Test: Quaternary Structure of Protein - MCAT MCQ

Quaternary structure of a protein is the arrangement of protein subunits in a multi-subunit complex. Many proteins are composed of more than one polypeptide chain. These polypeptide chains fold to form a subunit of a complex protein. All subunits combine to form a complete functional protein.

The interaction shown in the above figure is a disulphide bond. Two Cysteine residues are involved in the formation of a disulfide bond. It is covalent in nature.

The interaction shown in the above figure is ionic interactions. Ionic interaction is the interaction between two charged groups. If the two charged groups are oppositely charged then they will attract, while two similar charges repel each other.

Quaternary structure of a protein is made of a combination of any of the following interactions: i) Hydrogen bonds, ii) hydrophobic interactions, iii) disulfide bonds, iv) ionic interactions. Thus, the option containing all of the above options is correct.

Many proteins have a complex structure i.e. they have a multi-subunit structure. When some or all the sub-units of a complex protein are identical it is called oligomers.

Hydrogen bond, disulfide bond, an ionic bond are the interactions that make up the quaternary structure. The peptide bond is the basis of the primary structure of a protein. Hence, the peptide bond is the odd one out.

Beta-mercaptoethanol/dithiothreitol is used forthe reduction of disulfide bonds. Urea and guanidine hydrochloride are denaturating agents, while Iodoacetate is used for acetylation of sulfhydryl groups.

Generally, complex proteins that have more than one polypeptide chains adopt a quaternary structure. Thus, the option ‘single polypeptide chain’ is not suitable for the quaternary structure. All other options are suitable for the quaternary structure of a protein.

The interaction shown in the above figure is hydrophobic interactions. Hydrophobic interactions involve interaction between two non-polar hydrophobic residues. It is non-covalent in nature.

The interaction shown in the above figure is the hydrogen bond. A hydrogen bond is the interaction between a hydrogen atom attached to an electronegative atom with another electronegative atom like oxygen in its vicinity. It is non-covalent in nature.