In this process each molecule of glucose (a hexose sugar) is broken down in stepwise biochemical reactions under enzymatic control into two molecules of pyruvic acid. All these reactions take place in cytoplasm. Starch or other stored carbohydrates are converted to glucose before their utilization in this process.
(1) Glucose molecule is phosphorylated in presence of ATP to form glucose-6- phosphate. The reaction is catalysed by enzyme hexokinase which requires a divalent Mg++ as cofactor. ATP is converted into ADP in this reaction.
hexokinase
Glucose + ATP → Glucose-6-phosphate + ADP
(1 mol)
Glucose + ATP → Glucose-6-phosphate + ADP
(1 mol)
(2) Glucose-6-phosphate (Robinson's ester) is isomenzed to form fructose-6-phosphate in presence of enzyme phosphohexo isomerase.
phosphohexo isomerase
Glucose-6-phosphate ⇌ fructose-6-phosphate
(3) Fructose-6-phosphate (Newberg's ester) is then phosphorylated by ATP to form fructose-1, 6-disphosphate in presence of enzyme phosphofructokinase. The ATP is convened to ADP in the reaction./
Phosphofructokinase
Fructose-6-phosphate + ATP → Fructose-1, 6-diphosphate + ADP
(4) Fructose-1, 6-disphosphate is then cleaved to two triose phosphates; dihydroxy acetone phosphate and 3-phosphoglyceraldehyde. The reaction is catalysed by enzyme aldolase. The two trioses are isomeric and they may isomerise to each other in presence of enzyme triosephosphate isomerase.
(5) 3-Phosphoglyceraldehyde is convened to 1, 3-disphosphoglyceraldehyde in presence of inorganic phosphate (H3PO4).
3-phosphoglyceraidehyde + H3PO4 ⇌ 1,3-diphosphoglyceraldehyde
(2 mol) (2 mol) (2 mol)
(6) 1,3-diphosphoglyceraldehyde is oxidized to form 1, 3-disphosphoglycerie acid in presence of enzyme triosephosphate dehydrogenate and coenzyme NAD+. NAD+ acts H+ acceptor and reduced to NADH + H in reaction:
1,3-diphosphoglyceraldehyde + 2NAD+ ⇌ 1, 3-diphosphoglyceric acid +2 NAD.2H
(2mol) (2mol)
(7) 1,3-diphosphoglyceric acid is converted into 3-phosphoglyceric acid in presence of enzyme-phosphoglyceric acid kinase. One molecule of ADP is phosphorylated to ATP in the reaction.
phosphoglyceric acid kinase
1,3-disphosphoglyceric acid + 2ADP → 3-Phosphoglyceric acid + 2ATP
(8) 3-Phosphoglyceric acid is transformed to 2- phosphoglyceric acid in presence of enzyme phosphoglyceryl mutase.
phosphoglyceryl mutase
3-Phosphoglyceric acid ⇌ 2-Phosphoglyceric acid
(9) The next reaction involves the dehydration of 2-phosphoglyceric acid to produce 2-phosphoenol pyruvic acid in presence of enzyme-enolase.
enolase
2-Phosphoglyceric acid ⇌ 2-Phosphoenol Pyruvic acid
(10) 2 moles of phosphoenol pyruvic acid reacts with 2 moles of ADP forming 2 moles of pyruvic acid and 2 ATP in presence of enzyme-pyruvate kinase.
pyruvatekinase
2-phosphoenolpyruvic acid + 2 ADP → 2 Pyruvic acid + 2 ATP
(2 mol) (2 mol)
Summary of Glycolysis
(i) In glycolysis, from one molecule of glucose, two molecules of pyruvic are formed.
(ii) In this, four molecules of ATP are formed (2 ATP at stage 3, step 7 + 2ATP at stage 4, step 10). Because two molecules of ATP are consumed in phosphorylation reactions (1 ATP at stage1, step 1 + 1 ATP at stage 1, step 3 = 2 ATP molecules), therefore, during glycolysis there is net gain of 2 ATP molecules (4 ATP-2ATP = 2 ATP).
(iii) Two molecules of NAD are reduced to two molecules of NADH2 (stage 3, step 6) which later on oxidised aerobically to yield six molecules of ATP (one NAD molecule after oxidation produces 3 molecules of ATP). Thus, the total gain of ATP molecules during glycolysis in presence of oxygen will be increased to eight instead of two.
(iv) Thus, the energy of glucose become stored partly in ATP molecules and partly in NADH2 molecules.
(v) Oxygen is not required during glycolysis.
(vi) In glycolysis, CO2 is also not produced.
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