1. The structure formed by joining the amino acids by a peptide bond is called ________ structure of a protein.?
a) quaternary
b) tertiary
c) secondary
d) primary
Explanation: When amino acids are just linked together by a peptide bond to form a long linear chain of a polypeptide, it is called the primary structure of proteins. Secondary, tertiary, and Quaternary structure are all higher levels of organization and require non-covalent interactions.
2. Which of the following interactions is crucial for the primary structure of proteins?
a) Hydrogen bond
b) Di-sulfide bond
c) Vander Waals interactions
d) Peptide bond
Explanation: Protein’s primary structure is made of amino acids linked together by a peptide bond. Thus, the peptide bond is crucial for the primary structure of proteins. Hydrogen bond, disulfide bond, Vander Waals interactions are all required in a higher level of organization.
3. Which of the following represents the two-dimensional structure of proteins?
a) Quaternary
b) Tertiary
c) Secondary
d) Primary
Explanation: The primary structure of Protein represents the two-dimensional structure of proteins. The primary structure of proteins just contains amino acids linked together to form a long chain of a polypeptide. Quaternary, tertiary, and secondary structure refers to the three-dimensional structure of proteins.
4. For a protein with 100 amino acids, how many possible sequences are there?
a) (100)20
b) (2)100
c) (100)2
d) (20)100
Explanation: The general rule is, for a protein having ‘n’ residues, there are (20)n possible combinations. There are 20 different types of amino acids occurring naturally in proteins. Therefore, (20)100 is the correct answer.
5. The minimum number of residues that appear to be necessary for a polypeptide to fold into a discrete and stable shape that allows it to carry out a function is near 40.
a) False
b) True
Explanation: The above statement is true. Generally, proteins containing 40 or fewer residues are called peptides, whereas proteins containing more than 40 residues are called polypeptides. Multi-subunit protein is formed by joining identical and/or non-identical subunits.
6. In metal chelate affinity chromatography, divalent cations such as Zn+2 or Ni+2 is attached on the electrophoretic matrix and it binds to ___________
a) Trp tag
b) Ala tag
c) Gly tag
d) His tag
Explanation: When 6 histidine residues are inserted consecutively through recombinant DNA technology, it is called as His tag. It has a high affinity for nickel. This strategy is used for the purification of recombinant proteins.
7. The polyacrylamide gel electrophoresis (PAGE) of proteins is based on the following properties of proteins.
a) Size and charge only
b) Size and shape only
c) Electric charge only
d) Size, shape and electric charge
Explanation: The separation in PAGE is based on gel filtration i.e. the size and shape, as well as electrophoretic mobility i.e. electric charge. There are two types of PAGE viz. i) SDS PAGE and ii) Native PAGE.
8. __________ is used as a reducing agent in SDS PAGE.
a) Sodium dodecyl sulfate (SDS)
b) Ammonium persulphate (APS)
c) Bisacrylamide
d) 2-mercaptoethanol
Explanation: 2-mercaptoethanol (beta-mercaptoethanol) is used as a reducing agent in SDS PAGE. It breaks the disulfide bonds and denatures the protein molecule. SDS is not a reducing agent, it disrupts non-covalent interactions between peptides. Ammonium persulphate (APS) and Bisacrylamide do not show any reducing properties.
9. Trypsin cleaves peptide bonds on the ‘c’ terminal side of the positively charged residues Arg and Lys, if the next residues is not _____
a) His
b) Gly
c) Ala
d) Pro
Explanation: Trypsin is a proteolytic enzyme with high specificity. It cleaves the peptide bond on the ‘c’ terminal side of the positively charged residues Arg and Lys, if the next residues is not Pro. It is used in the cleavage of protein molecules for protein sequencing.
10. Cyanogen bromide (CNBr) cleaves the peptide bond on the ‘C’ terminal side of the _______ residues.
a) Trp
b) Ser
c) Ala
d) Met
Explanation: Cyanogen Bromide is an endopeptidase (a type of proteolytic enzyme). It cleaves the peptide bond on the ‘C’ terminal side of the Met residues. It is used to cleave recombinant proteins, therefore it finds wide application in gene cloning.