Steroid-A-isomerase

This enzyme [EC 5.3.3.1], also called steroid A-isomerase and A -3-ketosteroid isomerase, catalyzes the interconversion of a 3-oxo-A -steroid and a 3-oxo-A -steroid. 

An analogous approach has been used to inactivate the A -3-ketosteroid isom-erase (steroid A-isomerase) from Pseudomonas testosteroni, an enzyme which... 

In the next reaction steps, the enzymes 3)S-hydroxy-A -steroid dehydrogenase, steroid A-isomerase and steroid 17a-hydroxylase are involved. Oxidation at C-3 and double bond migration leads to progesterone, the hormone of the corpus luteum. Afterwards, the C-17 position is hydroxylated to give 17 -hydroxyprogesterone. The reverse reaction sequence is also known and... 

As a result of the prolongation of the deficiency of the 3/3-hydroxy-dehydrogenase, As — A isomerase system after birth, an important quantity of 3 -hydroxy-As steroids is excreted in the urine of newborns. As the 16a-hydroxylase is very active in the first days of life, very little if any dehydroepiandrosterone or dehydroepiandrosterone sulfate is excreted in newborn urine (Migeon et al., 1957b Vestergaard, 1965 Bertrand et al., 1966 Paulsen et al., 1966) on the other hand, the total excretion rate of urinary 16a-hydroxydehydroepiandrosterone is 300-600 )tig/24 hours (Bongiovanni, 1962 Mitchell and Shackleton, 1966 Mitchell, 1967). Cleary and Pion (1968) found these values to be between 0.7 and 3 mg/24 hours in the first 2 days of life. Alost of these steroids arc eliminated as ester sulfates. 

Gu Y, Guo J, Pal A, Pan SS, Zimniak P, Singh SV, et al. Crystal structure of human glutathione N-transferase A3-3 and mechanistic implications for its high steroid isomerase activity. Biochemistry 2004 43 15673-9. 

Pharmacologically active allenic steroids have already been examined intensively for about 30 years [5], Thus, the only naturally occurring allenic steroid 107 had been synthesized 3 years before its isolation from Callyspongia diffusa and it had been identified as an inhibitor of the sterol biosynthesis of the silkworm Bombyx mori (Scheme 18.34) [86d], At this early stage, allenic 3-oxo-5,10-secosteroids of type 108 were also used for the irreversible inhibition of ketosteroid isomerases in bacteria, assuming that their activity is probably caused by Michael addition of a nucleophilic amino acid side chain of the enzyme at the 5-position of the steroid [103, 104]. Since this activity is also observed in the corresponding /3,y-acetylenic ketones, it can be rationalized that the latter are converted in vivo into the allenic steroids 108 by enzymatic isomerization [104, 105],... 

Application of CBS extrapolations to the A5-ketosteroid isomerase-catalyzed conversion of A5-androstene-3,17-dione to the A4 isomer (Fig. 4.10) provides a test case for extensions to enzyme kinetics. This task requires integration of CBS extrapolations into multilayer ONIOM calculations [56, 57] of the steroid and the active site combined with a polarizable continuum model (PCM) treatment of bulk dielectric effects [58-60], The goal is to reliably predict absolute rates of enzyme-catalyzed reactions within an order of magnitude, in order to verify or disprove a proposed mechanism. 

Steroid Systems Labeling of steroid systems, 46, 447 irreversible inhibitors of A -3-ketosteroid isomerase acetylenic and allenic 3-0X0-5,10-secosteroids, 46, 461 labeling of AC3-ketosteroid isomerase by photoexcited steroid ketones, 46, 469. 

A great deal of research has been undertaken to determine if the dehydrogena-tion/isomerization reactions are properties of one system or of two separate enzymes (see Ref. 52) most of the evidence suggests that the former is true. Probably three, or even four, substrate-specific isomerases may occur in bovine adrenal cortex which can act on C19, C21 and C27 steroids. Likewise, separate 5-ene-3/3-HSDs may exist in the adrenal cortex for C19 and C21 steroids, because the latter did not compete with C19 steroids for active sites of the enzymes studied. 

During the period of time when the nature of the 19-nortestosterone acetate-dependent photoinactivation was under investigation, a new bacterial steroid isomerase was obtained from extracts of Pseudomonas putida (Biotype B) in nearly homogeneous form and some of its physical and enzymatic properties were characterized (64, 65, 66). The putida isomerase is similar in its molecular weight and quaternary structure to the testosteroni isomerase. Chemically, the most striking difference between the two isomerases is the presence of four residues of cysteine per polypeptide chain of the putida isomerase whereas no cysteine or cystine is present in the testosteroni isomerase. N-Terminal sequence analysis of the putida isomerase demonstrated substantial sequence homology between the two enzymes. 

The chemistry of the coupling reaction of the allenic secosteroids to isomerase has been under investigation by Penning and Talalay (82, 83, 84) who report that the site of attachment is in the tetrapeptide sequence 55-58. Identification of the actual site has been hampered by a lability of the bound steroid to both low and high pH. 

A single 3/8-hydroxy-steroid dehydrogenase A5-3-oxo-steroid isomerase complex is present in human adrenal301 but a similar complex isolated from rat mitochondria was thought to be a redistribution artefact.302 The dimeric form of the A5-3-oxo-steroid isomerase from Pseudomonas species has a molecular weight of 26 800 dalton and is unusually stable to dissociation on dilution 303 it may be the catalytically active species. The enzyme from a bacterial source is irreversibly inhibited by an acetylenic analogue of the natural substrate.304... 

P450s catalyze a number of other oxidative reactions, on substrates that range from alkanes to complex endogenous molecules such as steroids and fatty acids. Table 10.1 lists many of the oxidative reactions catalyzed by P450s. These enzymes are also known to catalyze non-oxidative dehydrase, reductase and isomerase reactions [15],... 

Chloro-9-cyclopentyl-8-azapurine inhibited synthesis of DNA, RNA, and protein in E. coli. Blockage of thymine-nucleotide formation was the first effect seen. Alkylation of enzymes by the 6-chloro substituent was suggested as a mechanism. This azapurine inhibited the RNA polymerase from E. coli, but not that from M. lysodeikticus. Formyltetrahydrofolate synthetases, of both mammalian and bacterial origins, were strongly inhibited. The same azapurine, at 0.3 mM, markedly inhibited the steroid-induced synthesis of A -3-ketosteroid isomerase in Pseudomonas testoster-oni ... 

In another variant on the use of paramagnetic species to probe protein structure, Zhao et al.2m determined the secondary structural features of A8-3-ketosteroid isomerase using 3D NMR techniques on the l5N,l3C-labelled protein. This enzyme catalyses the conversion of A5- to A4-3-ketosteroids and is a homodimer of 125 amino acids per subunit. The NMR studies were undertaken on the steroid-bound protein. The amino acids near to the steroid were confirmed by binding a steroid incorporating a spin label and monitoring the disappearance from the 15N- H HSQC spectrum of the cross-peaks associated with these residues. 

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