NAD-malic enzymes of Arabidopsis thaliana display distinct kinetic mechanisms that support differences in physiological control.
The Arabidopsis thaliana genome contains two genes encoding NAD-MEs [NAD-dependent malic enzymes; NAD-ME1 (TAIR accession number At4G13560) and NAD-ME2 (TAIR accession number At4G00570)]. The encoded proteins are localized to mitochondria and assemble as homo- and hetero- dimers in vitro and in vivo. In the present work, the kinetic mechanisms of NAD-ME1 and -ME2 homodimers and NAD-MEH (NAD-ME heterodimer) were studied as an approach to understand the contribution of these enzymes to plant physiology. Product-inhibition and substrate-analogue analyses indicated that NAD-ME2 follows a sequential ordered Bi-Ter mechanism, NAD being the leading substrate followed by L-malate. On the other hand, NAD-ME1 and NAD-MEH can bind both substrates randomly. However, NAD-ME1 shows a preferred route that involves the addition of NAD first. As a consequence of the kinetic mechanism, NAD-ME1 showed a partial inhibition by L-malate at low NAD concentrations. The analysis of a protein chimaeric for NAD-ME1 and -ME2 indicated that the first 176 amino acids are associated with the differences observed in the kinetic mechanisms of the enzymes. Furthermore, NAD-ME1, -ME2 and -MEH catalyse the reverse reaction (pyruvate reductive carboxylation) with very low catalytic activity, supporting the notion that these isoforms act only in L-malate oxidation in plant mitochondria. The different kinetic mechanism of each NAD-ME entity suggests that, for a metabolic condition in which the mitochondrial NAD level is low and the L-malate level is high, the activity of NAD-ME2 and/or -MEH would be preferred over that of NAD-ME1.
Centro de Estudios Fotosintéticos y Bioquímicos, Universidad Nacional de Rosario, Suipacha 531, Rosario, Argentina.
Marcos A
MA
Tronconi
Mariel C
MC
Gerrard Wheeler
Verónica G
VG
Maurino
María F
MF
Drincovich
Carlos S
CS
Andreo
D017360:Arabidopsis / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002384:Catalysis; D019281:Dimerization; D004795:Enzyme Stability; D007700:Kinetics; D008291:Malate Dehydrogenase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
D029681:Arabidopsis Proteins; D008291:Malate Dehydrogenase; C046446:malate dehydrogenase-(oxaloacetate-decarboxylating) (NAD+)