Identification of histidine-119 as the target in the site-specific inactivation of ribonuclease A by ferrate ion

Mechanism of adenylate kinase. Does adenosine 5'-triphosphate bind to the adenosine 5'-monophosphate site?

Although the substrate binding properties of adenylate kinase (AK) have been studied extensively by various biochemical and biophysical techniques, it remains controversial whether uncomplexed adenosine 5'-triphosphate (ATP) binds to the adenosine 5'-monophosphate (AMP) site of AK. We present two sets of experiments which argue against binding of ATP to the AMP site. (a) 31P nuclear magnetic resonance titration of ATP with AK indicated a 1:1 stoichiometry on the basis of changes in coupling constants and line widths. This ruled out binding of ATP to both sites. (b) ATP and MgATP were found to behave similarly by protecting AK from spontaneous inactivation while AMP showed only a small degree of protection. Such inactivation could also be protected or reversed by dithioerythritol and is most likely due to oxidation of sulfhydryl groups, one of which (cysteine-25) is located near the MgATP site. The results support binding of ATP to the MgATP site predominantly, instead of the AMP site, in the absence of Mg2+.

Effect of ferrate, a site-specific phosphate analog, on human deoxyhemoglobin

Ferrate ion, a phosphate analog and a potent oxidizing agent, is known to inactivate a number of enzymes which interact with phosphoryl compounds. In contrast, enzymes which do not interact with phosphoryl compounds are not affected by comparable concentrations of ferrate. To further explore the specificity of ferrate as a reagent which is specific for phosphoryl binding sites, a study of its effect on human hemoglobin A was undertaken. In the deoxy form, this protein is known to interact with 2,3-bisphosphoglycerate, its natural allosteric inhibitor of cooperative binding of oxygen, while as oxyhemoglobin it does not interact with the inhibitor. Treatment with ferrate ion caused the loss of approximately three amino acid residues per beta chain of human deoxyhemoglobin, His-2, His-143, and Tyr-145, and one residue, presumably Tyr-42, per alpha chain. Oxyhemoglobin was not affected by the reagent. 2,3-Bisphosphoglycerate was found to protect deoxyhemoglobin from the action of ferrate. His-2 and His-143 are among the residues reported to be implicated in the binding of 2,3-bisphosphoglycerate by deoxyhemoglobin [A. Arnone (1972) Nature (London) 237, 146-148].