isomerization process. Data are presented that indicate that under optimal concentrations of the efficient androst-5(6)-ene-3,17-dione substrate, the forward reaction for ES complex formation (as defined by k(+1)) is limited only by diffusion and the apparent K(m) does not approach the equilibrium constant, suggesting that the evolution of this enzyme has proceeded close to ;catalytic perfection'.