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Levels of Protein Structure — Identify which option best represents a tertiary structural feature.

Difficulty: Easy

A multimeric protein
An alpha-helix
A beta-pleated sheet
A globular domain
A primary amino acid sequence

Correct Answer: A globular domain

Explanation:

Introduction:
Protein structure is organized into levels: primary (sequence), secondary (local motifs), tertiary (overall fold of a single polypeptide), and quaternary (assembly of multiple subunits). This question asks you to identify a tertiary feature.

Given Data / Assumptions:

Alpha-helices and beta-sheets are secondary structures.
A globular domain describes the 3D fold of a single polypeptide chain or a distinct region within it.
Multimeric assembly corresponds to quaternary structure.

Concept / Approach:
Map each option to the appropriate structural level and select the one that reflects the 3D folding of one chain (tertiary structure).

Step-by-Step Solution:

1) Option a: Multimeric protein implies multiple chains interacting (quaternary).2) Option b: Alpha-helix is secondary.3) Option c: Beta-pleated sheet is secondary.4) Option d: A globular domain is the compact 3D fold of a single chain region (tertiary).5) Option e: Primary sequence is the linear order of amino acids.

Verification / Alternative check:
Examples: Myoglobin is essentially a single-chain globular protein (tertiary). Hemoglobin is tetrameric (quaternary).

Why Other Options Are Wrong:

a) Not tertiary; involves multiple chains.b,c) Local motifs, not overall fold.e) Linear sequence, not 3D structure.

Common Pitfalls:
Confusing secondary motifs with the full 3D folding of a domain; assuming 'larger' means quaternary even if it is a single chain.

Levels of Protein Structure — Identify which option best represents a tertiary structural feature.

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