Steric effect and effect of metal coordination on the reactivity of nitric oxide with cysteine-containing proteins under anaerobic conditions

A comparison is made between the reactivity of nitric oxide (NO) with cysteine, bovine serum albumin (BSA) and metallothionein-1 (MT1) at pH 7 under strictly anaerobic conditions. The rate of reaction of NO with these amino acid/proteins was found to be of the order: cysteine>BSA≫MT1, in clear disparity with the size of the reactants. The difference in the reaction rates is attributed to steric effects due to the high molecular size in the case of BSA and to effects of metal coordination proper as well as to steric effects associated with the closed dual shell-like structure resulting from the tight coordination of the thiolate groups with Zn2+ in MT1. The mechanisms of the reaction of NO with cysteine, BSA and MT and its possible implication for the rate of the respective reactions are discussed.