Human Steroid Biosynthesis Chapter 8 Human Steroid Biosynthesis © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 1 Conceptual model for steroidogenesis FIGURE 8.1 Conceptual model for steroidogenesis. (A) Cyclopentanoperhydrophenanthrene steroid nucleus, with boxed letters indicating rings A–D and numbers for carbon atoms 1–21. Substituents (see carbon 3) projecting in front of the page (dark wedge) are β-configuration, and those projecting behind the page (light wedge) are α-configuration. (B) The conversion of cholesterol to pregnenolone is the quantitative and acutely regulated step, involving StAR and CYP11A1. The downstream enzymes determine the major steroid products of each steroidogenic cell. The hydroxysteroid dehydrogenases dominate the terminal reactions in the steroidogenic cells and in peripheral tissues and target cells. Additional enzymes, including the sulfotransferases, 5α-reductases, and UGTs, participate in various biosynthetic and degradative processes, including enzymes not traditionally considered steroidogenic, such as CYP3A4. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 2 A canonical, composite steroidogenic pathway FIGURE 8.2 A canonical, composite steroidogenic pathway. In this rendering, progesterone and aldosterone pathways are at the top, cortisol synthesis is in the second row, and androgen and estrogens are in the lower half. The major enzyme is shown with each arrow, although additional enzymes might catalyze individual reactions. The dashed arrow indicates minor pathway. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 3 The conversion of cholesterol to pregnenolone FIGURE 8.3 The conversion of cholesterol to pregnenolone. The StAR protein (cloud-shaped image) is required to transfer cholesterol in the outer mitochondrial membrane (OMM) to the inner mitochondrial membrane (IMM), where CYP11A1 resides. Electrons flow (open block arrows) from NADPH to the flavin (three fused hexagons) of FDXR (oval) to the iron–sulfur cluster (rhombus with filled circles at vertices) of FDX1 (triangle) to heme (Maltese cross with central filled circle, representing iron atom) of CYP11A1 (circle with wedge-shaped indentation), to support catalysis. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 4 The cytochrome P450 reaction cycle FIGURE 8.4 The cytochrome P450 reaction cycle. Examples shown here are the final two steps in the biosynthesis of cortisol. (A) CYP21A2- catalyzed 21-hydroxylation of 17-hydroxyprogesterone (substrate in box, “R–H”) to 11-deoxycortisol (product in box, “R–OH”), a microsomal P450 reaction. (B) CYP11B1-catalyzed 11β-hydroxylation of 11-deoxycortisol (substrate in box, “R–H”) to cortisol (product in box, “R–OH”), a mitochondrial P450 reaction. The two reaction cycles are identical except for the electron transfer proteins, which are POR for CYP21A2 and FDX1 (which receives electrons from FDXR) for CYP11B1. In each case, the heme is shown as a thick, short horizontal line with Fe above. In the resting state, the sixth ligand of the ferric iron (Fe+3) is water. Substrate binding (R–H) displaces this water and lowers the redox potential of the heme, which favors one-electron reduction via POR or FDX1. In the ferrous (Fe+2) state, the iron binds molecular oxygen avidly, and the oxyferrous P450 receives a second electron. Protonation of the iron-peroxide allows cleavage of the O–O bond, releasing water, and generating the reactive iron-oxide species with radical or odd-electron character. Electrons are shown as dots (i.e., R·, :O), and single-barbed arrows or “fishhooks” show single-electron movements, such as cleavage of R–H bond to form R· and ·H. The reactive heme-oxygen species performs a hydrogen atom abstraction, removing H· from substrate and forming R· and an iron-bound hydroxide radical (Fe–·OH). The final step of the hydroxylation chemistry is radical recombination, in which R· reacts with ·OH to form R–OH, the hydroxylated product, 11-deoxycortisol or cortisol, respectively. The mechanism is the same for all steroid hydroxylation reactions. The mechanisms of the carbon–carbon bond cleavage reactions, which CYP11A1, CYP17A1, and CYP19A1 all catalyze, probably share some similarities with the normal reaction cycle but must also account for the unique products and transformations. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 5 Steroidogenic pathways of the adrenal FIGURE 8.5 Steroidogenic pathways of the adrenal. (A) The zona glomerulosa makes aldosterone and few other products. (B) The zona fasciculata makes cortisol using three steroid hydroxylases. Minor products and pathways are shown with dotted lines, although these pathways become significant in disease states such as CAH. (C) The zona reticularis makes primarily DHEAS, yet a small amount of testosterone and other 19-carbon steroid are variably produced as well. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 6 Steroidogenic pathways of the gonads and placenta FIGURE 8.6 Steroidogenic pathways of the gonads and placenta. (A) Steroidogenesis in the testis (Leydig cell) yields T, although several sequences of intermediates are likely. (B) E2 production in the ovary uses two cell types. The granulosa cells generate pregnenolone and complete the conversion of androgens to estrogens (reactions in clear area outside box), while the theca cells convert pregnenolone to androstenedione (reactions inside textured box). Theca cells also convert cholesterol to pregnenolone (label in white area adjacent to this reaction). (C) In the corpus luteum, steroidogenesis proceeds only to P4 using two enzymes CYP11A1 and 3βHSD2, and because the corpus luteum also expresses 5α-reductase type 1, a significant amount of P4 is reduced to 5α-pregnane-3,20-dione. (D) Placental steroidogenesis has two pathways. P4 production in the placenta mimics that of the corpus luteum, except using 3βHSD1 (not type 2). E2 and E3 synthesis in the fetoplacental unit is a complex interplay of three compartments. The fetal adrenal (textured square) generates the required 19-carbon precursor DHEAS, some of which is 16α-hydroxylated in the fetal liver (textured oval) via CYP3A7 to 16α-hydroxyDHEAS. The placental steroid sulfatase (STS) and 3βHSD1 enzymes remove the sulfate and oxidize/isomerize the steroid to yield 16α-hydroxyandrostenedione; parallel reactions also occur with DHEAS to form AD (STS reaction not shown in figure to avoid clutter). These 19-carbon, Δ4 steroids are substrates for aromatase (CYP19A1), and the products— 16α-hydroxyestrone and estrone—are substrates for 17βHSD1, yielding the potent estrogens E3 and E2, respectively. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 8. 7 The alternate or “backdoor pathway” to DHT FIGURE 8.7 The alternate or “backdoor pathway” to DHT. This pathway requires that 5α-reductase activity is present and abundant 17OHP. The canonical pathways are also shown for comparison. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition