Malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+)

Based on crystallography data of homologous NADP-dependent malic enzymes of mammalian origin, a 3D model for C4 pathway NADP-ME in plants has been developed, identifying the key residues involved in substrate-binding or catalysis.

[1] Valine substitution for glycine in either motif region rendered the enzyme completely inactive while spectral analysis indicated no major changes from wild-type form.

[8] Lysine residue 255 has been implicated as a catalytic base for the enzymes reactivity; however, further studies are still required to conclusively establish its biochemical role.

For CAM plants, drought conditions cause stoma to largely remain shut to avoid water loss by evapotranspiration, which unfortunately leads to CO2 starvation.

In compensation, closed stoma activates the translation of NADP-ME to reinforce high efficiency of CO2 assimilation during the brief intervals of CO2 intake, allowing for carbon fixation to continue.

Conversely, slowed light reactions leads to a rise in acidity within the stroma, promoting the inhibition of NADP-ME by malate.

Because the high energy products of the light reactions, NADPH and ATP, are required for the Calvin cycle to proceed, a buildup of CO2 without them is not useful, explaining the need for the regulatory mechanism.

In the cytosol, the enzyme existed as a series of housekeeping isoforms purposed towards a variety of functions including malate level maintenance during hypoxia, microspore separation, and pathogen defense.

Crystal structure of a homologous human malic enzyme highlights key residues involved in substrate binding and catalysis. Site II contains the GLGDLG motif, site V contains the other GXGXXG motif, the highlighted arginine residue interacts with both NADP + and malate, and the highlighted lysine may possibly be involved in base catalysis. PDB file identity for image is 2aw5.