Which amino acid plays a key role in disulfide bond formation?
Cysteine is the accurate answer because it correctly identifies the biological function or role described in this question. In Protein Solubility, understanding the specific functions of molecules, enzymes, or structures is fundamental. Cysteine fulfills this particular biological role through its specific structural properties, biochemical activity, or physiological mechanism. The other options (Methionine, Excessive hydrophobic collapse, and Alanine) serve different biological functions or are associated with other processes, pathways, or structural roles within the cell or organism.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4