Monday, October 1, 2012

GRE Biology Practice Test: Proteins


1. 4-hydroxy proline is present in
a)      Collagen
b)      Plant cell wall
c)       Keratin
d)      Both plant cell wall and collagen

2. Peptide bonds between amino acids are highly stable and have a half-life of seven years in intracellular condition. This is due to
a)      High activation energy required for hydrolysis
b)      Low activation energy required for hydrolysis
c)       Peptide bond is a covalent bond
d)      Peptide bond is rigid and planar

3. Tri-peptide consists
a)      3 amino acids and 3 peptide bonds
b)      2 amino acids and 3 peptide bonds
c)       3 amino acids and 2 peptide bonds
d)      3 amino acids and 4 peptide bonds

4. All the following statements are true except
a)      Ornithine and citrulline are uncommon aminoacids
b)      Ornithine is an  intermediates in Urea cycle
c)       Ormithine and citrulline are uncommon amino acids present in many proteins
d)      Both ornithine and citrulline are intermediates in urea cycle

5. The characteristic strong absorbance of 280nm of proteins is due to
a)      All amino acids can absorb at 280nm
b)      Only Tryptophan and tyrosine can absorb at 280 nm
c)       Tryptophan is responsible for the absorbance
d)      Tryptophan, tyrosine and phenylalanine can absorb at 280 nm

6. The formation of cystine occurs at
a)      ER
b)      Golgi
c)       Cytosol
d)      Mitochondria

 7. Which of the following interaction contributes most in protein folding
a)      Covalent bond
b)      Ionic bond
c)       Hydrophobic interaction
d)      Vander walls interaction

8. Which of the following aminoacid has pKa near neutrality
a)      Tryptophan
b)      Arginine
c)       Histidine
d)      Asparagine

9. Desmosine is an unusual amino acid found in
a)      Myosin
b)      Elastin
c)       Troponin
d)      Actin

10. How many small peptides are formed upon cleavage by trypsin if a protein has 5 lysine residues
a)      4
b)      5
c)       6
d)      7
                       
11. EF-1α and EF-Tu are
a)      Analogs
b)      Homologs
c)       Paralogs
d)      Syllogs

12. All the statements  regarding peptide bond are true except
a)      Peptide bond is a co-valent bond
b)      Peptide bond is rigid and planar
c)       Peptide bond has partial double bond character
d)      Peptide bond is formed by non-condensation reaction

13. Αlpha-helix has
a)      3.6 residues/turn and  is a right handed helix
b)      3.8 residues/turn and is a right handed helix
c)       3.6 residues/turn and is a left handed helix
d)      3.8 residues/turn and is a left handed helix

14.Which of the following amino acids are rarely present in alpha helix
a)      Glycine and proline
b)      Proline and tryptophan
c)       Tryptophan and glycine
d)      Proline only

15. PDI is an enzyme involve in
a)      Protein synthesis
b)      Protein degradation
c)       Protein folding
d)      Protein quaternery structure formation

Answers with explanation

 1-d) 
Both plant cell wall and collagen

Explanation: 4-hydroxyproline, 5-hydroxy lysine, selenocysteine, desmosine, carboxy glutamate all are uncommon  amino acids

2-a)

3-c) 
amino acids and 2 peptide bonds

 4-c) 
Ormithine and citrulline are uncommon amino acids present in many proteins
Remember, these two amino acids are not present in any proteins.

 5-d
Tryptophan, tyrosine and phenylalanine can absorb at 280 nm. These are aromatic amino acids. Tryptophan and tyrosine absorbs strongly at 280nm where as phenylalanine has comparatively low absorbance.

6-a) ER.
Two cysteine amino acids are linked by di-sulphide bond to form cystine. Di-sulphide bonds are formed only in the highly oxidizing environment of ER.

7-c Hydrophobic interaction

8-c histidine has a pKa of 6.0 and has buffering capacity

9-b) Elastin

10-c) 6 Trypsin cleave at C-terminal of lysine or arginine residue

11-b) homologs.
EF-Tu elongation factor is involved in protein synthesis of bacteria. The protein counterpart in eukaryote is EF-1α. Both are members of the same family with a common ancestry.

12-d Peptide bond is formed by non-condensation reaction. Actually Peptide bond is formed by condensation reaction.

13-a) 3.6 residues/turn and is a right handed helix in all proteins. You have to read the options carefully before picking one.

14-a) glycine and proline.
Proline is an alpha helix terminator as it cannot form H- bond with other residues. Glycine is the simplest amino acid and has high conformational flexibility. Polymers of glycine form coiled structures entirely different from alpha helix.

15-c 
PDI (protein di sulphide isomerase) is an enzyme involved in shuffling of di sulphide bonds in protein folding. Refer our post protein folding.


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