Steroid Protein biosynthesis

Stocco, D. M. 2001. StAR protein and the regulation of steroid hormone biosynthesis. Annu. Rev. Physiol., 63 193-213. 

In the absence of steroid hormones the receptors remain in an inactive complex, designated the apo-receptor complex (review Pratt, 1993 Bohen, 1995). In the apore-ceptor complex the receptor is boimd to proteins belonging to the chaperone class. Chaperones are proteins whose levels are increased as a result of a stress situation, such as a rise in ambient temperature. The chaperones assume a central function in the folding process of proteins in the cell. Chaperones aid proteins in avoiding incorrectly folded states. They participate in the folding of proteins during and after ribosomal protein biosynthesis, during membrane transport of proteins, as well as in the correct assembly of protein complexes. 

Conversion of cholesterol to pregnenolone is the rale-limiting step in steroid hormone biosynthesis. It is not ihccnry-malic transformation itself that is rate limiting, however, the translocation of cholesterol to the inner milochondrui membrane of steroid-synihesi/.ing cells is rate limiting. A key protein involved in the translocation is the 5Kruidugcmc... 

Anabolic steroids increase the deposition of protein as muscle or protein biosynthesis. 

The steroid hormones act on target cells to regulate gene expression and protein biosynthesis via the formation of steroid-receptor complexes, as outlined in Figure 33.5. The lipophilic steroid hormones are carried in the bloodstream, with the majority of the hormones reversibly bound to serum carrier proteins. The free steroids... 

General metabolic significance. Vitamin D stimulates intestinal absorption of calcium and phosphate, renal reabsorption of these ions, deposition and mobilization of minerals in the hard tissue, controlling normal calcium and phosphate blood level by means of these processes. Molecular mechanism of the vitamin D effects most frequently conform to the effect of steroid hormones (induction of protein biosynthesis). 

Nevertheless, the sequence of events in the uteri of estrogen-stimulated animals argues against a primary effect of the hormone on the bioenergetic pathway because the changes in the increase in the rate of biosynthesis—especially that of proteins—usually precede the changes in the rate of respiration. An alternative hypothesis suggests that the primary effect of the steroid hormone is on protein biosynthesis. 

Catelli MG, Binart N, Jung Testas I, Renoir JM, Baulieu EE, Feramisco JR, Welch WJ (1985) The common 90-kd protein component of non-transformed 8S steroid receptors is a heat-shock protein. EMBO J 4 3131-3135 Chambers I, Harrison PR (1987) A new puzzle in seleno-protein biosynthesis selenocysteine seems to be encoded by a stop codon UGA. Trends Biochem Sci 12 255-256... 

NAD and NADP are required as redox coen2ymes by a large number of enzymes and ia particular dehydrogenases (Fig. 6). NAD" is utilized ia the catabohe oxidations of carbohydrates, proteins, and fats, whereas NADPH2 is the coenzyme for anaboHc reactions and is used ia fats and steroid biosynthesis. NADP+ is also used ia the cataboHsm of carbohydrates (2). 

B-79MI10405)) and to attempt to present a comprehensive list would be a mammoth task. A number of general introductory texts are available which provide an overview of the subject and an entrance to the more specialist literature (e.g. B-78MI10401, B-78MI10402, B-81MI10402, B-81MI10403). A more comprehensive view of the literature from 1971 to 1978 is provided by The Royal Society of Chemistry series Biosynthesis (Specialist Periodic Reports), volumes 1 to 6, and by various chapters of the companion series Terpenoids and Steroids , Alkaloids and Peptides and Proteins . 

An example is adrenodoxin reductase (see chapter banner, p. 764), which passes electrons from NADPH to cytochrome P450 via the small redox protein adrenodoxin. This system functions in steroid biosynthesis as is indicated in Fig. 22-7.209a b Other flavin-dependent reductases have protective functions catalyzing the reduction of ascorbic acid radicals,210 211 toxic quinones,212-214 and peroxides.215-218... 

Cytochromes P450 are monooxygenases whose cosubstrates, often NADH or NADPH, deliver electrons to the active center heme via a separate flavoprotein and often via an iron-sulfur protein as well 476a b A typical reaction (Eq. 18-55) is the 11 (3-hydroxylation of a steroid, an essential step in the biosynthesis of steroid hormones (Fig. 22-11). The hydroxyl group is introduced without inversion of configuration. The same enzyme converts unsaturated derivatives to epoxides (Eq. 18-56), while other cytochromes P450... 

The key step in the biosynthesis of brassinosteroids is the conversion of castasterone to brassinolide. This reaction is a lactonization of the steroidal B-ring or a Bayer-Villiger-oxidation. Tomato cell suspension cultures have been extensively studied in respect to the metabolism of 24-< / i-castasterone and 24-ep/-brassinolide [21, 22, 32, 33]. A microsomal fraction of tomato cell cultures, induced by 24-epi-castasterone, was able to convert this substrate into 24-epi-brassinolide (Fig. (11)). The specific enzyme activity was determined to be 230 fkat/mg protein with NADPH serving as the only accepted electrondonor [24],... 

Biosynthesis of Polyketides Phenolic Compounds derived from Shikimate. The Biosynthesis of CB - Cjg Terpenoid Compounds Trrterpenoids Steroids, and Carotenoids Non-protein Amino-acids, Cyanogenic Glycosides, and Glucosinolates Biosynihesis of Alkaloids. 

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