Describe the process of glycolysis with the help of schematic representation.
Glycolysis consists of two major phases:
a. Preparatory phase and cleavage. b. Oxidative and payoff phase.
a. Preparatory phase and cleavage:
In this step, glucose molecule is activated by phosphorylation and then cleaved into two molecules of triose phosphates, namely 3-PGAL and DHAP. In the next step, only 3-PGAL participates and hence DHAP gets converted into 3-PGAL. Thus, 2 molecules of 3-PGAL are formed.
It includes the following reactions:
i. Phosphorylation I: A glucose molecule is phosphorylated to glucose 6-phosphate in presence of enzyme hexokinase. In this reaction, ATP supplies the phosphate and gets converted into ADP.
ii. Isomerisation: Glucose-6-phosphate is isomerised to fructose-6-phosphate in presence of enzyme phosphoglucoisomerase.
iii. Phosphorylation II: Fructose-6-phosphate is phosphorylated to fructose-1,6- diphosphate. Phosphate is supplied by ATP which gets converted to ADP. Reaction is catalyzed by enzyme phosphofructokinase.
iv. Cleavage: Fructose-1,6-diphosphate splits up into two inter-convertible compounds, i.e. Dihydroxy acetone phosphate (DHAP) and 3-phosphoglyceraldehyde (3-PGAL). Reaction occurs in presence of enzyme aldolase.
v. Isomerization : DHAP is isomerised into 3PGAL with the help of enzyme phosphotriose isomerase, thus two molecules of PGAL are obtained.
b. Oxidative and payoff phase:
In this phase, oxidation or removal of hydrogen takes place and then there is ATP generation.
i. Oxidative Phosphorylation: 3-PGAL undergoes oxidation and phosphorylation to form 1,3-diphosphoglyceric acid (1,3-DPGA). Hydrogen released combines with NAD to form NADH2. Phosphate is supplied for the reaction by phosphoric acid (H3PO4).
ii. ATP generation I / Dephosphorylation I: 1,3-diphosphoglyceric acid is dephosphorylated to 3-phosphoglyceric acid (3-PGA). Phosphate released combines with ADP to form ATP.
Reaction takes place in presence of enzyme phosphoglycero-kinase.
iii. Isomerisation: In presence of enzyme phosphoglycero-mutase, 3-Phosphoglyceric acid is isomerised to 2-phosphoglyceric acid.
iv. Dehydration: 2-Phosphoglyceric acid loses a water molecule to form phosphoenol pyruvic acid. The enzyme enolase catalyses the reaction.
v. ATP generation II / Dephosphorylation II: Phosphoenol pyruvic acid is dephosphorylated to pyruvic acid in presence of enzyme pyruvate kinase. Phosphate released combines with ADP to form ATP.