Which amino acid combination is formed when two cysteine amino acids link together?

When two cysteine amino acids link together, they form a compound known as cystine. This occurs through a process where the thiol groups (-SH) present in the cysteine amino acids undergo oxidation, resulting in the formation of a disulfide bond (S-S bond). Cystine is significant in protein structures, as these disulfide bonds provide stability and strength to proteins, particularly those like keratin, which is a structural protein found in hair, skin, and nails.

The presence of cystine is crucial in the formation and maintenance of the three-dimensional structure of various proteins, contributing to their overall function. This explains why cystine is the correct answer, highlighting the relationship between cysteine and its oxidized form when discussing amino acids and proteins in biochemical contexts.