Which statement regarding primary protein structure is false?

• A. Amide linkages covalently keep amino acid residues in their particular order.
• B. Protein composition entails both the quantity and types of amino acids, but not the order.
• C. Amino acid sequences are always considered from N-terminal to C-terminal residues.
• D. Each amino acid residue contains both a free/ionizable amino & carboxyl group.

Answer: (D)

Verified step by step guidance

1. Understand that primary protein structure refers to the linear sequence of amino acids in a polypeptide chain.
2. Recognize that amide linkages, also known as peptide bonds, are covalent bonds that connect amino acids in a specific sequence, forming the backbone of the protein structure.
3. Acknowledge that the primary structure of a protein is defined by the specific order of amino acids, which is crucial for the protein's function and properties.
4. Note that amino acid sequences are conventionally read from the N-terminal (amino end) to the C-terminal (carboxyl end), reflecting the direction of protein synthesis.
5. Clarify that in a polypeptide chain, the amino acids are linked such that only the terminal ends have free amino and carboxyl groups, while the internal residues are involved in peptide bonds, thus not having free/ionizable groups.