FIGURE 2.

Different functional categories of Cys residues. A schematic representation of Cys functionality is shown. Starting from the top of the rhombus, which gives the molecular structure of Cys, and going clockwise: catalytic residues (red circle; representing a nucleophilic thiolate), metal-binding Cys (orange circles; representing a zinc-Cys₄-binding site), structural disulfides (yellow circles; representing a covalent bond between two Cys residues), and regulatory Cys (violet circle; representing an S-nitrosylation site). As discussed in text, not all functional Cys can be reliably categorized. For example, some catalytic Cys residues can also be S-nitrosylated or oxidized to sulfenic acid, and some metal-binding Cys residues can, in certain situations, turn to cystine residues. To represent this complexity and an occasional interplay of functions, the rhombus connecting different functional Cys categories is shown with a dashed line.