Test: Primary Structure of Protein - MCAT MCQ

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.

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.

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.

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.

Mass spectrometry is a technique that measures the mass to charge (m/z) ratio for the ions in the gas phase. Therefore, mass spectrometry is used for the sequencing of peptides. Mass spectrometry can also be used to identify a protein; hence, it is also called protein mass fingerprinting.

During translation, the first amino acid attached has an amino free end and the last amino acid has a carboxyl free end. Hence, the N terminal is represented on the left side and C terminal on the right side.

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.

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.

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.

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.