title: Phosphoenolpyruvate carboxykinase assayed at physiological concentrations of metal ions has a high affinity for CO2.
abstract: The effect of Mn2+/Mg2+ concentration on the activity of intact, homogeneous phosphoenolpyruvate carboxykinase (PEPCK) from leaves of the C4 grass, Guinea grass (Panicum maximum), have been investigated. Assay conditions were optimized so that PEPCK activity could be measured at concentrations of Mn2+/Mg2+ similar to those found in the cytosol (low micromolar Mn2+ and millimolar Mg2+). PEPCK activity was totally dependent on Mn2+ and was activated at low micromolar concentrations of Mn2+ by millimolar concentrations of Mg2+. Therefore, at physiological concentrations of Mn2+, PEPCK has a requirement for Mg2+. Assay at physiological concentrations of Mn2+/Mg2+ led to a marked decrease in its affinity for ATP and a 13-fold increase in its affinity for CO2. The Km (CO2) was further decreased by assay at physiological ATP to ADP ratios, reaching values as low as 20 microM CO2, comparable with the Km (CO2) of ribulose 1,5-bisphosphate carboxylase-oxygenase. This means that PEPCK will catalyze a reversible reaction and that it could operate as a carboxylase in vivo, a feature that could be particularly important in algal CO2-concentrating systems.
authors.affiliation: Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, United Kingdom.
authors.forename: Zhi-Hui
authors.initials: ZH
authors.lastname: Chen
authors.forename: Robert P
authors.initials: RP
authors.lastname: Walker
authors.forename: Richard M
authors.initials: RM
authors.lastname: Acheson
authors.forename: Richard C
authors.initials: RC
authors.lastname: Leegood
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism* / Q000494:pharmacology; D003600:Cytosol / Q000201:enzymology; D003653:Decarboxylation; D008274:Magnesium / Q000494:pharmacology*; D008345:Manganese / Q000494:pharmacology*; D008897:Panicum / Q000187:drug effects / Q000201:enzymology*; D019764:Phosphoenolpyruvate Carboxykinase (ATP) / Q000302:isolation & purification / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology; D010940:Plant Proteins / Q000378:metabolism
chemical_list: D010940:Plant Proteins; C054765:rca protein, plant; D002245:Carbon Dioxide; D008345:Manganese; D002244:Carbon; D000255:Adenosine Triphosphate; D019764:Phosphoenolpyruvate Carboxykinase (ATP); D008274:Magnesium