Collagen triple helix and higher-order assembly

Steps:

• Recall that three polypeptide chains form a triple helix via interchain hydrogen bonds, so X = hydrogen.
• Note that triple helices assemble into microfibrils through intermolecular interactions, making Y = microfibrils.
• Recognize that microfibrils aggregate into larger fibres, so Z = fibres.
• Match these to option B, which correctly identifies the bonds and structures.

Why B is correct:

• Hydrogen bonds stabilize the triple helix (X) per collagen's quaternary structure definition, while microfibrils (Y) and fibres (Z) describe the hierarchical assembly from tropocollagen units.

Why the others are wrong:

• A: Disulphide bonds are absent in collagen triple helices, unlike in keratin.
• C: Peptide bonds are intrachain, not for holding separate chains together (X); Z reverses the assembly order.

Final answer: B