title: Reactivity of Cdc25 phosphatase at low pH and with thiophosphorylated protein substrate.
abstract: Cdc25s, dual-specificity phosphatases that dephosphorylate and activate cyclin-dependent kinases, are important regulators of the eukaryotic cell cycle. Herein, we probe the protonation state of the phosphate on the protein substrate of Cdc25 by pH-dependent studies and thiosubstitution. We have extended the useable range of pH for this enzyme substrate pair by using high concentrations of glycerol under acidic conditions. Using the protein substrate, we find a slope of 2 for the acidic side of the bell-shaped pH-rate profile, as found with other protein tyrosine phosphatases. Using thiophosphorylated protein substrate, we find no change in the basic side of the pH-rate profile, despite a large reduction in activity as measured by kcat/Km (0.18%) or kcat (0. 11%). In contrast, the acidic side of the profile changes shows a slope of 1, consistent with the 1.5 pH unit shift associated with thiosubstitution. Thus, Cdc25, like other protein phosphatases, uses a dianionic phosphorylated substrate.
authors: Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA. rudolph@biochem.duke.edu
authors: Johannes
authors: J
authors: Rudolph
mesh_terms: D019925:Cyclin A / Q000494:pharmacology; D004795:Enzyme Stability; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D015394:Molecular Structure; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism*; D020687:cdc25 Phosphatases / Q000737:chemistry / Q000378:metabolism*
chemical_list: D019925:Cyclin A; D010750:Phosphoproteins; D013438:Sulfhydryl Compounds; D020687:cdc25 Phosphatases