Food Chemistry and Fish in Nutrition

6.3.1.1. Glycolysis

Glucose is converted into two molecules of pyruvate, chemical energy in the form of ATP is produced, and NADH- reduced coenzymes are produced. This metabolic pathway takes place in almost all cells. All of the enzymes of the glycolysis pathway are found in the extramitochondrial soluble fraction of the cell, the cytosol. They catalyze the reactions under aerobic and anaerobic conditions.

 

The over all reactions of glycolytic reactions are presented in below.

Glycolysis

Anaerobic condition

Glucose → Pyruvate → Lactic acid

 

Aerobic condition

Pyruvate → CO₂ + H₂O

 

Glycolysis is a ten step process in which every step is enzyme catalyzed. Details of individual steps with in the glycolytic pathway are now considered.

Step1: Phosphorylation of Glucose

Glucose enters into the glycolytic pathway by phosphorylation to glucose 6- phosphate, accomplished by the enzyme hexokinase. However, in liver parenchyma cells and in pancreatic islet cells, this function is carried out by glucokinase. ATP is required as phosphate donor, and it reacts as the Mg-ATP complex.

The terminal high-energy phosphate of ATP is utilized, and ADP is produced. The reaction is accompanied by considerable loss of free energy as heat and therefore, under physiologic conditions, may be regarded as irreversible.

Hexokinase is inhibited in an allosteric manner by the product, glucose 6- phosphate. The function of glucokinase is to remove glucose from the blood following digestion and absorption. It is specific for glucose.

Step 2: Converstion of Glucose 6 Phosphate to Fructose 6-Phosphate

Glucose 6 phosphate is converted to fructose 6-phosphate by phosphoglucose isomerase, which involves an aldose-ketose isomerization.

Step 3: Phosphorlyation of Fructose 6-Phosphate to Fructose 1, 6-Bisphosphate

This reaction is catalyzed by the enzyme phosphofructokinase to produce fructose 1, 6-bisphosphate from Fructose 6 phosphate. The phosphofructokinase reaction is irreversible under physiologic conditions.

Step 4: Cleavage of Fructose 1,6-Bisphosphate

Fructose 1, 6-bisphosphate is split by aldolase (fructose1, 6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde -3-phosphate and dihydroxyacetone phosphate.

Step 5: Inter conversion of triose phosphates

Glyceraldehyde-3-phosphate and dihydroxy acetone phosphate are interconverted by the enzyme phosphotriose isomerase.

At this stage 2 molecules glyceraldehyde 3-phosphate are formed. Dihydroxyacetone phosphate is also formed from glycerol of fat, which is phosphorylated to glycerol 3 phosphate and then to dihydroxy acetone phosphate. 

Step 6: Oxidation of Glyceraldehyde 3-Phosphate to 1, 3 Bisphosphoglycerate

Glyceraldehyde-3-phosphate is converted to 1,3 bisphosphglycerate by glyceraldehyde 3-phosphate dehydrogenase using NAD⁺ as the coenzyme. Finally, by phosphorolysis, inorganic phosphate (pi) is added, forming1, 3 bisphosphoglycerate, and the free enzyme.

Energy released during the oxidation is conserved by the formation of a high-energy sulfur group that becomes, after phosphorolysis, a high- energy phosphate group in position 1 of 1, 3 bisphosphoglycerate.

Step 7: Transfer of phosphate group from 1, 3 bisphosphoglycerate

1, 3-bisphosphoglycerate is oxidized to 3- phosphoglycerate by phosphoglycerate kinase. This high- energy phosphate is captured as ATP in a further reaction with ADP.

Since two molecules of triose phosphate are formed per molecule of glucose undergoing glycolysis, two molecules of ATP are generated at this stage per molecule of glucose.

Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

In the next reaction 3-phosphoglycerate is converted to 2-phosphoglycerate by the enzyme phosphoglycerate mutase.

Step 9: Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate

The subsequent step is catalyzed by enolase that promotes the reversible removal of a water molecule from 2-Phosphoglycerate to form phosphoenolpyruvate.

Step 10: Conversion of Phosphoenol pyruvate to pyruvate

The high-energy phosphate of phosphoenol pyruvate is transferred to ADP by the enzyme pyruvate kinase to generate, at this stage, two molecules of ATP per molecule of glucose oxidized and enolpyruvate is formed. Enolpyruvate formed is converted spontaneously to the keto form pyruvate. This is an irreversible step.