Which of the following is NOT true about proteins?

• A. All proteins function as enzymes in biological systems.
• B. Proteins are composed of amino acids linked by peptide bonds.
• C. The three-dimensional structure of a protein determines its function.
• D. Proteins can serve as structural components in cells.

Answer: (A)

Verified step by step guidance

1. Step 1: Begin by understanding the role of proteins in biological systems. Proteins are macromolecules composed of amino acids linked by peptide bonds, and they perform a wide variety of functions, including enzymatic, structural, signaling, and transport roles.
2. Step 2: Analyze the statement 'All proteins function as enzymes in biological systems.' This is incorrect because not all proteins act as enzymes. While enzymes are a subset of proteins that catalyze biochemical reactions, many proteins serve other roles, such as structural components (e.g., collagen), transport molecules (e.g., hemoglobin), or signaling molecules (e.g., hormones).
3. Step 3: Evaluate the statement 'Proteins are composed of amino acids linked by peptide bonds.' This is true. Proteins are polymers of amino acids, and the amino acids are connected by peptide bonds formed during translation.
4. Step 4: Examine the statement 'The three-dimensional structure of a protein determines its function.' This is true. The specific folding and shape of a protein, determined by its primary amino acid sequence, are critical for its biological activity and interaction with other molecules.
5. Step 5: Assess the statement 'Proteins can serve as structural components in cells.' This is true. Proteins like actin, tubulin, and collagen provide structural support to cells and tissues, playing essential roles in maintaining cellular integrity and function.