Which of the following statements concerning protein structure is true?

• A. All globular proteins, including myoglobin and its 8 α-helices, have quaternary structure.
• B. Disulfide bond formation in fibrous proteins is incredibly rare due to the lack of a compact fold.
• C. Enzyme structures tend to have significantly more beta turns than silk fibroin or α-keratin.
• D. Hemoglobin is more soluble than collagen as a result of having a lower variety of motifs.

Answer: (C)

Verified step by step guidance

1. Understand the four levels of protein structure: primary, secondary, tertiary, and quaternary. Primary structure refers to the sequence of amino acids, secondary structure includes α-helices and β-sheets, tertiary structure is the overall 3D shape of a single polypeptide, and quaternary structure involves the assembly of multiple polypeptide chains.
2. Recognize that not all globular proteins have quaternary structure. Quaternary structure is only present if a protein is composed of more than one polypeptide chain. Myoglobin, for example, is a single polypeptide chain and does not have quaternary structure.
3. Consider the role of disulfide bonds in protein structure. Disulfide bonds are covalent bonds that can stabilize protein structure, particularly in extracellular proteins. In fibrous proteins, which often have repetitive sequences and extended structures, disulfide bonds may be less common due to the lack of compact folding.
4. Examine the structural features of enzymes compared to fibrous proteins like silk fibroin and α-keratin. Enzymes, which are typically globular proteins, often have complex tertiary structures with many turns and loops, including beta turns, to facilitate their catalytic functions.
5. Evaluate the solubility of hemoglobin versus collagen. Hemoglobin, a globular protein with quaternary structure, is more soluble in water than collagen, a fibrous protein, due to its compact and varied structure, which allows it to interact more favorably with water molecules.