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Biochemistry

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Lipid

Question
35 out of 63
 

The regulatory enzyme in the biosynthetic pathway of cholesterol is



A Hydroxymethylglutaryl Co. A synthetase

B Hydroxy methyl glutaryl Co-A reductase

C Mevalonate kinase

D Isopentenyl pyrophosphoisomerase

Ans. B Hydroxy-methyl - glutaryl CoA reductase

Cholesterol synthesis: -

a. Cholesterol is derived about equally from diet and Biosynthesis 50%

b. Synthesis: - 500mg/day, 10% of total in Liver, 15% in gut and rest are in skin. The microsomal

c. endoplasmic reticulum and cytosol fraction of the cell is responsible for synthesis.

d. Acetyl-CoA is the source of all ‘C’-atoms in cholesterol

e. Synthesis in 5-stages

[I] 1st Acetyl CoA Forms HMG-CoA and Mevalonate:

* HMG —CoA reductase

a. is a microsomal enzyme, rate limiting step in the pathway of cholesterol synthesis

b. it activity is decreased by fasting, LDL cholesterol, Mevastatin, Lovastatin, Mevalonate.

c. Insulin and thyroid hormones increase its activity.

d. Glycogen and glucocorticoids decrease its activity

e. The Enzyme is in active form when dephosphorylated and in inactive form when phosphorylated.

[II] 2ndMevalonate forms active Isoprenoid units: - By means of a decarboxylation, the active isoprenoid units, Isopentenyldiphosphate, is formed

[III] 3rd Six — Isoprenoid units form squalene

[IV] 4thSqualene is converted into Lanosterol

[V] 5thLanosterol is converted into cholesterol

** Farnesylpyrophosphate, that gives rise to squalene, also gives rises other imp. isoprenoidseg. Dolichol, Ubiquinone.

Lipid Flashcard List

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