AIIMS PG 2004 BIO.pdf

7/28/2019 AIIMS PG 2004 BIO.pdf

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AIIMS PG NOVEMBER 2004: SUB JECT: BIOCHEMISTRY

Q 1 .The normal vale of P50 on the oxy haemoglobin dissociation

curve in an adult is

a) 1.8 kPa

b) 2.7c) 3.6

d) 4.5

Q 2 .Which of the following amino acid is most likely to be found

in the trans membrane region of a protein

a) Lysine

b) Arginine

c) Leucine

d) Aspartate

Q 3 . Which of the following statement about HbS is not true :

a) HbS differs from HbA by the substitution of Valine forGlutamate in position 6 of Beta chain

b) One altered peptide HbS migrate faster toward the threshold

(-) than the corresponding peptideof HbS

c) Binding of HbS to the deoxygenated HbS can extend the

polymer nd cause sickling of RBC

d) Lowering the concentration of deoxygenated HbS can

prevent sickling

Q 4 .During the dark phase of visual cycle ,which form of

Vitamn-A combine with Opsin to make Rhodopsin .

a) All trans retinal dehyde

b) All trans retinol

c) 11-Cis retinal dehyde


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d) 11- Cis retinol

Q 5 .The molecular weight of protein can be determined by

a) Native PAGEb) Sodium dodecyl sulphate h-PAGEc) Isoelectric focussing

d) Ion exchange chromatography

Q 6 .Ladder pattern of DNA electrophoresis in apoptosis is

caused by the action of following enzyme:

a) Endonucleaseb) Trans glutaminase

c) DNAse

d) Caspase

ANSWERS AND EXPLANATIONS

1. c, Explanation: In the haemoglobin-oxygen dissociation curve, the

haemoglobin tetramer can bind up to 4 molecules of oxygen in the iron

containing sites of the heme molecule. When acid is produced in thetissue, the dissociation curve shifts to the right. Alkalosis has the oppositeeffect, reducing oxygen delivery. So, P50= 26mm Hg= 3.5 kPa.

2. c, Explanation: The transmembrane region of a protein includes most

of the non-polar amino acids. Hence the answer to the question is leucine

which is the most non-polar of the choice given.

3. c, Explanation:

In HbS, the nonpolar amino acid valine is replaced by Glutamineof the beta subunit, generating a hydrophobic sticky patch on thesurface of the beta subunit of both oxy-HbS and deoxy-HbS.

Both HbA and HbS have a complementary sticky patch on theirsurfaces that is exposed only if they are deoxygenated.

That is, at low PO2, deoxy HbS is polymerized to form long,insoluble fibres.


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So, binding of deoxy-HbA terminates fibre polymerization, soHbA lacks the second sticky patch necessary to bind another Hbmolecule.

These twisted helicle fibres distort the erythrocyte into acharacteristic sickle shape, so it becomes vulnerable to spleen

sinusoids.

A low PO2 such as high altitude accelerates the tendency topolymerize.

4. c, Explanation:

When vitamin A is taken, it is in all trans retinol form. It goes to pigmentepithelium from systemic circulation. There it is isomerized spontaneously to

form 11 cis retinol and oxidized to 11 cis retinal. This end product iscombined with opsin to form rhodopsin which is deposited in the cell

membrane of pigment epithelium of retina. Due to this holoenzyme formation, the chemical equilibrium of [All trans----- 11 cis ] always tilt to the right to facilitate 11 cis conformation

formation.

When photon excites Rhodopsin, 11 cis retinal is converted to all trans formand for this retinal is released from apoprotein initiating a nerve impulse.

Then a cascade of reaction forming rhodopsin, bathorhodopsin occurs in pico

seconds. It results in ultimate formation of metarhodopsin 2 that causes c-

GMP formation as second messenger and initiate a nerve impulse via a

intracellular signal transducing pathway. And the final step is hydrolysis ofAll trans retinal.

So the key form of vitamin-A which combines with opsin, to initiate theWalds-Visual cycle, is 11 cis retinal.

5. B, SDS PAGE: Sodium dodesyl sulfate polyaccrilamide gel

electrophoresis. Explanation: Here SDS is amphipathic in nature with negative

charge at one end due to Sulfate Group. So it coats the polar peptides in an

approximate proportion of one SDS per Two peptide bonds. This imparts hugenegative charge on the protein irrespective of its amino acid sequence. Also due to

huge amount of SDS combined with a specific protein, the mobility of

electrophoresis depends on the number of SDS bound with a single protein which

is also determined by the number of amino acid of the protein, a rough

determinant of the molecular weight of the protein.Special note: If beta marcapto ethanol/ guanidium chloride is given along with

SDS PAGE to disrupt the disulfide bonds and tertiary structures, the molecular

weight of the peptides instead of the total protein is determined.


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6. a, Explanation: The biochemical hallmark of apoptosis is

fragmentation of the genomic DNA, an irreversible event that commits the cell todeath. This fragmentation of DNA is achieved by Endoclease, which causes

endogenous activation of Ca And Mg channels.

Special note: The enzyme endonuclease cleaves the DNA between two linker DNA

segments generating mono and di oligonucleosomal units.

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AIIMS PG 2004 BIO.pdf