Biochemistry—Secondary Structure In the β-pleated sheet, which of the following statements best describes its key features?

Introduction / Context:
β-Pleated sheets are one of the two principal protein secondary structures, stabilized by inter-strand hydrogen bonding. This question tests recognition of hydrogen-bonding patterns, strand orientation, and chain conformation in β-sheets.

Given Data / Assumptions:

• β-Sheets comprise at least two β-strands.
• Hydrogen bonds form between C=O and N-H groups of adjacent strands.
• Strands can be parallel or antiparallel.
• Each strand is extended (zig-zag), not tightly coiled.

Concept / Approach:
Identify the defining characteristics: inter-strand hydrogen bonds, orientation variability (parallel/antiparallel), and extended backbone geometry. If all listed statements are standard features, “All of the above” is correct.

Step-by-Step Solution:

Verification / Alternative check:
Ramachandran plots show β-strand phi/psi angles consistent with extended conformations. Structural databases showcase both parallel and antiparallel sheets.

Why Other Options Are Wrong:

• Options A–C are not wrong; they collectively describe β-sheets. Hence D is correct and E is incorrect.

Common Pitfalls:
Confusing intra-strand with inter-strand hydrogen bonds; assuming only antiparallel sheets exist; picturing sheets as flat without pleats (the pleating is subtle but real).

Final Answer:
All of the above.