Cystine is dimer of Cystein amino acid formed by oxidation reaction. Its main function is to provide mechanical strength to proteins and allow them to retain their 3-D structures and also serves as a substrate for the cystine-glutamate antiporter.
As shown in figure cystine has two amino groups (highlighted blue) and two carboxylic groups (highlighted green). In its original structure cystine is neutral in nature as it has equal number of basic (NH₂) and acidic (COOH) moieties along with two chiral centers (asymmetric carbons) highlighted with red spots.
When one -NH₂ group is replaced by -COOH group the cystine is converted into an acidic compound with three COOH groups and a NH₂ group. Also, one asymmetric carbon will convert into a symmetrical carbon with a loss of one chiral carbon.
In second step, when another NH₂ is replaced by COOH, the acidic strength of resulting compound will increase along with conversion of last chiral carbon into symmetric carbon.
Therefore, the final structure will be acidic in nature with zero chiral carbons as shown in figure attached below.
