GLYCERALDEHYDE 3-PHOSPHATE (G3P): STRUCTURE, FUNCTIONS - BIOLOGY - 2025

The glyceraldehyde 3-phosphate (GAP) is a metabolite of the glycolysis (the name comes from the Greek; glicos = sweet or sugar; lysis = rupture), which is a metabolic pathway that converts glucose molecule into two molecules of pyruvate to produce energy in the form of adenosine triphosphate (ATP).

In cells, glyceraldehyde 3-phosphate connects glycolysis with gluconeogenesis and the pentose phosphate pathway. In photosynthetic organisms, glyceraldehyde 3-phosphate, coming from the fixation of carbon dioxide, is used for the biosynthesis of sugars. In the liver, fructose metabolism produces GAP, which is incorporated into glycolysis.

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Structure

Glyceraldehyde 3-phosphate is a phosphorylated sugar that has three carbons. Its empirical formula is C ₃ H ₇ O ₆ P. The aldehyde group (-CHO) is carbon 1 (C-1), the hydroxymethylene group (-CHOH) is carbon 2 (C-2) and the hydroxymethyl group (-CH ₂ OH) is carbon 3 (C3). The latter forms a bond with the phosphate group (phosphoester bond).

The configuration of glyceraldehyde 3-phosphate at chiral C-2 is D. By convention, with respect to chiral carbon, in the Fischer projection the aldehyde group is represented upward, the hydroxymethylphosphate group downward, the hydroxyl group downward. right and the hydrogen atom to the left.

characteristics

Glyceraldehyde 3-phosphate has a molecular mass of 170.06 g / mol. The standard Gibbs free energy change (ΔGº) for any reaction must be calculated by adding the variation of the free energy of the products, and subtracting the sum of the variation of the free energy of the reactants.

In this way, the free energy variation (ΔGº) of formation of glyceraldehyde 3-phosphate is determined, which is -1,285 KJ × mol ^-1. By convention, in the standard state of 25ºC and 1 atm, the free energy of the pure elements is zero.

Features

Glycolysis and gluconeogenesis

Glycolysis is present in all cells. It is divided into two phases: 1) phase of energy investment and synthesis of metabolites with high phosphate group transfer potential, such as glyceraldehyde 3-phosphate (GAP); 2) ATP synthesis step from molecules with high phosphate group transfer potential.

Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are formed from fructose 1,6-bisphosphate, a reaction catalyzed by the enzyme aldolase. Glyceraldehyde 3-phosphate is converted to 1,3-bisphosphoglycerate (1,3BPG), through a reaction catalyzed by the enzyme GAP dehydrogenase.

GAP dehydrogenase catalyzes the oxidation of the aldehyde's carbon atom and transfers a phosphate group. Thus, a mixed anhydride (1,3BPG) is formed in which the acyl group and the phosphorus atom are prone to the nucleophilic attack reaction.

Next, in a reaction catalyzed by 3-phosphoglycerate kinase, 1,3BPG transfers the phosphate group from carbon 1 to ADP, forming ATP.

Because the reactions catalyzed by aldolase, GAP dehydrogenase and 3-phosphoglycerate kinase are in equilibrium (ΔGº ~ 0), they are reversible, thus being part of the gluconeogenesis pathway (or the new synthesis of glucose).

The pentose phosphate pathway and the Calvin cycle

In the pentose phosphate pathway, glyceraldehyde 3-phosphate (GAP) and fructose 6-phosphate (F6P) are formed by cutting reactions and formation of CC bonds, from pentoses, xylulose 5-phosphate and ribose 5 -phosphate.

Glyceraldehyde 3-phosphate can follow the gluconeogenesis pathway and form glucose 6-phosphate, which continues the pentose phosphate pathway. Glucose can be completely oxidized to produce six CO ₂ molecules through the oxidative step of the pentose phosphate pathway.

In the Calvin cycle, CO ₂ is fixed as 3-phosphoglycerate, in a reaction catalyzed by ribulose bisphosphate carboxylase. The 3-phosphoglycerate is then reduced by NADH through the action of an enzyme called GAP dehydrogenase.

2 GAP molecules are needed for the biosynthesis of a hexose, such as glucose, which is used for the biosynthesis of starch or cellulose in plants.

Fructose metabolism

The fructokinase enzyme catalyzes the phosphorylation of fructose by ATP at C-1, forming fructose 1-phosphate. Aldolase A, found in muscle, is specific for fructose 1,6-bisphosphate as a substrate. Aldolase B is found in the liver and is specific for fructose 1-phosphate as a substrate.

Aldolase B catalyzes the aldol breakdown of fructose 1-phosphate and produces dihydroxyacetone phosphate and glyceraldehyde. Glyceraldehyde kinase catalyzes the phosphorylation of glyceraldehyde by ATP, forming a glycolytic intermediate, glyceraldehyde 3-phosphate (GAP).

In a different way, glyceraldehyde is transformed into glycerol by an alcohol dehydrogenase that uses NADH as an electron donor substrate. Glycerol kinase then phosphorylates glycerol through ATP, forming glycerol phosphate. The latter metabolite is reoxidized, forming dihydroxyacetone phosphate (DHAP) and NADH.

DHAP is converted to GAP by the enzyme triose phosphate isomerase. In this way, fructose is converted into metabolites of glycolysis. However, fructose given intravenously can cause serious damage, consisting of drastic depletion of intracellular phosphate and ATP. Lactic acidosis even occurs.

The damage of fructose is due to the fact that it does not have the set points that glucose catabolism normally has. First, fructose enters the muscles through GLUT5, which is independent of insulin.

Second, fructose is directly converted to GAP and thus bypasses the regulation of the enzyme phosphofruct kinase (PFK) at the beginning of glycolysis.

Via Entner-Doudoroff

Glycolysis is the universal pathway for glucose catabolism. However, some bacteria alternatively use the Entner-Doudoroff pathway. This pathway involves six steps catalyzed by enzymes, in which glucose is transformed into GAP and pyruvate, which are two end products of this pathway.

GAP and pyruvate are transformed into ethanol by alcoholic fermentation reactions.

References

1. Berg, JM, Tymoczco, JL, Stryer, L. 2015. Biochemistry. A short course. WH Freeman, New York.
2. Miesfeld, RL, McEvoy, MM 2017. Biochemistry. WW Norton, New York.
3. Nelson, DL, Cox, MM 2017. Lehninger principles of biochemistry. WH Freeman, New York.
4. Salway JG 2004. Metabolism at a glance. Blackwell, Malden.
5. Voet, D., Voet, JG, Pratt, CW 2008. Fundamentals of biochemistry: life at the molecular level. Wiley, Hoboken.