Question

what is an identifying characteristic of the chemical structure of proteins? a. proteins always contain nitrogen atoms. b. proteins have identical repeating units within the structure. c. all carbon atoms in proteins are bonded to at least three hydrogen atoms. d. amine groups are found at both ends of the chemical backbone in proteins. e. the side chains in amino acids have either a positive charge or negative charge.

Explanation:

• Proteins are made of amino acids. Amino acids have a general formula \(R - CH(NH_2)-COOH\), which shows that nitrogen (\(N\)) is always present in proteins.
• For option b, proteins are made of different amino acids (not identical repeating units). For option c, in amino - acid structure (the building blocks of proteins), the central carbon is bonded to an amino group (\(-NH_2\)), a carboxyl group (\(-COOH\)), a hydrogen, and an \(R\) group (so not all carbons are bonded to three hydrogens). For option d, in a polypeptide chain (a protein chain), there is an amino (\(-NH_2\)) group at one end and a carboxyl (\(-COOH\)) group at the other end. For option e, side - chains in amino acids can be non - polar (no charge), polar (no charge), acidic (negative charge), or basic (positive charge).

Answer:

A. Proteins always contain nitrogen atoms.