Glycolysis

1) Balance

Glycolysis is a catabolic pathway in the cytoplasm that is found in almost all organisms – irrespective of whether they live aerobically or anaerobically. The balance of glycolysis is simple: glucose is broken down into two molecules of pyruvate, and in addition two molecules of ATP and two molecules and two of NADH+H⁺ are formed.

In the presence of oxygen, pyruvate and NADH+H⁺  reach the mitochondria, where they undergo further transformation(aerobic glycolysis). In anaerobic conditions, fermentation products such as lactate or ethanol have to be formed in the cytoplasm from pyruvate and NADH+ H⁺  , in order to regenerate NAD+ so that glycolysis can continue(anaerobic glycolysis). In the anaerobic state, glycolysis is the only means of obtaining ATP that animals cells have.

2) Reactions

Glyclolysis involves ten individual steps, including three isomerizations and four phosphate transfers. The only redox reaction takes place in step 6:

               I.      Glucose, which is taken up by animal cells from the blood and other sources, is first phosphorylated to glucose-6-phosphate, with ATP being consumed. The glucose 6-phosphate is not capable of leaving the cell.

            II.      In the next step, glucose 6-phosphate is isomerized into fructose 6-phosphate.

         III.      Using ATP again, another phosphorylation takes place, giving rise to fructose 1,6-biphosphate. Phosphofructokinase is the most important key enzyme in glycolysis.

         IV.      Fructose 1,6-biphosphate is broken down by aldolase into the C₃ compounds glyceraldehyde 3-phosphate(also known as glycerol 3-phosphate) and glycerone 3-phosphate(dihydroxyacetone 3-phosphate).

            V.      The latter two products are placed in fast equilibrium by triosephospinate isomerase.

         VI.      Glyceraldehyde 3-phosphate is now oxidized by glyceraldehyde-3-phosphate dehydrogenase, with NADH+H⁺ being formed. In this reaction, inorganic phosphate is taken up into the molecule(substrate-level phosphorylation), and 1,3-biphosphoglycerate is produced. This intermediate contains a mixed acid-anhydride bond, the phosphate part of which is at a high chemical potential.

      VII.      Catalyzed by phosphoglycerate kinase, this phosphate residue is transferred to ADP, producing 3-phosphoglycerate and ATP. The ATP balance is thus once again in the equilibrium.

   VIII.      As a result of shifting of the remaining phosphate residue within the molecule, the isomer 2-phosphoglycerate is formed.

         IX.      Elimination of water from 2-phosphoglycerate produces the phosphate ester of the enol form of pyruvate – phosphoenolpyruvate(PEP). This reaction also raises the second phosphate residue to a high potential.

            X.      In the last step, pyruvate kinase transfers this residue to ADP. The remaining enol pyruvate is immediately rearranged into pyruvate, which is much more stable. Along with step VII and the thiokinase reaction in the  tricarboxylic acid cycle, the pyruvate kinase reaction is one of the three reactions in animal metabolism that are able to produce ATP independently of the respiratory chain.

In glycolysis, two molecules of ATP are initially used for activation(I,III). Later, two ATPs are formed per C₃ fragment. Overall, therefore, there is a small net gain of 2 mol ATP per mol of glucose. 

3) Energy profile

The energy balance of metabolic pathways depends not only on the standard changes in enthalpy ΔG°, but also on concentrations of metabolites. Figure below shows the actual enthalpy changes ΔG for the individual steps of glycolysis in erythrocytes.

As spotted, only three reactions(I,III,X) are associated with large changes in free enthalpy. In this case, the equilibrium lies well on the side of the products. All of the other steps are freely reversible. The same steps are also followed – in the reverse direction – in gluconeogenesis, with same enzymes being activated as in glucose degradation. The non-reversible steps(I,III,X) are bypassed in glucose biosynthesis.

“Coloured atlas of biochemistry”, second edition; J. Koolman, K.H. Roehm