Gluconeogenesis in Plants: A Key Interface between Organic Acid/Amino Acid/Lipid and Sugar Metabolism

Abstract

Gluconeogenesis is a key interface between organic acid/amino acid/lipid and sugar metabolism. The aims of this article are four-fold. First, to provide a concise overview of plant gluconeogenesis. Second, to emphasise the widespread occurrence of gluconeogenesis and its utilisation in diverse processes. Third, to stress the importance of the vacuolar storage and release of Krebs cycle acids/nitrogenous compounds, and of the role of gluconeogenesis and malic enzyme in this process. Fourth, to outline the contribution of fine control of enzyme activity to the coordinate-regulation of gluconeogenesis and malate metabolism, and the importance of cytosolic pH in this.

Keywords: gluconeogenesis; malate; malic enzyme; nitrogen metabolism; organic acids; phosphoenolpyruvate carboxykinase; pyruvate orthophosphate dikinase; vacuole.

Figure 1

The PEPCK and PPDK gluconeogenesis pathways. OAA = oxaloacetate; PEP = phosphoenolpyruvate; MDH = malate dehydrogenase; ME = malic enzyme; PEPCK = phosphoenolpyruvate carboxykinase; PPDK = pyruvate orthophosphate dikinase.

Figure 2

The utilisation of vacuolar-released malate by gluconeogenesis and other processes such as the Krebs cycle. OAA = oxaloacetate; PEP = phosphoenolpyruvate; MDH = malate dehydrogenase; ME = malic enzyme; PEPCK = phosphoenolpyruvate carboxykinase; PK = pyruvate kinase; PPDK = pyruvate orthophosphate dikinase.

Figure 3

Gluconeogenesis from malate and its association with nitrogen metabolism. Simplified scheme depicting gluconeogenesis associated with the metabolism of asparagine or ammonium: storage phase of these metabolites (A); utilisation phase (B) (Reproduced from Walker et al. [7]). Fluxes through different reactions in the schemes will differ according to factors such as which nitrogenous compound (e.g., asparagine [e.g., maize pedicel], glutamine or ammonium [e.g., maize root fed ammonium]) is the major input. Glutamate, aspartate, pyruvate and PEP are the precursors of most amino acids [105], and in both situations these metabolites could be produced. GDH = glutamate dehydrogenase; GOGAT = glutamine oxoglutarate aminotransferase; GS = glutamine synthase; ME = malic enzyme; OAA = oxalacetate; PEP = phosphoenolpyruvate; PEPC = phosphoenolpyruvate carboxylase.

Figure 4

Gluconeogenesis from vacuolar-released alanine. Simplified scheme outlining the functions of PPDK in gluconeogenesis and nitrogen metabolism (Reproduced from Walker et al. [7]). GDH = glutamate dehydrogenase; GOGAT = glutamine oxoglutarate aminotransferase; ME = malic enzyme; OAA = oxaloacetate; 2-OG = 2-oxoglutarate; PEP = phosphoenolpyruvate; PEPC = phosphoenolpyruvate carboxylase.

Figure 5

Phylogenetic analysis of PEPCK proteins from three domains of life. Several representative PEPCK protein sequences were selected for each phylum of three domains of life [130] including bacteria [131,132], protozoans and fungi [133], plants [134] and metazoans in the NCBI database. Protein sequences were aligned using Clustal W [135]. After deletion of segments with poor consensus alignment, sequences were subjected to Bayesian inference for establishment of phylogenetic relationships between proteins [136]. Analysis were run for 5 million generations under a mixed amino-acid model with rate variation between sites estimated by a gamma distribution. Bayesian inference posterior probabilities (BIPPs) of tree nodes are indicated by coloured dots. Gene identifiers of the proteins are color-coded to represent the phyla from which they are derived. Green for plants, blue for fungi, pink for protozoans, purple for bacteria and red for metazoans. Corresponding species names are listed by the side of accession number on each branch of the tree. Note for the plant species there are very little differences in the amino acid sequence of the protein apart from in the c12 kD N-terminal extension. Thus, the reconstructed phylogeny of the plant enzyme is based largely on this part of the protein.

Figure 6

Alignment of N-terminal extension sequences of plant PEPCKs. Several representative PEPCK protein sequences, from the phylum of plants and algae [134], were selected from the NCBI database. Protein sequences were aligned using Clustal W [135]. The alignment of N-terminal extension sequences was edited and extracted using the BioEdit programme. Species names are listed by the side of accession number. The phosphorylation motif (KK/RXSXPT) or its absence is shown in the red box.