Which elements make up proteins?

• A. C, H, O, N, P, S
• B. C, H, O
• C. C, H, O, N
• D. C, H, O, N, P

Answer: (A)

Proteins are built from amino acids, which all contain carbon, hydrogen, and oxygen, plus nitrogen in the amino group. In many amino acids sulfur is present (as in cysteine and methionine), and sulfur-containing amino acids help form disulfide bonds that stabilize protein structure. Phosphorus isn’t part of every amino acid, but proteins can carry phosphate groups through phosphorylation, so phosphorus can appear in proteins as well. Because proteins can include C, H, O, N, S, and sometimes P, this set is the most complete description of the elements that can make up proteins. The other options miss one or more essential elements (for example, nitrogen is essential for amino groups; sulfur is present in important amino acids; phosphorus appears in phosphorylated proteins), so they’re less accurate overall.