Molecular Forms and Kinetic Properties of Pyruvate, P_I Dikinase From Two Populations of Barnyard Grass (Echinochloa crus-galli) From Sites of Contrasting Climates

Abstract

Plants from two populations of the C₄ barnyard grass (Echinochloa crus-galli (L.) Beauv.) from Québec (QUE) and Mississippi (MISS) were acclimated under controlled conditions to 26/20 and 14/8ºC daylnight. The apparent energy of activation (E_a, K_m for pyruvate, V_max/K_m ratios, K_cat (substrate turnover number) and specific activity of pyruvate, P_I dikinase (PPDK, EC 2.7.9.1) were analysed from partially purified Sephadex G-25 extracts of PPDK from leaves and from highly purified PPDK. PPDK from both populations consisted of one isomorph with the same electrophoretic mobility in polyacrylamide gels and similar molecular weights for the native enzyme (385 kDa) and for the subunit of the tetramer (94.8 kDa). No significant differences were observed for any of the kinetic properties of partially purified or purified PPDK or for the specific activity per mg protein of purified PPDK extracted from plants of the two populations and acclimated to the two thermoperiods. Net photosynthetic rates (Ps) were positively correlated with PPDK activity levels (E) but ElPs ratios were lower than 1.0, ranging from 0.43 to 0.67. Results indicate that differences in activity levels, thermal properties and in the kinetics of light activation and dark inactivation of PPDK extracted from cold-acclimated MISS and QUE plants, as reported in earlier studies, are due to causes other than kinetic properties or electrophoretic characteristics of PPDK.