Given two proteins extracted from connective tissue and blood respectively, which structural features would help you identify the connective tissue protein as collagen and the blood protein as hemoglobin?

• A. Collagen shows long, repetitive glycine-proline-hydroxyproline sequences forming a triple-helical fibrous structure that aggregates into fibrils, while hemoglobin is a compact tetrameric globular protein with four noncovalent subunits each containing a heme group.
• B. Collagen forms a compact globular tetramer with heme prosthetic groups for oxygen binding, while hemoglobin forms long rope-like fibrils that give tensile strength to tissues.
• C. Both collagen and hemoglobin are single alpha-helical monomers with identical tertiary folding and no quaternary assembly, so only molecular weight can distinguish them.
• D. Collagen contains many tryptophans that bind oxygen cooperatively whereas hemoglobin contains repeating glycine-proline motifs that form collagen-like triple helices.

Answer: (A)