Which of the following is not a residue at which proteins can be phosphorylated?

Proteins can be modified by the addition of a phosphate group to certain amino acid residues, a process known as phosphorylation. The residues that are typically phosphorylated include serine, threonine, and tyrosine. These amino acids have hydroxyl (-OH) groups in their side chains, which are chemically reactive and can form a bond with a phosphate group.

Alanine, on the other hand, does not have a hydroxyl group in its side chain; it contains a methyl group instead. This structural difference means that alanine cannot participate in phosphorylation in the same way that serine, threonine, and tyrosine can. Therefore, it is correct to state that alanine is not a residue at which proteins can be phosphorylated.

Understanding the functional implications of phosphorylation is critical in the study of cell signaling and regulation of various biological processes, as phosphorylation can dramatically alter protein function, stability, or interactions with other biomolecules.