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Protein Structure

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This quiz is based on the various aspects of the structure of protein molecules and amino acids.

1. The sequence of different amino acids in the polypeptide chain of a protein is called its :
• secondary structure
• tertiary structure
• primary structure
• Quaternary structure
Answer: primary structure
The primary structure of a protein is its amino acid sequence.


2. The Quaternary structure of a protein is :
• its structure resulting from interactions between amino acid side chains.
• its structure resulting from hydrogen bonds between the C=O and N-H groups of different amino acids.
• its amino acid sequence
• its structure resulting from the union of more than one protein molecule, called subunit proteins.
Answer: its structure resulting from the union of more than one protein molecule, called subunit proteins.
Almost all proteins are made up of more than one polypeptide chain. These different peptide chains are called subunits and the spatial arrangement of these subunits gives rise to the quaternary structure of a protein. A famous example of quaternary structure is hemoglobin !


3. Successive amino acids in the polypeptide chains that make up a protein are held together by :
• N-glycosidic bonds
• interprotamine disulfide bonds
• peptide bonds
• phosphodiester bonds
Answer: peptide bonds
The formation of a peptide bond between two amino acids is an example of a condensation reaction between an amino group and a carboxylic acid group.



4. How many different amino acids are found in proteins ?
• infinite
• 27
• 30
• 20
Answer: 20
There are 20 different amino acids which are found in proteins. Note that there are more than 20 amino acids in existence. For Example : L-Thyroxine, D-Glutamic acid, etc that do not appear in proteins but have some biological significance.


5. Each protein has a particular 3D structure which is decided by its:
• secondary structure
• tertiary structure
• primary structure
• Quaternary structure
Answer: primary structure
Each protein has a particular 3D structure which is decided by the order of amino acids in its chain.


6. Infectious self-reproducing agents consisting only of protein, with no nucleic acids (hypothesized in 1982 by Nobel Laureate Stanley B. Prusiner) are called :
• exteins
• Prions
• proteomes
• proteinoids
Answer: Prions
Prions are pathogens consisting only of protein and no nucleic acids (which enable reproduction). Prions are responsible for mad cow disease (bovine spongiform encephalopathy).


7. Which of the following is not an aromatic amino acid ?
• Serine
• Phenylalanine
• Tryptophan
• Tyrosine
Answer: Serine
Phenylalanine, Tryptophan and Tyrosine are the only three amino acids (out of the 20 that appear in proteins) that have a Benzene ring in their side chain.


8. Which of the following amino acids is polar ?
• Histidine
• Valine
• Leucine
• Isoleucine
Answer: Histidine
Out of the 20 amino acids that appear in proteins, exactly half are non-polar molecules. Also, among the polar amino acids, with the exception of Arginine, Lysine and Histidine, all others are negatively charged.


9. Which of the following is not a type of protein secondary structure ?
• third level folding to form small globules
• â-pleated sheet
• á-helix
• Turns
Answer: third level folding to form small globules
The two most common folding patterns in proteins are the á-helix and the â-pleated sheet.



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