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Biochemistry

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Enzymes

Question
11 out of 20
 

NO is formed in the body by (AIIMS May 2009)



A L-Arginine
B Uracil

C Guanine
D Aspartate

Ans. A L-Arginine

Arginine serves as the precursor of the potent muscle relaxant nitric oxide (NO) in a Ca2+-dependent reaction catalyzed by NO synthase

Nitric oxide (NO) / Endothelium derived relaxing factor (EDRF)

Synthesis:

a. NO is formed by the action of cytosolic enzyme NO synthase in endothelial & neuronal cells.

b. Substrate: Arginine, O2, NADPH.

c. Coenzyme: NADPH, FAD, FMN, Heme, tetrahydrobiopterin

d. Product: Citrulline, NO

Subtype

Name

Comments

Result of Gene knockout in Mice

1

nNOS

Activity depends on elevated Ca2+. First identified in neurons.

Calmodulin-activated

Pyloric stenosis, resistance to vascular stroke, aggressive sexual behaviour (males)

2

iNOS

Independent of elevated Ca2+. Prominent in macrophages.

More susceptible to certain types of infection

3

eNOS

Activity depends on elevated Ca2+, First identified in endothelial cells.

Elevated mean blood pressure.

Function

a. Relaxes smooth muscle

b. Prevents platelet aggregation

c. Functions as neurotransmitter in brain

d. Mediates tumor – Cidal, bactericidal actions of macrophages.

e. Involved in penile erection, [Sildenafil citrate (Viagra) affects this process by inhibiting a cGMPphosphodiesterase]

Features:

No has a very short t ½ (approx 3-4 seconds) in tissue because it react with oxygen & superoxide producing peroxynitrite which decomposes to form highly reactive OH radicle (Catalytic action).

Enzymes Flashcard List

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