por The triose They are three-carbon monosaccharides whose empirical chemical formula is C3H6O6. There are two trioses: glyceraldehyde (an aldose) and dihydroxyacetone (a ketose). Trioses are important in metabolism because they connect three metabolic pathways: glycolysis, gluconeogenesis, and the pentose phosphate pathway.
During photosynthesis, the Calvin cycle is a source of trioses that serve for the biosynthesis of fructose-6-phosphate. This sugar, in a phosphorylated manner, is converted by enzymatically catalyzed steps into storage or structural polysaccharides.
Trioses participate in the biosynthesis of lipids that are part of cell membranes and adipocytes.
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Characteristics
Glyceraldehyde aldose has one chiral carbon atom and therefore has two enantiomers, L-glyceraldehyde and D-glyceraldehyde. Both D and L enantiomers have different chemical and physical characteristics.
D-glyceraldehyde rotates plane-polarized light to the right (+) and has a rotation [α]D, at 25°C, of +8.7°, while L-glyceraldehyde rotates the plane of polarized light to the left (-) and has a rotation [α]D, at 25°C, of -8.7°.
The chiral carbon of glyceraldehyde is carbon 2 (C-2), which is a secondary alcohol. The Fischer projection depicts the hydroxyl group (-OH) of D-glyceraldehyde on the right and the OH- group of L-glyceraldehyde on the left.
Dihydroxyacetone lacks chiral carbons and does not have enantiomeric forms. The addition of a hydroxymethylene (-CHOH) group to glyceraldehyde or dihydroxyacetone allows for the creation of a new chiral center. Consequently, sugar is a tetrose because it has four carbons.
The addition of a -CHOH group to the tetrose creates a new chiral center. The sugar formed is a pentose. -CHOH groups can continue to be added up to a maximum of ten carbons.
Functions in the organism
Trioses as intermediates in glycolysis, gluconeogenesis and the pentose phosphate pathway
Glycolysis consists of the breakdown of the glucose molecule into two pyruvate molecules to produce energy. This path involves two phases: 1) preparatory phase, or energy consumption; 2) power generation phase. The first is the one that produces the trioses.
In the first phase, the free energy content of glucose is increased, through the formation of phosphoesters. In this phase, adenosine triphosphate (ATP) is the phosphate donor. This phase culminates in the conversion of fructose phosphoester 1,6-bisphosphate (F1,6BP) into two phosphate trioses, glyceraldehyde 3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP).
Gluconeogenesis is the biosynthesis of glucose from pyruvate and other intermediates. It uses all the glycolysis enzymes that catalyze reactions whose biochemical standard Gibbs energy variation is in equilibrium (ΔGº’~ 0). Because of this, glycolysis and gluconeogenesis have common intermediaries, including GA3P and DHAP.
The pentose phosphate pathway consists of two stages: an oxidative phase of glucose-6-phosphate and another for the formation of NADPH and ribose-5-phosphate. In the second phase, ribose 5-phosphate is converted to glycolysis intermediates, F1,6BP and GA3P.
Trioses and the Calvin Cycle
Photosynthesis is divided into two stages. In the first, light-dependent reactions occur that produce NADPH and ATP. These substances are used in the second, in which there is fixation of carbon dioxide and formation of hexoses from trioses through a pathway known as the Calvin cycle.
In the Calvin cycle, the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the covalent attachment of CO2 to the pentose ribulose 1,5-bisphosphate and breaks the unstable six-carbon intermediate into two three-carbon molecules. carbon atoms: 3-phosphoglycerate.
Through enzymatic reactions that include the phosphorylation and reduction of 3-phosphoglycerate, using ATP and NADP, GA3P is produced. This metabolite is converted to fructose 1,6-bisphosphate (F1,6BP) via a metabolic pathway similar to gluconeogenesis.
Through the action of a phosphatase, F1,6BP is converted to fructose-6-phosphate. A phosphohexose isomerase then produces glucose 6-phosphate (Glc6P). Finally, an epimerase converts Glc6P to glucose 1-phosphate, which is used for starch biosynthesis.
Trioses and lipids of biological membranes and adipocytes
GA3P and DHAP can form glycerol phosphate which is a necessary metabolite for the biosynthesis of triacylglycerols and glycerolipids. This is because both triose phosphates can be interconverted by a reaction catalyzed by triose phosphate isomerase, which keeps both trioses in equilibrium.
The enzyme glycerol-phosphate dehydrogenase catalyzes an oxidation-reduction reaction, in which NADH donates an electron pair to DHAP to form glycerol 3-phosphate and NAD+. L-glycerol 3-phosphate is part of the skeleton of phospholipids that are a structural part of biological membranes.
Glycerol is prochiral, lacking asymmetric carbons, but when one of its two primary alcohols forms a phosphoester, it can be correctly named L-glycerol 3-phosphate, or D-glycerol 3-phosphate.
Glycerophospholipids are also called phosphoglycerides, being named as derivatives of phosphatidic acid. Phosphoglycerides can form phosphoacylglycerols by forming ester bonds with two fatty acids. In this case, the resulting product is 1,2-phosphodiacylglycerol, which is an important component of membranes.
A glycerophosphatase catalyzes the hydrolysis of the phosphate group of glycerol 3-phosphate, producing glycerol plus phosphate. Glycerol can serve as a starting metabolite for the biosynthesis of triacylglycerides, which are common in adipocytes.
Trioses and the membranes of archaebacteria
Similar to eubacteria and eukaryotes, glycerol 3-phosphate is formed from triose phosphates (GA3P and DHAP). However, there are differences: the first is that the glycerol 3-phosphate in the membranes of archaebacteria is of the L configuration, while in the membranes of eubacteria and eukaryotes it is of the D configuration.
A second difference is that the membranes of archaebacteria form ester bonds with two long hydrocarbon chains of isoprenoid groups, whereas in eubacteria and eukaryotes glycerol forms ester bonds (1,2-diacylglycerol) with two hydrocarbon chains of fatty acids.
A third difference is that, in the membranes of archaebacteria, the substituents on the phosphate group and glycerol 3-phosphate are different from those of eubacteria and eukaryotes. For example, the phosphate group is attached to the disaccharide α-glucopyranosyl-(1®2)-β-galactofuranose.
References
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Nelson, DL, Cox, MM 2017. Lehninger Principles of Biochemistry. W. H. Freeman, New York.
Sinnott, ML 2007. Carbohydrate chemistry and biochemistry structure and mechanism. Royal Society of Chemistry, Cambridge.
Stick, RV, Williams, SJ 2009. Carbohydrates: the essential molecules of life. Elsevier, Amsterdam.
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