n→π* Interactions Modulate the Properties of Cysteine Residues and Disulfide Bonds in Proteins

Noncovalent interactions are ubiquitous in biology, taking on roles that include stabilizing the conformation of and assembling biomolecules, and providing an optimal environment for enzymatic catalysis. Here, we describe a noncovalent interaction that engages the sulfur atoms of cysteine residues and disulfide bonds in proteins-their donation of electron density into an antibonding orbital of proximal amide carbonyl groups. This n ->pi* interaction tunes the reactivity of the CXXC motif, which is the critical feature of thioredoxin and other enzymes involved in redox homeostasis. In particular, an n ->pi* interaction lowers the pK(a) value of the N-terminal cysteine residue of the motif, which is the nucleophile that initiates catalysis. In addition, the interplay between disulfide n ->pi* interactions and C5 hydrogen bonds leads to hyperstable beta-strands. Finally, n ->pi* interactions stabilize vicinal disulfide bonds, which are naturally diverse in function. These previously unappreciated n ->pi* interactions are strong and underlie the ability of cysteine residues and disulfide bonds to engage in the structure and function of proteins.