Glycolysis

 Glycolysis (Gk. glycos – sugar, lysis – splitting,):

It is also called ЕМР pathway because it was discovered by three German scientists— Gustav Embden, Otto Meyerhof and J. Pamas in 1930.

Glycolysis is the process of partial oxidation of glucose or similar hexose sugar into two molecules of pyruvic acid through a series of ten enzyme mediated reactions releasing some energy (as ATP) and reducing power (as NADH2). It occurs in cytosol or cytoplasm. Glycolysis is common to both aerobic and anaerobic modes of respiration.

It is the first stage of breakdown of glucose in aerobic respiration and the only step in glucose breakdown in anaerobic respiration. Glycolysis has two phases, preparatory and pay off. In the preparatory phase glucose is broken down to glycerealdehyde 3-phosphate. In the pay off phase the latter is changed into pyruvate pro­ducing NADH and ATP.

Preparatory Phase (Energy Spending Phase):

1. Phosphorylation of Glucose:

Respiratory substrate (glucose or fructose) is formed by hydrolysis of starch or sucrose. Hydrolysis of starch occurs with the help of enzymes amylase and maltase. It yields glucose. Sucrose is hydrolysed by enzyme invertase to form glucose and fructose. Glucose is phosphorylated to glucose-6-phosphate by ATP in the presence of enzyme hexokinase (Meyerhof, 1927) or glucokinase (e.g., liver) and Mg2+.

2. Synthesis of Fructose 6-phosphate:

Glucose-6-phosphate is changed to its isomer fructose-6-phosphate with the help of enzyme phosphohexose isomerase.

Fructose 6-phosphate can also be produced directly by phosphorylation of fructose with the help of enzyme fructokinase.

3. Formation of Fructose 1, 6-Bi-phosphate:

Fructose 6-phosphate is further phosphorylated by means of ATP in presence of enzyme phosphofructo-kinase and Mg2+. The product is fructose 1: 6 bi-phosphate.

In plants a pyrophosphate (ppi) dependent phosphofructokinase has been discovered which carries out conversion of fructose 6-phosphate into fructose 1, 6- bi-phosphate.

4. Splitting:

Fructose 1: 6 bi-phosphate Aldolase → splits up enzymatically to form one molecule each of 3-carbon compounds, glyceraldehyde 3- phosphate (= GAP or 3-phosphoglyceral- dehyde =PGAL) and dihydroxy acetone 3-phosphate (DiHAP).

5. Isomerisation of DiHAP:

Dehydroxyacetone 3-phosphate is isomerised to glycer­aldehyde 3-phosphate with the help of enzyme triose phosphate isomerase.

Pay Off Phase (Energy Conserving Phase):

6. Oxidation and Phosphorylation:

In the presence of enzyme glyceraldehyde 3- phosphate dehydrogenase, the glyceraldehyde 3-phosphate is oxidised through removal of hydrogen and addition of phosphate from inorganic source to form 1: 3 biphosphoglycerate. NAD+ is hydrogen acceptor. It produces NADH.

7. Substrate Level Phosphorylation (Formation of ATP):

One of the two phosphates of biphosphoglycerate is linked by high energy bond. It can synthesise ATP and form 3- phosphoglycerate. The enzyme is phosphoglycerate kinase. The direct synthesis of ATP from metabolites is called substrate level phosphorylation.

8. Isomerization:

3-phosphoglycerate is changed to its isomer 2-phosphoglycerate by enzyme phosphoglyceromutase.

9. Dehydration:

Through the agency of enzyme enolase, 2-phosphoglycerate is con­verted to phosphoenol pyruvate (PEP). A molecule of water is removed in the process. Mg2+ is required.

10. Formation of Pyruvate:

During formation of phosphoenol pyruvate, the phosphate radical picks up energy. It helps in the production of ATP by substrate level phosphory­lation. The enzyme is pyruvate kinase. It produces pyruvate from phosphoenol pyruvate.

Net Products of Glycolysis:

In glycolysis two molecules of ATP are consumed during double phosphorylation of glucose to form fructose 1: 6 biphosphate.

In return four mol­ecules of ATP are produced by substrate level phosphorylation (conversion of 1: 3 biphosphoglycerate to 3-phosphoglycerate and phosphenol pyruvate to pyruvate). Two molecules of NADH2 are formed at the time of oxidation of glyceraldehyde 3-phosphate to 1: 3 biphosphoglycerate.

The net reaction is as follows:

Each NADH is equivalent to 3 ATP, so that the net gain in glycolysis is 8 ATP. Intermediates of glycolysis are used for synthesis of important bio-chemicals.

For ex­ample, phosphoenol pyruvate yields shikimic acid which is used in synthesis of amino acids, tryptophan, tyrosine and phenylalanine. Tryptophan is raw material for IAA synthesis. The amino acids are employed for synthesis of proteins, alkaloids, flavonoids and lignin. Similarly, pyruvic acid forms amino acid alanine.