Detection of a transient enzyme-steroid complex during active-site-directed irreversible inhibition of 3-oxo-delta 5-steroid isomerase

The isomerization of Δ5-androstene-3,17-dione by the human glutathione transferase A3-3 proceeds via a conjugated heteroannular diene intermediate

The seemingly simple proton abstraction reactions underpin many chemical transformations, including isomerization reactions, and are thus of immense biological significance. Despite the energetic cost, enzyme-catalyzed proton abstraction reactions show remarkable rate enhancements. The pathways leading to these accelerated rates are numerous and on occasion partly enigmatic. The isomerization of the steroid Δ(5)-androstene-3,17-dione by the glutathione transferase A3-3 in mammals was investigated to gain insight into the mechanism. Particular emphasis was placed on the nature of the transition state, the intermediate suspected of aiding this process, and the hydrogen bonds postulated to be the stabilizing forces of these transient species. The UV-visible detection of the intermediate places this species in the catalytic pathway, whereas fluorescence spectroscopy is used to obtain the binding constant of the analog intermediate, equilenin. Solvent isotope exchange reveals that proton abstraction from the substrate to form the intermediate is rate-limiting. Analysis of the data in terms of the Marcus formalism indicates that the human glutathione transferase A3-3 lowers the intrinsic kinetic barrier by 3 kcal/mol. The results lead to the conclusion that this reaction proceeds through an enforced concerted mechanism in which the barrier to product formation is kinetically insignificant.

Nature of the intermediate in the 3-oxo-.DELTA.5-steroid isomerase reaction

The role of Tyr-14 of 3-oxo-delta 5-steroid isomerase (KSI) was probed by analysis of the spectra of 3-amino-1,3,5(10)-estratrien-17 beta-ol (4) and equilenin (5) bound to the active site of KSI. The ultraviolet spectrum of 4 bound to KSI is identical to that for 4 in neutral solution. This observation indicates that Tyr-14 does not protonate the amine group of 4 at the active site. By analogy, it is argued that the 3-oxo group of steroid substrates for KSI is not protonated during the reaction. In contrast, the fluorescence excitation spectra of 5 bound to KSI show characteristics of an ionized phenol, even at pH values as low as 3.8. It is concluded that the pKa of equilenin is perturbed from its value in solution of 9 to less than or equal to 3.5 at the active site of KSI. Similarly, the pKa of the intermediate dienol in the KSI reaction should be lowered to less than or equal to 4.5 when it is bound to KSI. Thus, the function of Tyr-14 as an electrophilic catalyst is likely the stabilization of the anion of the dienol by hydrogen bonding rather than by proton transfer.

Structure-function aspects and inhibitor design of type 5 17beta-hydroxysteroid dehydrogenase (AKR1C3)

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) type 5 has been cloned from human prostate and is identical to type 2 3alpha-HSD and is a member of the aldo-keto reductase (AKR) superfamily; it is formally AKR1C3. In vitro the homogeneous recombinant enzyme expressed in Escherichia coli functions as a 3-keto-, 17-keto- and 20-ketosteroid reductase and as a 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidase. The enzyme will reduce 5alpha-DHT, Delta(4)-androstene-3,17-dione, estrone and progesterone to produce 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxprogesterone, respectively. It will also oxidize 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxyprogesterone to produce 5alpha-androstane-3,17-dione, Delta(4)-androstene-3,17-dione, and progesterone, respectively. Many of these properties are shared by the related AKR1C1, AKR1C2 and AKR1C4 isoforms. RT-PCR shows that AKR1C3 is dominantly expressed in the human prostate and mammary gland. Examination of k(cat)/K(m) for these reactions indicates that as a reductase it prefers 5alpha-dihydrotestosterone and 5alpha-androstane-3,17-dione as substrates to Delta(4)-androstene-3,17-dione, suggesting that in the prostate it favors the formation of inactive androgens. Its concerted reductase activity may, however, lead to a pro-estrogenic state in the breast since it will convert estrone to 17beta-estradiol; convert Delta(4)-androstene-3,17-dione to testosterone (which can be aromatized to 17beta-estradiol); and it will reduce progesterone to its inactive metabolite 20alpha-hydroxyprogesterone. Drawing on detailed structure-function analysis of the related rat 3alpha-HSD (AKR1C9), which shares 69% sequence identity with AKR1C3, it is predicted that AKR1C3 catalyzes an ordered bi bi mechanism, that the rate determining step is k(chem), and that an oxyanion prevails in the transition state. Based on these relationships steroidal-based inhibitors that compete with the steroid product would be desirable since they would act as uncompetitive inhibitors. With regards to transition state analogs steroid carboxylates and pyrazoles may be preferred while 3alpha, 17beta or 20alpha-spiro-oxiranes may act as mechanism-based inactivators.

Evaluation of the internal equilibrium constant for 3-oxo-delta 5-steroid isomerase using the D38E and D38N mutants: the energetic basis for catalysis

The dissociation constant (KD) for the complex of the intermediate dienol (2) and the D38N mutant of 3-oxo-delta 5-steroid isomerase (D38N.2) has been determined for the isomerization of 5-androstene-3,17-dione (1). KD for D38N.2 is pH-dependent, with values of 6 nM at pH 6.9, 51 nM at pH 5.8, and 59 nM at pH 5.2. These values of KD are used to estimate the pH-independent dissociation constant (0.7 +/- 0.3 microM) for the complex of dienol and wild-type (WT) enzyme. The internal equilibrium constant (Kint = 0.3 +/- 0.2) for the interconversion of bound substrate (WT.1) and bound intermediate (WT.2) was then calculated for WT using its KD, the values for the external equilibrium constant for 1<-->2, and the dissociation constant of the enzyme substrate complex (KS). The dissociation constant (KD) for the complex of equilenin (4) with WT, D38E, and D38N enzymes was also determined at pH values from 4 to 7. For the complex of 4 with D38N (D38N.4), KD is pH-dependent with an apparent pKa of about 4.5, whereas KD for both WT.4 and D38E.4 is pH-independent. These values are used to give two additional estimates of the internal equilibrium constant for WT (Kint = 0.5 and 0.01). Analysis of these results in terms of Marcus formalism leads to the conclusion that the primary function of the enzyme is to decrease the thermodynamic barrier to formation of the intermediate by lowering delta Gzero by about 10 kcal/mol. In contrast, the intrinsic free energy of activation (delta G++int) is only decreased by about 3 kcal/mol. These results are discussed in terms of competing theories of enzymatic enolization.

Stereochemistry of the concerted enolization catalyzed by delta 5-3-ketosteroid isomerase

The reaction catalyzed by delta 5-3-ketosteroid isomerase has been shown to occur via the concerted enolization of the delta 5-3-ketosteroid substrate to form a dienolic intermediate, brought about by Tyr-14, which hydrogen bonds to and protonates the 3-keto group, and Asp-38, which removes and axial (beta) proton from C-4 of the substrate, in the same rate-limiting step [Xue, L., Talalay, P., & Mildvan, A.S. (1990) Biochemistry 29, 7491-7500; Kuliopulos, A., Mildvan, A.S., Shortle, D., & Talalay, P. (1989) Biochemistry 26, 3927-3937]. Since the axial C-4 proton is removed by Asp-38 from above the substrate, a determination of the complete stereochemistry of this rapid, concerted enolization requires information on the direction of approach of Tyr-14 to the enzyme-bound steroid. The double mutant enzyme, Y55F + Y88F, which retains Tyr-14 as the sole Tyr residue, was prepared and showed only a 4.5-fold decrease in kcat (12,000 s-1) and a 3.6-fold decrease in KM (94 microM) for delta 5-androstene-3, 17,dione, in comparison with the wild-type enzyme. Deuteration of the aromatic rings of the 10 Phe residues further facilitated the assignment of the aromatic proton resonances of Tyr-14 in the 600-MHz TOCSY spectrum at 6.66 +/- 0.01 ppm (3,5H) and at 6.82 +/- 0.01 ppm (2,6H). Variation of the pH from 4.9 to 10.9 did not alter these shifts, indicating that the pKa of Tyr-14 exceeds 10.9. Resonances assigned to the three His residues titrated with pKa values very similar to those found with the wild-type enzyme. The binding of 19-nortestosterone, a product analogue and substrate of the reverse isomerase reaction, induced downfield shifts of -0.12 and -0.06 ppm of the 3,5-and 2,6-proton resonances of Tyr-14, respectively, possibly due to deshielding by the 3-keto group of the steroid, but also induced +0.29 to -0.41 ppm changes in the chemical shifts of 8 of the 10 Phe residues and smaller changes in 10 of the 12 ring-shifted methyl resonances, indicating a steroid-induced conformation change in the enzyme. NOESY spectra in H2O revealed strong negative Overhauser effects from the 3,5-proton resonance of Tyr-14 to the overlapping 2 alpha-, 2 beta-, or 6 beta-proton resonances of the bound steroid but no NOE's to the 4- or 6 alpha-protons of the steroid.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of microbial cholesterol oxidases by dimethylmorpholines

Cholesterol oxidase is a potentially important enzyme in steroid transformations, catalysing the conversion of 3-hydroxy-5-ene steroids to 3-keto-4-ene derivatives via a 3-keto-5-ene intermediate. Morpholine derivatives, especially fenpropimorph and tridemorph, were found to block selectively the isomerisation activity of cholesterol oxidases isolated from Nocardia erythropolis, Streptomyces sp., Pseudomonas testosteroni and Schizophyllum commune. These enzymes differ strongly in physical characteristics and catalytic behaviour. The effectiveness of the inhibitors varied with the cholesterol oxidase tested. Fenpropimorph was most effective with each of the 4 enzymes, 50 mg/l inhibiting about 50% of the enzyme activity. Inhibition was instantaneous and followed a reversible competitive mechanism in Streptomyces sp. and a reversible non-competitive mechanism in Nocardia erythropolis and Schizophyllum commune. An irreversible type of inhibition was observed for P. testosteroni cholesterol oxidase.