Question

there are four levels of protein structure. these figures show primary, secondary, tertiary, and quaternary protein structure. which level(s) of protein structure may be stabilized by covalent bonds?
primary, tertiary and quaternary levels of protein structure.
secondary level of protein structure.
none of the levels of protein structure is stabilized by covalent bonds.

Explanation:

Primary structure is stabilized by peptide bonds (a type of covalent bond). Tertiary structure can be stabilized by disulfide bridges (covalent bonds between cysteine residues). Quaternary structure can also be stabilized by disulfide - like covalent bonds between different polypeptide chains. Secondary structure is stabilized by hydrogen bonds, not covalent bonds.

Answer:

Primary, tertiary and quaternary levels of protein structure.