Biomedical subjects

S X Lin

Publications and source records attributed to S X Lin.

75 records · Page 5Linked to original sources

PubMed1982

A thermal-variation method for analysing the rate constants of the Michaelis--Menten mechanism.

By analysing the variations of saturation velocity and Michaelis constant with temperature and invoking the mathematical constraint represented by the Arrhenius equation, it becomes possible to estimate k+2 and indistinguishably k+1 and k-1 for the Michaelis--Menten mechanism of one-substrate enzyme reactions. Distinction between k+1 and k-1 may be obtained through the determination of isotopic rate effects. This procedure thus provides a basis for evaluating all three rate constants of the one-substrate mechanism, and disproves the suggestion that k+1 and k-1 are intrinsically unobtainable from steady-state kinetic measurements.

Enzymes↗

PubMedDate not supplied

3D-structure of human estrogenic 17beta-HSD1: binding with various steroids.

Human estrogenic dehydrogenase (17beta-HSD1) catalyses the last step in the biosynthesis of the active estrogens that stimulate the proliferation of breast cancer cells. While the primary substrate for the enzyme is estrone, the enzyme has some activity for the non-estrogenic substrates. To better understand the structure function relationships of 17beta-HSD1 and to provide a better ground for the design of inhibitors, we have determined the crystal structures of 17beta-HSD1 in complex with different steroids. The structure of the complex of estradiol with the enzyme determined previously (Azzi et al., Nature Structural Biology 3, 665-668) showed that the narrow active site was highly complementary to the substrate. The substrate specificity is due to a combination of hydrogen bonding and hydrophobic interactions between the steroid and the enzyme binding pocket. We have now determined structures of 17beta-HSD1 in complex with dihydrotestosterone and 20alpha-OH-progesterone. In the case of the C19 androgen, several residues within the enzyme active site make some small adjustments to accommodate the increased bulk of the substrate. In addition, the C19 steroids bind in a slightly different position from estradiol with shifts in positions of up to 1.4 A. The altered binding position avoids unfavorable steric interactions between Leu 149 and the C19 methyl group (Han et al., unpublished). The known kinetic parameters for these substrates can be rationalized in light of the structures presented. These results give evidence for the structural basis of steroid recognition by 17beta-HSD1 and throw light on the design of new inhibitors for this pivotal steroid enzyme.

17-Hydroxysteroid Dehydrogenases↗

PubMedDate not supplied

Crystallization and preliminary crystal structure of the complex of 17beta-hydroxysteroid dehydrogenase with a dual-site inhibitor.

Human estrogenic 17beta-hydroxysteroid dehydrogenase (17beta-HSD1) catalyzes the synthesis of 17beta-estradiol (E2) from estrone, in the ovary and peripheral tissues. While the structures of 17beta-HSD1 alone and in complex with E2 have been determined (D. Ghosh, V. Pletnev, D.-W. Zhu, Z. Wawrzak, W.-L. Duax, W. Pangborn, F. Labrie, S.-X. Lin, Structure of human 17beta-hydroxysteroid dehydrogenase at 2.20 A resolution, Structure 3 (1995) 503-513), no structures of inhibitor/enzyme complex, either modeled or from crystallography, have been reported before the submission of the present paper. The best available inhibitors are among the 'dual-site inhibitors', blocking estrogenic 17beta-HSD and the estrogen receptor. These compounds belong to a family of estradiol analogues having an halogen atom at the 16alpha position and an extended alkyl-amide chain at the 7alpha position (C. Labrie, G. Martel, J.M. Dufour, G. Levesque, Y. Merand, F. Labrie, Novel compounds inhibit estrogen formation and action, Cancer Res. 52 (1992) 610-615). We now report the crystallization of this enzyme/inhibitor complex. The complex of the best available dual-site inhibitor, EM-139, with 17beta-HSD1 has been crystallized using both cocrystallization and soaking methods. Crystals are isomorphous to the native crystals grown in the presence of 0.06% beta-octyl-glucoside and polyethyleneglycol 4000, with a monoclinic space group C2. Data at 1.8 A have been collected from a synchrotron source. Even though the size of the inhibitor is greater than that of the substrate, our preliminary X-ray-diffraction study shows that EM-139 fits into the active site in a position similar to that of estrogen. The availability of such structural data will help design more potent inhibitors of estrogenic 17beta-HSD.

Apoenzymes↗