Question

met-ala-cys-his-ala-pro-met-ala-phe-cys
imagine you have an altered version of this protein that changes the first cysteine (cys) to a leucine (leu), as shown below:
met-ala-leu-his-ala-pro-met-ala-phe-cys
review the structures of leucine and cysteine:
which levels of protein structure could be affected by changing the cysteine to a leucine in the altered polypeptide?
quaternary structure only
primary structure only
both primary and tertiary structure
tertiary structure only

Explanation:

The primary structure is the sequence of amino - acids. Changing cysteine to leucine alters the amino - acid sequence, thus affecting primary structure. Tertiary structure is determined by interactions between side - chains of amino - acids. Cysteine has a sulfhydryl group ($-SH$) which can form disulfide bridges, while leucine has a non - polar side - chain. This change in side - chain properties can affect tertiary structure. Quaternary structure involves interactions between multiple polypeptide chains, and there is no indication this is relevant here.

Answer:

C. Both primary and tertiary structure