What Amino Acid Is Made to Replace Another Amino Acid $\alpha$

Multiple Choice

What amino acid is made to replace another amino acid in membrane-spanning $\alpha$ -helices by site-directed mutagenesis of the gene coding for the protein? This is done in order to determine how close together two such $\alpha$ -helices are in the structure of an integral protein.The amino acid is added to facilitate the formation of crosslinks between $\alpha$ -helices that are in close proximity.

A) phenylalanine
B) alanine
C) cysteine
D) methionine
E) proline