Oryza sativa Japonica Group glycolysis IV (plant cytosol)

Pathway diagram: glycolysis IV (plant cytosol)

If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Locations of Mapped Genes:

Schematic showing all replicons, marked with selected genes

Synonyms: glycolysis 4

Superclasses:

Generation of Precursor Metabolites and Energy → Glycolysis

Summary:
General Background

Glycolysis, which was first studied as a pathway for the utilization of glucose, is one of the components of central metabolism, the other two being the pentose phosphate pathway and the TCA cycle. As such, its functioning is essential under all conditions of growth because it produces six (β-D-glucose-6-phosphate, fructose-6-phosphate, dihydroxy-acetone-phosphate, 3-phosphoglycerate, phosphoenolpyruvate, and pyruvate) of the 13 precursor metabolites that are the starting materials for the biosynthesis of building blocks for macromolecules and other needed small molecules. Glycolysis can be found, if at least in part, in all organisms.

Glycolysis has evolved as a catabolic anaerobic pathway that fulfills two essential functions:

i) it oxidizes hexoses to generate ATP, reductants and pyruvate, and

ii) it is an amphibolic pathway (pathway that involves both catabolism and anabolism) because it can reversibly produce hexoses from various low-molecular weight molecules.

Because various degradation pathways feed into glycolysis at many different points, glycolysis or portions of it run in the forward or reverse direction, depending on the carbon source being utilized, in order to satisfy the cell's need for precursor metabolites. This switching of direction is possible because all but two of the enzymatic reactions comprising glycolysis are reversible, and the conversions catalyzed by the two exceptions are rendered functionally reversible by other enzymes (fructose-1,6-bisphosphatase I and phosphoenolpyruvate synthetase) that catalyze different irreversible reactions flowing in the opposite direction.

About This Pathway

In plants, glycolysis is the predominant pathway fueling respiration (see TCA cycle) because, unlike animal mitochondria, plant mitochondria rarely respire fatty acids. In plants, this pathway occurs in two different subcellular locations: the cytosol and plastids, which are the sites of sucrose degradation III and starch degradation, respectively. Whereas the plastidic glycolysis pathway (see glycolysis I) is identical to the conventional microbial glycolysis, the cytosolic pathway (i.e. this pathway) is slightly modified. These pathways can interact with one another though the action of highly selective transporters present in the inner plastid envelope [Emes93]. In chloroplasts in the dark, as well as in plastids of non-photosynthetic tissues, the primary function of glycolysis I is the degradation of starch to generate carbon skeletons, reductants and ATP for anabolic pathways such as that of fatty acid biosynthesis initiation I via pyruvate dehydrogenase complex. In the cytosol, the same products are generated from the degradation of sucrose.

In the cytosol, there are two alternative reactions to the plastidic pathway: EC 2.7.1.90 and EC 1.2.1.9. The former represents an alternative to EC 2.7.1.11; this reaction utilizes diphosphate rather than ATP to convert fructose-6-phosphate to fructose-1,6-bisphosphate. The plant cytosol lacks soluble inorganic alkaline pyrophosphatases (PPiases) and, consequently, contains higher concentrations of diphosphate (up to 0.3 mM [Weiner87]). It has been proposed that the utilization of pyrophosphate rather than that of ATP in glycolysis is favored under nutritional Pi deprivation or oxygen deficiency [Davies93, Plaxton96]. The latter reaction, EC 1.2.1.9, provides a bypass between D-glyceraldehyde-3-phosphate and 3-phosphoglycerate, which allows for their conversion without phosphorylation by a non-phosphorylating glyceraldehyde 3-P dehydrogenase [Plaxton96, Rius06]. This reaction produces NADPH but not ATP.