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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.

ABSTRACT: 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'.

SUBMITTER: Weintraub H

PROVIDER: S-EPMC1161450 | biostudies-other | 1980 Mar

REPOSITORIES: biostudies-other

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Project description:Several steroid analogues containing conjugated acetylenic ketone groups as part of a seco-ring structure or as substituents on the intact steroid system are irreversible inhibitors of delta 5-3-oxo steroid isomerase (EC 5.3.3.1) from Pseudomonas testosteroni. Thus 10 beta-(1-oxoprop-2-ynyl)oestr-4-ene-3,17-dione (I), 5,10-seco-oestr-4-yne-3,10,17-trione (II), 17 beta-hydroxy-5,10-seco-oestr-4-yne-3,10-dione (III) and 17 beta-(1-oxoprop-2-ynyl)androst-4-en-3-one (IV) irreversibly inactivate isomerase in a time-dependent manner. In all cases saturation kinetics are observed. Protection against inactivation is afforded by the powerful competitive inhibitor 19-nortestosterone. The inhibition constants (Ki) for 19-nortestosterone obtained from such experiments are in good agreement with those determined from conventional competitive-inhibition studies of enzyme activity. These compounds thus appear to be active-site directed. In every case the inactivated enzyme could be dialysed without return of activity, indicating that a stable covalent bond probably had formed between the steroid and enzyme. Compound (I) is a very potent inhibitor of isomerase [Ki = 66.0 microM and k+2 = 12.5 x 10(-3) s-1 (where Ki is the dissociation constant of the reversible enzyme-inhibitor complex and k+2 is the rate constant for the inactivation reaction of the enzyme-inhibitor complex)] giving half-lives of inactivation of 30-45 s at saturation. It is argued that the basic-amino-acid residue that abstracts the intramolecularly transferred 4 beta-proton in the reaction mechanism could form a Michael-addition product with compound (I). In contrast, although compound (IV) has a lower inhibition constant (Ki = 14.5 microM), it is a relatively poor alkylating agent (k+2 = 0.13 x 10(-3) s-1). If the conjugated acetylenic ketone groups are replaced by alpha-hydroxyacetylene groups, the resultant analogues of steroids (I)-(IV) are reversible competitive inhibitors with Ki values in the range 27-350 microM. The enzyme binds steroids in the C19 series with functionalized acetylenic substituents at C-17 in preference to steroids in the C18 series bearing similar groups in the ring structure or as C-10 substituents. In the 5,10-seco-steroid series the presence of hydroxy groups at both C-3 and C-17 is deleterious to binding by the enzyme.

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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.

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