Signaling functions of superoxide and hydrogen peroxide in enzymatic phosphorylation/dephosphorylation reactions are now well documented, but their mechanisms are still not always clear. Now we propose the novel signaling mechanisms, by which superoxide and hydrogen peroxide mediate the activation and inhibition of phosphorylation/dephosphorylation catalyzed by protein kinases and protein phosphatases. We suggest that as a powerful nucleophile, superoxide is able to mediate phosphorylation of numerous proteins by protein kinases through the deprotonation of protein serine or threonine residues that sharply accelerates the rates of nucleophilic reaction between kinases and phosphorylating proteins. Furthermore the role of superoxide is enhanced due to its "chain" formation in the O(2)(-)--> PI 3-kinase --> protein kinases --> NADPH oxidase --> O(2)(-) cycle. Furthermore we suggest that hydrogen peroxide signaling in the dephosphorylation reactions by protein phosphatases and in the activation of protein kinases is actually mediated by superoxide formed during the conversion of H(2)O(2) into superoxide by the oxidized superoxide dismutase. This proposal is supported by the high rates of superoxide reactions with an anion of the catalytic cysteine residue of protein tyrosine phosphatases and the inability of hydrogen peroxide to react directly with protein serine and threonine residues in the reactions of protein kinases. Understanding of specific role of superoxide in the reactions catalyzed by protein kinases and protein phosphatases can be of importance for the selection of inhibitors of these enzymes playing a big role in numerous physiological and pathological processes.