Steroid-binding enzymes

The crystal structures of 3 a, 20)ff-hydroxysteroid dehydrogenase [58], J -3-keto steroid isomerase [56], and cholesterol oxidase [57] have been reported. 

The arrangement of monomers of 3a,20yS-HSD to form a tetramer differs significantly from those observed in other NAD- and NADP-dependent dehydrogenase enzymes. The cofactor binding sites and the putative substrate binding cavities are near 

Biochemical modification of certain amino acids in 3a,20)ff-HSD reduces or eliminates activity. These residues and others that are conserved in the short-chain dehydrogenase family may be involved in the mechanism of action of 3a,20/3-HSD. The crystal structure provides the opportunity to explain why certain residues are conserved and to identify residues in the cofactor and substrate binding sites. Because different members of this enzyme family have signiHcantly different substrates, 

There is ample room for a 3,17-androstenedione molecule to be placed in the putative active site with its C(3) end toward the nicotinamide ring. When the C(3) carbonyl is oriented to achieve the appropriate stereochemistry of the reaction product, the angular methyl groups are similarly oriented relative to those in the model for the reaction at C(20). A structure determination of an enzyme-substrate/inhibi-tor complex could cast further light upon details of the mechanism. 

J -3-Keto isomerase catalyzes the isomerization of J -3-ketosteroids to zl -3-ketosteroids by stereospecific transfer of a hydrogen atom from C(4) to C(6). There is considerable evidence that it is the 40- and 6/5-hydrogens that are involved and that the reaction proceeds via an enolic intermediate. A low resolution (6 A) crystal structure determination has been published and the probable steroid-binding site identified via a bound inhibitor, 4-acetoxy-mercuric estradiol. The results of a higher resolution study (2.5 A) combined with the results of NMR studies and analysis of activity of mutant forms of the enzyme have helped to further define the probable active site of the enzyme [64]. 

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Polanski, J. and Gieleciak, R. (2003) The comparative molecular surface analysis (CoMSA) withmodified uniformative variable elimination-PLS (UVE-PLS) method application to the steroids binding the aromatase enzyme. J. Chem. Inf. Comput. Sci., 43, 656—666. 

The isomerase enzyme is an elongated dimer in which the proposed steroid binding site is surrounded by a distorted barrel of eight 0 strands and a single a helix. Six 0 strands and the a helix are from one monomer, and two strands are contributed from the second. The dimer interface is made up of the meeting of the barrel heads. While there are two similar shaped barrels in the dimer only one appears to contain the substrate, with one end of the steroid at the surface of the barrel top (Figure 14.16). 

It may be worth noting a common feature of all three of the enzymes for which steroids are substrates and X-ray structures have been determined the presence of a tyrosine in the postulated steroid-binding pockets. 

Recent X-ray studies of enzymes for which steroids are primary substrates, steroid binding globulins, and fragments of steroid receptors with amino acids and nucleic acids are just beginning to provide additional insight into the nature of steroid receptor binding and macromolecular interactions, and the mechanism by which steroids exert their hormonal functions. 

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