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 L-arginine into L-citrulline by the enzyme NO synthase, has been propose d to exert its action in several physiological and pathological events. The great propensity for nitrosothiol formation and breakdown represents a mec hanism which modulates the action of macromolecules containing NO-reactive Cys residues at their active centre and/or allosteric sites. Based on the h uman haemoglobin (Hb) structure and accounting for the known acid-base cata lysed Cys beta 93-nitrosylation and Cys beta 93NO-denitrosylation 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 m otif for Cys-NO 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 positio n has been observed in some NO-reactive protein sequences (e.g., NMDA recep tors). However, the most important component of the tri- or tetra-peptide c onsensus motif has been recognised as the Cys(Asp/Glu) pair [Stamler et at, Neuron (1997) 18, 691-696]. Here, we analyse the three-dimensional structu re of several proteins containing NO-reactive Cys residues, and show that t heir nitrosylation and denitrosylation processes may depend on the Cys-Sy a tomic structural microenvironment rather than on the tri- or tetra-peptide sequence consensus motif.