Question

The helix and pleated sheet form of a protein is its__________ structure. a. secondary b. tertiary c. primary d. quaternary

Short answer

The helix and pleated sheet form of a protein is its \(\textbf{secondary}\) structure.

Step-by-step solution

Understanding the four levels of protein structure

The protein structure can be classified into four levels: primary, secondary, tertiary, and quaternary. Each level represents a different aspect of the protein's organization and folding.

Primary structure

The primary structure of a protein refers to the linear sequence of amino acids in a polypeptide chain. It is the basic, one-dimensional structure held together by covalent peptide bonds between adjacent amino acids. In this exercise, we are looking for the structure corresponding to the helix and pleated sheet form, which is not the primary structure.

Secondary structure

The secondary structure of a protein refers to the local folding of the polypeptide chain into more ordered, stable structures like alpha-helices and beta-pleated sheets. These structures are formed due to hydrogen bonding between the backbone atoms of the peptide chain. Since this level includes the helix and pleated sheet form, it could be the correct answer.

Tertiary structure

The tertiary structure of a protein refers to the overall three-dimensional folding of the polypeptide chain due to the interactions between the side chains (R-groups) of amino acids. These interactions include hydrogen bonding, hydrophobic interactions, ionic bonds, and disulfide bridges. The tertiary structure is not the helix and pleated sheet form, so this is not the correct answer.

Quaternary structure

The quaternary structure of a protein refers to the assembly and interaction of multiple polypeptide chains to form a single, functional protein complex. This level is only applicable to proteins with more than one polypeptide chain (subunits). The quaternary structure doesn't directly relate to the helix and pleated sheet form, so this option is also not the correct answer.

Conclusion

Based on our analysis, the correct answer is the secondary structure, which includes the helix and pleated sheet form of a protein. Therefore, the answer to the given exercise is (a) secondary structure.