title: 3-Mercaptopicolinate. A reversible active site inhibitor of avian liver phosphoenolpyruvate carboxykinase.
abstract: The inhibition of chicken liver phosphoenolpyruvate carboxykinase by 3-mercaptopicolinic acid (3-MP) has been investigated. Kinetic studies show 3-MP to be a noncompetitive inhibitor relative to all substrates and to the activator, Mn2+. EPR studies demonstrate that Mn2+ binding to the enzyme is unaffected by 3-MP. Proton relaxation rate studies demonstrate that 3-MP binds to the binary E X Mn complex with a KD of 0.5 X 10(-6) M and gives a ternary enhancement of 8.0. Additional proton relaxation rate studies detected formation of the quaternary complexes E X Mn X IDP X 3-MP, E X Mn X ITP X 3-MP, and E X Mn X CO2 X 3-MP. High resolution 1H nuclear relaxation rate studies suggest that 3-MP binds in close proximity to the activator cation, Mn2+, but not in the first coordination sphere. Active site models suggest that the 3-MP-binding site may partially overlap the phosphoenolpyruvate-binding site. The NMR studies, which detected formation of the quaternary E X Mn X 3-MP X phosphoenolpyruvate complex, also demonstrated that the binding of one of these ligands affects the interactions of the other ligand with E X Mn. Calorimetric studies of the E X Mn complex demonstrated that 3-MP causes an increase in the transition temperature midpoint without an increase in enthalpy. These results indicate that 3-MP causes a conformational change in the enzyme but does not increase the thermostability of the ternary complex. The experiments reported herein suggest that inhibition by 3-MP is due to specific and reversible binding within the active site of phosphoenolpyruvate carboxykinase.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002645:Chickens; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008345:Manganese / Q000494:pharmacology; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000037:antagonists & inhibitors*; D010848:Picolinic Acids / Q000494:pharmacology*; D011489:Protein Denaturation; D013696:Temperature
chemical_list: D010848:Picolinic Acids; D008345:Manganese; C008363:3-mercaptopicolinic acid; D010729:Phosphoenolpyruvate Carboxykinase (GTP)