Structure analogy — Cellulose fibers most closely resemble which level of protein structural motif?

Introduction:
Cellulose is a linear β-1,4-linked D-glucose polymer. Although carbohydrates and proteins are chemically distinct, supramolecular arrangements can be compared. This question asks which protein secondary structure best analogizes the extended, sheet-like arrangement of cellulose microfibrils.

Given Data / Assumptions:

• Cellulose chains are extended and form extensive interchain hydrogen bonding.
• Microfibrils pack into sheets that give mechanical strength to plant cell walls.
• Protein secondary structures include helices, sheets, and turns.

Concept / Approach:
Like β-sheets in proteins, parallel arrays of extended cellulose chains are stabilized by a dense network of hydrogen bonds between adjacent strands, yielding rigid, sheet-like assemblies. In contrast, α-helices are compact and helical; β-turns are local directional reversals rather than extended arrays.

Step-by-Step Solution:

Verification / Alternative check:
X-ray diffraction and solid-state NMR analyses reveal crystalline cellulose I with layered sheet-like packing, paralleling the way β-sheets generate rigid protein fabrics (e.g., silk fibroin).

Why Other Options Are Wrong:

• α-helices: helical, not sheet-like.
• β-turns: short motifs, not fiber-forming arrays.
• Coiled-coils: intertwined α-helices, unlike extended polysaccharide sheets.
• None of these: incorrect because β-sheets provide a clear analogy.

Common Pitfalls:
Assuming any extended polymer resembles an α-helix due to common textbook images; cellulose is not helical like α-keratin.

Final Answer:
β-sheets.