Photosynthesis in Phosphoenolpyruvate Carboxykinase-Type C₄ Species: Properties of Nad-Malic Enzyme From Urochloa panicoides

Abstract

NAD-malic enzyme (EC 1.1.1.39) was purified from bundle sheath strands of Urochloa panicoides (a phosphoenolpyruvate carboxykinase-type C₄ plant) and its kinetic and regulatory properties were investigated. The native enzyme has a molecular weight of about 470 000 and is an octomer composed of two slightly different monomers which occur in a 1 : 1 ratio. The enzyme has an absolute requirement for Mn²⁺, is stimulated by CoA, acetyl CoA, fructose 1,6-bisphosphate and SO₄^2- and is inhibited by HCO₃, oxaloacetate, 2-oxoglutarate and pyruvate. The enzyme is shown to be localised in the mito- chondria. The purified NAD-malic enzyme is unable to catalyse the carboxylation of pyruvate according to the reverse reaction. These findings are discussed in relation to the C₄ photosynthetic pathway and its possible role in PEP carboxykinase-type C₄ plants.