Structure/function relationships responsible for the kinetic differences between human type 1 and type 2 3beta-hydroxysteroid dehydrogenase and for the catalysis of the type 1 activity.
Two distinct genes encode the 93% homologous type 1 (placenta, peripheral tissues) and type 2 (adrenals, gonads) 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD/isomerase) in humans. Mutagenesis studies using the type 1 enzyme have produced the Y154F and K158Q mutant enzymes in the Y(154)-P-H(156)-S-K(158) motif as well as the Y269S and K273Q mutants from a second motif, Y(269)-T-L-S-K(273), both of which are present in the primary structure of the human type 1 3beta-HSD/isomerase. In addition, the H156Y mutant of the type 1 enzyme has created a chimera of the type 2 enzyme motif (Y(154)-P-Y(156)-S-K(158)) in the type 1 enzyme. The mutant and wild-type enzymes have been expressed and purified. The K(m) value of dehydroepiandrosterone is 13-fold greater, and the maximal turnover rate (K(cat)) is 2-fold greater for wild-type 2 3beta-HSD compared with the wild-type 1 3beta-HSD activity. The H156Y mutant of the type 1 enzyme has substrate kinetic constants for 3beta-HSD activity that are very similar to those of the wild-type 2 enzyme. Dixon analysis shows that epostane inhibits the 3beta-HSD activity of the wild-type 1 enzyme with 14-17-fold greater affinity compared with the wild-type 2 and H156Y enzymes. The Y154F and K158Q mutants exhibit no 3beta-HSD activity, have substantial isomerase activity, and utilize substrate with K(m) values similar to those of wild-type 1 isomerase. The Y269S and K273Q mutants have low, pH-dependent 3beta-HSD activity, exhibit only 5% of the maximal isomerase activity, and utilize the isomerase substrate very poorly. From these studies, a structural basis for the profound differences in the substrate and inhibition kinetics of the wild-type 1 and 2 3beta-HSD, plus a catalytic role for the Tyr(154) and Lys(158) residues in the 3beta-HSD reaction have been identified. These advances in our understanding of the structure/function of human type 1 and 2 3beta-HSD/isomerase may lead to the design of selective inhibitors of the type 1 enzyme not only in placenta to control the onset of labor but also in hormone-sensitive breast, prostate, and choriocarcinoma tumors to slow their growth.
Thomas
James L
JL
Division of Basic Medical Sciences, Mercer University School of Medicine, Macon, Georgia 31210, USA. Thomas_J@Mercer.edu
Mason
J Ian
JI
Brandt
Stacey
S
Spencer
Byron R
BR
Norris
Wendy
W
D015096:3-Hydroxysteroid Dehydrogenases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001483:Base Sequence; D002384:Catalysis; D017931:DNA Primers; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011375:Progesterone Reductase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013251:Steroid Isomerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
C021425:3 beta-hydroxysteroid oxidoreductase-delta(5) 3-ketosteroid isomerase; D017931:DNA Primers; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D015096:3-Hydroxysteroid Dehydrogenases; D011375:Progesterone Reductase; D013251:Steroid Isomerases