Re-Evaluation of Amino Acid Sequence and Structural Consensus Rules for Cysteine-Nitric Oxide Reactivity

Nitric oxide (NO), produced in different cell types through the conversion of Larginine into Lcitrulline by the enzyme NO synthase, has been proposed to exert its action in several physiological and pathological events. The great propensity for nitrosothiol formation and breakdown represents a mechanism which modulates the action of macromolecules containing NOreactive Cys residues at their active centre and/or allosteric sites. Based on the human haemoglobin (Hb) structure and accounting for the known acidbase catalysed Cys?93-nitrosylation and Cys?93NOdenitrosylation processes, the putative amino acid sequence (Lys/Arg/His/Asp/Glu)Cys(Asp/Glu) (sites 1, 0, and + 1, respectively) has been proposed as the minimum consensus motif for CysNO reactivity. Although not found in human Hb, the presence of a polar amino acid residue (Gly/Ser/Thr/Cys/Tyr/Asn/Gln) at the 2 position has been observed in some NOreactive protein sequences (e.g., NMDA receptors). However, the most important component of the tri or tetrapeptide consensus motif has been recognised as the Cys(Asp/Glu) pair [Stamler et al., Neuron (1997) 18, 691 696]. Here, we analyse the threedimensional structure of several proteins containing NOreactive Cys residues, and show that their nitrosylation and denitrosylation processes may depend on the CysS? atomic structural microenvironment rather than on the tri or tetrapeptide sequence consensus motif.