A computational investigation of the reactions of tyrosyl, tryptophanyl, and cysteinyl radicals with nitric oxide and molecular oxygen

Proteins are main targets of oxidants in biological systems. This oxidation may occur in the protein backbone as well as in certain amino acid side chains, depending on the oxidant and amino acid intrinsic reactivity. Moreover, many enzymes are capable of generating stable amino acid radicals, such as tyrosyl, tryptophanyl and cysteinyl radicals. These species react very rapidly (many times as diffusion-controlled reactions) with relevant cellular open-shell species such as nitric oxide (center dot NO) or molecular oxygen (O-2). The exception to this apparent rule is tyrosyl radical, that reacts at diffusion rates with center dot NO, but shows very slow reactivity towards O-2 (rate constant <10(3) M-1 s(-1)). In this work, we provide a comparative molecular-level description of the reaction mechanisms involved in the reactions of tyrosyl, tryptophanyl and cysteinyl radicals towards center dot NO and O-2, through quantum mechanics simulations which allow us to obtain relevant energetic and structural parameters, proposing a molecular explanation to this tyrosyl discrimination capability, namely, its marginal reactivity with O-2.