This set of Structural Biology Multiple Choice Questions & Answers (MCQs) focuses on “Three Dimensional Structure of Proteins”.
1. Who predicted alpha helix can make the proteins stable and energetically favourable?
a) Gerardus Mulder
b) Dan Reynolds
c) Linus Pauling
d) Frederick Banting
View Answer
Explanation: Mulder died experiments on the egg albumin and he described the protein. He was a Dutch scientist and was working in the field of organic chemistry. He also the first one to use the term “protein”. But Pauling discovered that some proteins will form helical chains which will stabilize the protein. Pauling was an American scientist. He has done significant work in the field of biochemistry.
2. Who verified and confirmed the alpha-helical structure completely?
a) Max Perutz
b) Frederick Banting
c) Gerardus Mulder
d) John Kendrew
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Explanation: Pauling predicted the structure of an alpha helix. This prediction is confirmed when J. Kendrew obtained clear patterns of myoglobulin from diffraction experiments. Those patterns clearly showed that myoglobulin has alpha helices as secondary structures.
3. Which one of the following is not a standard helix?
a) Alpha helix
b) Beta helix
c) Gamma helix
d) Pi helix
View Answer
Explanation: Beta helix is a domain where the beta sheets are arranged or coiled in the form of a helix. It is not a standard helix like alpha, pi, 310 and gamma helices.
4. What is the approximate value of angle pair, phi and psi of alpha helices?
a) -65 and -55
b) -60 and -50
c) -70 and -60
d) -70 and -80
View Answer
Explanation: When plotted by taking all 4 quadrants of Ramachandran plot into consideration, we get the -60 and -50 in the third quadrant. Hence for alpha helices will be positioned at the bottom left of Ramachandran plot.
5. What types of bonds are mainly responsible for alpha helices?
a) Sulphur bridges
b) Ionic interactions
c) Van der Waal’s forces
d) Protium bonds
View Answer
Explanation: The secondary structures are stabilized by protium bonds. In case of alpha helices the hydrogen formed a bond between distant amino acids and be responsible for that kind of shape.
6. In alpha helices, protium bonds are formed between _________
a) CO of nth residue and NH of n+3th residue
b) NH of n+4th residue and CO of n+6th residue
c) CO of nth residue and NH of n+6th residue
d) NH of n+4th residue and CO of nth residue
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Explanation: The hydrogen bonds will be formed between the carboxyl group of 1st amino acid residue and the amine group of 4th amino acid residue. This is applicable only for alpha helices.
7. Calculate the total number of residues present in an alpha helix with 6 turns.
a) Approximately 18
b) Approximately 19
c) Approximately 21
d) Approximately 22
View Answer
Explanation: An alpha helix has a total of 3.6 amino acid residues per turn. Given that, there are 6 turns in the alpha helix. So, 6 × 3.6 = 21.6 which is approximately 22.
8. The ends of alpha helices will be at the surface of alpha helices because ___________
a) They have positive charges at the ends
b) They have negative charges at the ends
c) They have free groups at the ends
d) They are non-polar
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Explanation: In alpha helices, not all NH and CO are involved in the hydrogen bonding. The first amine group and the last carboxyl group of the polypeptide chain are free and are not involved in H-bonding. These free groups make them polar and help them to be at the surface all the time.
9. If hydrogen bonds are formed between n+2th and n+7th amino acid residues that the helix is called as ____________
a) Alpha helix
b) Phi helix
c) 310 helix
d) Beta helix
View Answer
Explanation: In phi helix, the hydrogen bond is formed between the first amino acid and the fifth amino acid. In the question above, the hydrogen donor is 2nd amino acid and hydrogen acceptor is the 7th amino acid. So, the difference between them is 5. So, it’s a phi helix.
10. In a helix, if there is only one amino acid between hydrogen donor and hydrogen acceptor such a kind of helix is called as _____________
a) Alpha helix
b) Beta helix
c) Gamma helix
d) 310 helix
View Answer
Explanation: If the hydrogen bond is formed between the first amino acid and third amino acid of a polypeptide chain then it forms a 310 helix. Hence there will be a single amino acid between H donor and H acceptor.
11. Most of the times, phi helices and 310 helices occur only at the ends of alpha helices.
a) True
b) False
View Answer
Explanation: Phi helix is so loosely packed with H-bond between first and fifth amino acid whereas 310 helices is so tightly packed with H-bond between first and third amino acid. Hence, they are energetically unfavourable.
12. What is the average length of an alpha helix?
a) 20 amino acid residues
b) 10 amino acid residues
c) 30 amino acid residues
d) 40 amino acid residues
View Answer
Explanation: It is experimentally observed that the length of the alpha helix was different from one protein to another. But the average was found to be 10 in several proteins.
13. An alpha helix is 45 × 10-10 meters in length. Calculate the total number of amino acid residues in the alpha helix.
a) 10
b) 20
c) 30
d) 40
View Answer
Explanation: The helix rise per amino acid residue is 1.5 × 10-10 meters in an alpha helix. Hence (45 × 10-10) ÷ (1.5 × 10-10) = 30 amino acids.
14. Glycine induces significant bends in alpha helices.
a) True
b) False
View Answer
Explanation: Even though glycine is a very poor former of helices, it is not the one which introduces bends in helices. Proline is an amino acid responsible for bends. These bends are because of the pyrrole ring which does not support the formation of H-bonds and causes steric hindrance.
15. Which if these are good helix formers?
a) A, E, Y, M
b) P, G, Y, S
c) A, E, L, M
d) S, G, E, L
View Answer
Explanation: The good helix formers are the amino acids which favour the formation of helix if they are in a polypeptide chain. The amino acids like alanine, glutamate, leucine and methionine help to form alpha helices.
Sanfoundry Global Education & Learning Series – Structural Biology.
To practice all areas of Structural Biology, here is complete set of Multiple Choice Questions and Answers.
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