This set of Molecular Endocrinology Multiple Choice Questions & Answers (MCQs) focuses on “Glucocorticoids”.
1. In the diagram which shows the first step in the synthesis of corticosteroids given below, cholesterol is being converted to which product?
a) Pregnenolone
b) 17-OH progesterone
c) Steroid
d) Deoxy cortisol
View Answer
Explanation: Corticosteroids are synthesized from cholesterol in the adrenal cortex through a common pathway. Cholesterol is first modified to form pregnenolone in all three adrenal cortex zones. Pregnenolone itself is not a hormone, but is the immediate precursor of all steroid hormones being synthesized.
2. Which enzyme helps in the conversion of cholesterol to Pregnenolone in the steroid hormone biosynthesis pathway?
a) Cytochrome-P470- sidechain cleavage enzyme
b) Cytochrome-P450-sidechain cleavage enzyme
c) NADH dehydrogenase
d) Carboxylase
View Answer
Explanation: An enzyme called the cytochrome-P450 sidechain cleavage enzyme that hydroxylates cholesterol at C22 and C20 is present in the inner mitochondrial membrane. In order to form pregnenolone and isocaproic aldehyde, this enzyme cleaves the side chain of pregnenolone.
3. Which hormone helps in the conversion of Pregnenolone to 17-OH pregnenolone in the synthesis of glucocorticoids?
a) 17-beta-hydroxylase
b) 18-alpha-hydroxylase
c) 17-alpha-hydroxylase
d) 18-beta-hydroxylase
View Answer
Explanation: Pregnenolone is transported to the smooth reticulum endoplasmic reticulum in the first step. It is then converted to 17-OH-pregnenolone and is catalyzed by the 17-alpha-hydroxylase enzyme. Cytochrome P450 17A1, also known as steroid 17 alpha-monooxygenase, 17 alpha-hydroxylase, 17,20-lyase, or 17,20-desmolase, is a hydroxylase type enzyme that is encoded on chromosome 10 by the CYP17A1 gene in humans.
4. The three hydroxylase enzymes in the glucocorticoid formation, are all Cyt-P450-containing mono-oxygenases.
a) True
b) False
View Answer
Explanation: The three enzymes take part in all the three steps of glucocorticoid formation are hydroxylase enzymes. All three hydroxylases are Cyt-P450-containing mono-oxygenases and NADPH and molecular oxygen are needed for their action.
5. It is possible to synthesise Pregnenolone and progesterone from acetate.
a) True
b) False
View Answer
Explanation: Evidence exists that it is possible to synthesise pregnenolone and progesterone from acetate. This is achieved by a mechanism other than cholesterol, probably through 24 dehydrocholesterol.
6. In the second step of glucocorticoid synthesis 17-OH pregnenolone is converted to which molecule?
a) 11-deoxycortisol
b) 17-OH progesterone
c) Corticosterone
d) Retinoid
View Answer
Explanation: In the second step of glucocorticoid synthesis 17-OH pregnenolone is converted to 17-OH progesterone. This is carried out by two enzymes, they are NAD+ dependent 3 β-OH-steroid dehydrogenase and the other is Δ4,5-isomerase.
7. Which enzyme helps in the conversion of 17-OH-progesterone to 11-deoxycortisol in the third step of glucocorticoid synthesis?
a) 21-hydroxylase enzyme
b) 17-beta-hydroxylase
c) 17-alpha-hydroxylase
d) 18-beta-hydroxylase
View Answer
Explanation: 17-OH-progesterone is converted to 11-deoxycortisol by the enzyme 21-hydroxylase. This enzyme is present in the endoplasmic reticulum of the cell. The 21-hydroxylase steroid, also known as 21-monooxygenase, 21-alpha-hydroxylase, and, less generally, 21β-hydroxylase, is an enzyme that hydroxylates steroids at the C21 position and is involved in aldosterone and cortisol biosynthesis.
8. Which enzyme helps in the conversion of 11-deoxycortisol to cortisol in the final step of glucocorticoid synthesis?
a) 17-beta-hydroxylase
b) 17-alpha-hydroxylase
c) 18-beta-hydroxylase
d) 11-β-hydroxylase
View Answer
Explanation: 11-deoxycortisol is transported to mitochondrion and is converted to ‘cortisol’ by the enzyme 11-β-hydroxylase. This is the final step toward glucocorticoid formation. Glucocorticoids (GCs) are immunoregulatory hormones synthesized in the adrenal cortex under physiological and stress conditions and secreted into the blood in a circadian manner.
9. What is the role of ACTH in the formation of cortisol?
a) Stimulates the synthesis of glucocorticoids
b) Inhibits the synthesis of glucocorticoids
c) Inhibits the synthesis of mineralocorticoids
d) Stimulates the synthesis of mineralocorticoids
View Answer
Explanation: ACTH promotes glucocorticoid synthesis and secretion. This works in many ways. One method is by increasing the availability of free cholesterol in cells of the fasciculata. Adrenocorticotropic hormone (ACTH) is a hormone produced in the pituitary gland of the anterior or front portion of the brain.
10. Which hormone increases the conversion of cholesterol to pregnenolone in glucocorticoid synthesis?
a) Insulin
b) ACTH
c) Cortisol
d) ADH
View Answer
Explanation: ACTH improves the conversion of cholesterol to pregnenolone which is the “rate-limiting” phase. By the activity of G-6-PD and phosphogluconate dehydrogenase, ACTH also stimulates the HMP-shunt pathway.
Sanfoundry Global Education & Learning Series – Molecular Endocrinology.
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