- characteristics
- Functions in the body
- Trioses as intermediates in glycolysis, gluconeogenesis, and the pentose phosphate pathway
- The Trioses and the Calvin Cycle
- Trioses and lipids of biological membranes and adipocytes
- Trioses and membranes of archaebacteria
- References
The triose are monosaccharides three carbon whose chemical formula is empirical C 3 H 6 O 6. There are two trioses: glyceraldehyde (an aldose) and dihydroxyacetone (a ketosis). 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 reserve or structural polysaccharides.
Source: Wesalius
Trioses participate in the biosynthesis of lipids that are part of cell membranes and adipocytes.
characteristics
Aldose glyceraldehyde 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 the plane of polarized light to the right (+) and has a rotation D, at 25 ° C, of + 8.7 °, while L-glyceraldehyde rotates the plane of polarized light to the left (-) and has a rotation D, at 25 ° C, of -8.7 °.
The chiral carbon in glyceraldehyde is carbon 2 (C-2), which is a secondary alcohol. The Fischer projection represents 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 has no enantiomeric forms. The addition of a hydroxymethylene group (-CHOH) to glyceraldehyde or dihydroxyacetone allows the creation of a new chiral center. Consequently, sugar is tetrose because it has four carbons.
The addition of a -CHOH group to tetrose creates a new chiral center. The sugar formed is a pentose. You can keep adding -CHOH groups until you reach a maximum of ten carbons.
Functions in the body
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 route 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 the phosphoester fructose 1,6-bisphosphate (F1,6BP) into two triose phosphate, glyceraldehyde 3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP).
Gluconeogenesis is the biosynthesis of glucose from pyruvate and other intermediates. It employs 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 for glucose-6-phosphate and another for the formation of NADPH and ribose-5-phosphate. In the second phase, ribose 5-phosphate is converted into glycolysis intermediates, F1,6BP and GA3P.
The 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 bonding of CO 2 to pentose ribulose 1,5-bisphosphate and breaks the unstable six-carbon intermediate into two molecules of three carbon atoms: 3-phosphoglycerate.
Through enzymatic reactions that include phosphorylation and reduction of 3-phosphoglycerate, using ATP and NADP, GA3P is produced. This metabolite is converted to fructose 1,6-bisphosphate (F1,6BP) by a metabolic pathway similar to gluconeogenesis.
Through the action of a phosphatase, F1,6BP is converted to fructose-6-phosphate. Then a phosphohexose isomerase 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 trioses phosphate can be interconverted by a reaction catalyzed by triose phosphate isomerase, which maintains 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 phospholipid skeleton that is a structural part of biological membranes.
Glycerol is prochiral, it lacks asymmetric carbons, but when one of its two primary alcohols forms a phosphoester, it can be correctly called 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 the starting metabolite for the biosynthesis of triacylglycerides, which are common in adipocytes.
Trioses and membranes of archaebacteria
Similar to eubacteria and eukaryotes, glycerol 3-phosphate is formed from triose phosphate (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, while 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 of 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
- Cui, SW 2005. Food carbohydrates: chemistry, physical properties, and applications. CRC Press, Boca Raton.
- de Cock, P., Mäkinen, K, Honkala, E., Saag, M., Kennepohl, E., Eapen, A. 2016. Erythritol is more effective than xylitol and sorbitol in managing oral health endpoints. International Journal of Dentistry.
- Nelson, DL, Cox, MM 2017. Lehninger Principles of Biochemistry. WH 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.
- Voet, D., Voet, JG, Pratt, CW 2008. Fundamentals of biochemistry - life at the molecular level. Wiley, Hoboken.