| Popis: |
Spinach leaf phosphoribulokinase is sensitive to modification by ATP analogs that react with lysine residues. The 2',3'-dialdehyde derivative of ATP (oATP) inactivates enzyme in a slow, time-dependent fashion. The process follows first-order kinetics (kinact = 0.07 min-1), and the concentration dependence of inactivation indicates tight inhibitor binding (Ki = 106 microM). ATP offers good protection against inactivation (Kd = 67 microM), suggesting that oATP is directed toward the catalytic site. This conclusion is supported by the fact that oATP functions as an alternate substrate (Km = 0.55 mM). Inactivation of phosphoribulokinase by [14C]oATP results in a modification stoichiometry of 0.7/site. The 14C-labeled enzyme is stable to dialysis, suggesting that the covalent adduct formed between protein and oATP is not a simple Schiff's base. Adenosine di- and triphosphopyridoxals (Ado-P2-Pl and Ado-P3-Pl, respectively) also inhibit spinach phosphoribulokinase in a time-dependent fashion. In this case, activity loss is reversible unless the inhibited species is borohydride-reduced, suggesting that Ado-P2-Pl and Ado-P3-Pl form Schiff's bases with an amino group on the enzyme. Protection is afforded by the substrate ATP, suggesting that modification is active site-directed. Prolonged incubation of enzyme with these inhibitors does not result in complete inactivation of phosphoribulokinase. Residual activity is dependent on inhibitor concentration, as would be expected if equilibrium is established between the noncovalent E.I complex and the covalent (Schiff's base) E-I species. Kinetic data analysis indicates Ki values of 175 and 11 microM for Ado-P2-Pl and Ado-P3-Pl, respectively. Thus, the ATP-binding domain can easily accommodate the pyridoxal moiety which is tethered to the polyphosphate chain. The phosphorylated ATP analogs employed in this study exhibit substantially tighter binding to phosphoribulokinase than does fluorosulfonyl-benzoyladenosine (Ki = 4.8 mM), which we have previously demonstrated to be useful in selectively modifying the ATP-binding domain (Krieger, T. J., and Miziorko, H. M. (1986) Biochemistry 25, 3496-3501; Krieger, T. J., Mende-Mueller, L. M., and Miziorko, H. M. (1987) Biochim. Biophys. Acta 915, 112-119). Although the adduct formed between oATP and enzyme was unsuitable for structural analysis, borohydride reduction of the Schiff's base formed between enzyme and Ado-P3-[3H]Pl produced a species useful for investigation by protein chemistry techniques. A radiolabeled tryptic peptide was prepared, isolated, and sequenced; the data indicate that lysine 68 is the residue modified by Ado-P3-[3H]Pl. |
