esterol to the site of the first enzymatic reaction. The rapid flux of cholesterol from the outer (OMM) to the inner (IMM) mitochondrial membrane is mediated by the StAR protein. The IMM is the site where Cyp11a1, the enzyme catalyzing the first and rate-limiting step in the steroid biosynthetic pathway, is located. Overall there are two major classes of enzymes: the heme-containing cytochrome P450 and the hydroxysteroid dehydrogenase enzymes. The members of the P450 superfamily that participate in the pathway are membrane-bound mitochondrial or endoplasmic reticulum (microsome) enzymes that utilize NADPH as an electron donor. Two proteins mediate the mitochondrial electron transfer - the flavin-containing Fdxr and the iron-sulfur Fdx1; only one protein mediates the microsomal electron transfer - the P450 (cytochrome) oxidoreductase Por with two flavin cofactors. Dehydrogenases are membrane-bound mitochondrial or microsomal enzymes that utilize NAD or NADPH depending on whether the reaction is oxidation or reduction.  The 27-carbon cholesterol is converted to the C₂₁ pregnenolone in a series of three chemical reactions that together constitute the step catalyzed by Cyp11a1. From this point, the pathway diverges into the individual branches that lead to the synthesis of particular types of steroids at specific adrenal and gonadal locations. Pregnenolone is converted to progesterone, itself an important hormone, or to 17OH-pregnenolone by Hsd3b1 or Hsd3b2, and Cyp17a1, respectively. In the adrenal zona glomerulosa, progesterone is converted to the C₂₁ mineralocorticoid aldosterone via a number of steps in the aldosterone biosynthetic pathway. Either progesterone or 17OH-pregnenolone can lead to the formation of the C₂₁ glucocorticoid cortisol in the cortisol biosynthetic pathway in zona fasciculata. Cyp17a1 catalyzes both 17-hydroxylation and 17,20 lyase reactions. The lyase activity of the enzyme accounts for the formation of the C₁₉ androgen dehydroepiandrosterone DHEA in the zona reticularis. In the testosterone biosynthetic pathway, DHEA is converted to testosterone in the testis; in the beta-estradiol biosynthetic pathway, testosterone is converted to the C₁₈ estrogen estradiol in the ovaries.To see the ontology report for annotations, GViewer and download click here
[click to see the ontology report for related GO term - GO:0006694, associated KEGG map - map00140 and related entry at Reactome - REACT_11057.2]

The steroid hormone biosynthetic pathway is part of the Steroid Hormone Biosynthesis Pathway Suite and the Prostate Cancer Pathway Suite.  Click the name of the suite to view the related pathways.
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