Midterm II: Steroid Hormone Synthesis (Ben) Flashcards by Kiva Mcfetridge

What kind of reactions are most of the reactions in steroid synthesis?

Catalyzed by what type of enzymes?

And what is the general formula for these rxns?

Hydroxylations** catalyzed by **CYP450 enzymes

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Where are steroid hormone synthesis reactions located within the cell?

ER or Mitochondria

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What is the 2nd most frequent reaction type in steroid hormone synthesis?

Via what kind of enzyme + co-factor?

Oxidoreduction reactions

Via dehyrogenase** enzymes using **NAD⁺

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What is the electron donor molecule in the CYP450 enzymes?

And what special part of the enzyme transfers the electrons?

NADPH donates electrons

Heme-iron in CYP450 enzyme transfers electrons twice during the hydroxylation reactions

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CYP450 needs another enzyme to transfer key elements of the reaction to it.

What is the enyzme?

What does it transfer and how?

CYP450 Reductase

transfers 2 electrons** **one by one from NADPH

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What is the general name of the entire enzyme complex shown here?

Microsomal Electron Transport Chain

because it consists of the CYP450 reductase enzyme which transfers electrons to the CYP450 enzyme

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What are the components of the mitochondrial (not microsomal) electron transport chain involved in some CYP450 steroid synthesis reactions?

In what organ is this chain found?

Found on adrenal mitochondria…

Adrenodoxin Reductase - takes electrons from NADPH to adrenodoxin

Adrenodoxin - an iron-sulfur protein which transfers electrons to the CYP450

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What is the first step in the synthesis of all steroid hormones?

Reactants?

Enyzme?

Product?

(Include # of Cs)

Cleavage of cholesterols side chain…

Reactants: Cholesterol (27C) + 3 NADPH + 3 O²

Enzyme: P450_SCC Side Chain Cleavage Enyzme

Product: Pregnenolone + Isocaproaldehyde + 3 NADP⁺ + 3 H₂O

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Why are 3 NADPH used in the side chain cleavage enzyme reaction?

Because it is a 3 step reaction:

Hydroxylation of C22
Hydroxylation of C20
Cleavage of bond between C20 and 22

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Where is P450_SCC located?

the inner mitochondrial membrane

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How is cholesterol transported into the mitochondria for steroid hormone synthesis?

What is the name of the condition resulting from a lack of the molecule necessary for this?

StAR

(Steroidogenic Acute Regulatory Protein)

if this is missing = Congenital Lipoid Adrenal Hyperplasia

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What two hormones signal increased steroid hormone production?

In which tissues?

ACTH - adrenal gland
LH - testes, ovaries

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How do LH and ACTH induce steroid hormone synthesis?

(One route leading to 2 effects)

induction of cAMP increase

induces StAR activation
activates PKA which phosphorylates/activates cholesterol esterase

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What two actions does the P450c17 enyzme have?

17-hydroxylase

and

17,20 lyase

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What two reactions are performed by the 17 hydroxylase activity of the P450c17 enzyme?

Pregnenolone —-> 17OH Pregnenolone

Progesterone —-> 17OH Progesterone

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What two reactions are performed by the 17,20-lyase activity of P450c17?

17OH Pregnenolone —> DHEA (dehydroepiandrosterone)

17OH Progesterone —> androstenedione

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What 3 reactions are catalyzed by 3-Hydroxysteroid Dehydrogenase (3OHSDH)?

And what happens in these reactions?

Transfers double bond from B ring to A ring + dehydrogenates the 3-OH

Pregnenenolone —> Progesterone

17OH Pregnen —> 17 OH Progest

DHEA —> Androstenedione

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What is the activity of the P450c21 enyzme?

21 hydroxylase

hydroxylates the LAST carbon of progesterone and 17OH progesterone

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What two reactions does P450c21 perform via its 21-OHase activity?

Progesterone —> 11-Deoxycorticosterone

17OH Progesterone —> Deoxycortisol

What one enzyme catalyzes two different reactions with 3 different activities to make a final product of steroid hormone synthesis?

Aldosterone Synthase

Reactions:
- 11-deoxycorticosterone > corticosterone
- corticosterone > aldosterone
Activities:
- 11-hydroxylase
- 18-hydroxylase
- 18-dehydrogenase

Where is aldosterone synthase found (specific cell layer) and what kind of hormone does it produce?

What gene encodes it?

found in the zona glomerulosa
produces mineralocorticoid (specifically aldosterone)
encoded by CYP11B2

Lack of which enzyme/enzyme activities determines the production of only mineralocorticoids by the zona glomerulosa?

P450c17

with its 17-OHase and 17,20 Lyase activities

this keeps steroid synthesis in the glomerulosa to the pregnenolone-to-aldosterone path only

What gene/enyzme performs the last reaction in the glucocorticoid path?

11-Hydroxylase

encoded by CYP11B1

(does not have 18-OHase/DHase activity of CYP11B2)

What is the last reaction in the glucocorticoid pathway?

11-Deoxycortisol —> Cortisol

via CYP11B1 gene’s 11-Hydroxylase activity

In what cells does the CYP11B1 gene express the glucocorticoid-producing enzyme with only 11-OHase activity?

**Zona Fasciculata** and **Zona Reticularis**

Where do the reactions for the synthesis of _mineralocorticoids_ take place? (starting from pregnenolone) (2 locations)

1. **ER surface** * Pregnen \> Progest * Progest \> 11-deoxycorticosterone (DOC) 2. **Inner Mitochondrial Membrane** * DOC \> Corticosterone * Corticosterone \> Aldosterone

Which enzyme activities are preferred in the zona fasciculata? And thus what are its primary products?

**21-OHase / P450c21** - so reactions proceed in the direction of _glucocorticoids_ (cortisol) - weak mineralocorticoids (ie corticosterone) can also be made, BUT the zona fasciculata lacks 18-OHase/Dehydrogenase activity so aldosterone can't be made

Which enyzme activity is preferred in the _zona reticularis_? And thus what are its primary products?

**17,20 Lyase** activity - reactions proceed in the direction of _weak androgens_

Where within the cells of the zona fasciculata/reticularis are the main products of these cells made?

* **Fasciculata** - cortisol/corticosterone are completed on the _mitochondrial inner membrane_ (but made on the ER until their DOC/11-deoxycortisol intermediates) * **Reticularis** - weak androgens are completed on the _ER surface_

What is _adrenarche_?

development of the _zona reticularis_ around age 10-11 - this is seen enzymatically as an _increase in 17,20-Lyase activity_ - results in 100-fold increase in weak androgen (DHEA) production

What are the 3 mechanisms that favor **17,20-Lyase** activity?

1. High **_[CYP450 Reductase]/[P450c17]_** ratio 2. **_Cytochrome B5_** - allosterically increases the reductase-to-P450c17 electron donation efficiency 3. **_Phosphorylation_ -** of Ser/Thr residues on P450c17 * only 17,20-lyase, not 17-OHase enhancement

What are the general effects of aldosterone? What organ/part of the organ and how?

**Kidney Collecting Ducts** Increases expression of... * Na/K-ATPase * Na + K Channels * H⁺-ATPase Resulting in... * Na⁺ reabsorption * K⁺ secretion * H⁺ secretion

What are signs of aldosterone excess? And deficiency?

Excess: * hypertension * hypokalemia * low H+ Deficiency: * water/salt loss * hyperkalemia * acidosis (high H⁺)

What is the receptor for aldosterone?

**Mineralocorticoid Receptor** | (Steroid Receptor Type I)

How is the problem of steroid receptor "promiscuity" solved? Hint: an enzyme

**11**β**OH-steroid dehydrogenase 2** (on the ER's cytosolic surface) - present in mineralocorticoid target cells - inactivates _cortisol_ (--\> cortisone) and _corticosterone_ (---\> 11-dehydrocorticosterone) using NAD⁺ - doesn't affect aldosterone

What is steroid receptor promiscuity?

tendency of multiple different steroid hormones to bind to the mineralocorticoid receptor

What is the condition which results from deficiency of **11βOH Steroid Dehydrogenase 2**?

**Apparent Mineralocorticoid Excess** - hypertension + hypokalemia, BUT... low aldosterone levels + high cortisol/cortisone in urine

What enzyme opposes the "prereceptor specificity" effects of the enzyme protecting mineralocorticoid receptors from excess stimulation? What is its purpose?

**_11βOH Steroid Dehydrogenase 1_** (on the ER lumen surface) - reduces inactive cortisol to active cortisol within cortisol target cells - uses NADPH and H⁺

What 3 factors contribute to aldosterone production and secretion?

1. **Stress** - via ACh + ACTH (Ca²⁺ + cAMP increases) 2. **Hypovolemia** - _angiotensin II_ ---\> Ca²⁺ increase 3. **[K⁺]_EC Increase** - Ca²⁺ influx

What are some effects of cortisol? On carbohydrate metabolism? Lipid metabolism? Protein metabolism? Other effects?

Carbs: * **glucose oxidation _decrease_** * **gluconeogenesis _increase_ (via _PEPCK_)** * **liver glycogen _increase_** Lipids: * **plasma FFA** **_increase_** Proteins: * **proteolysis** **_increase_** Other: * **immunosuppresion** * **osteoporosis** (negative Ca²⁺ balance) ****

What is the condition of _excess_ cortisol called? Symptoms?

**Cushing Syndrome** - symptoms basically relate to cortisol effects * impaired glucose tolerance * visceral obesity * muscle atrophy * osteoporosis * infections (due to immunosuppression)

What is the condition of _deficiency_ of cortisol called? Symptoms?

**Adrenal Cortex Insufficiency** * Low BP * Dehydration * K+ increase * Na+ decrease * Exhaustion * Impaired hair growth

What two main factors stimulate cortisol production? Via what hormone? And what 2nd messenger?

1. **Stress** 2. **CRH** from hypothalamus - Both release ACTH - ACTH increases z. fascicularis cAMP

What 3 mechanisms promote _tissue-specific_ action of glucocorticoids?

1. **_Pre-receptor Specificity_ -** via 11β-HSD1/2 2. **_Different Receptor Isoforms_** - of glucocorticoid receptor 3. **_Co-activators/Repressors_** - modify effects of hormone in different tissues

What is the condition resulting from a hereditary defect in cortisol synthesis?

**Congenital Adrenal Hyperplasia** * most commonly... deficienies of 21-OHase or 11-β Hydroxylase * no cortisol to suppress ACTH secretion leads to adrenal hyperplasia