Cystine is formed by
Hydroxylation of cysteine molecule
Carboxylation of cysteine molecule
Peptide bond between two cysteine molecule
Disulfide bond between cysteine molecule
The difference in structure of cysteine and cystine. Two molecules of cysteine are joined together by S--S bond to form one molecule of cystine. Ref: M.N. Chatterjea - Textbook of Biochemistry, 8th edition, page no: 504
Metabolism of protein and amino acid