- Glucose is the primary component in the metabolism of plants, animals, and many microorganisms due to the rich potential energy and good fuel.
- Glucose stores as a starch or glycogen, when energy demands these are released as glucose to produce ATP either aerobically or anaerobically.
- In animal and plants, there are four major pathway of glucose utilization such as; the synthesis of complex polysaccharides destined for the extracellular space; stored in cells (as a polysaccharide or as sucrose); oxidized to a three-carbon compound (pyruvate) via glycolysis to provide ATP and metabolic intermediates, or oxidized via the pentose phosphate (phosphogluconate) pathway to yield ribose 5-phosphate for nucleic acid synthesis and NADPH for reductive biosynthetic processes.
- The photosynthetic organisms get their glucose by reducing atmospheric CO2 to triose and then triose to glucose.
- The non-photosynthetic organisms get their glucose from simpler three or four carbon precursors via gluconeogenesis. It is reverse glycolysis with the helps of different glycolytic enzymes.
Glycolysis Pathway Definition
- Glycolysis is a type of metabolic pathway where one molecule of glucose degraded into 2 molecules of 3 carbon-containing pyruvate molecules through a series of enzyme-catalyzed reactions.
- Glycolysis pathway also known as Embden-Meyerhof-Parnas Pathway.
- Glycolysis is a Greek Word where Glykys means Sweet and Lysis means Splitting.
- The glycolysis is a 10 step process, where the first 5 steps is known as Preparatory phase and the last 5 steps known as payoff phase.
- In Preparatory phase Glucose is converted into Glyceraldehyde 3 phosphate and Dihydroxyacetone phosphate. In the payoff phase, the Glyceraldehyde 3 phosphate is converted into 3 carbon-containing pyruvate.
- In Preparatory phase 2 molecules of ATP are used up while in payoff phase 4 molecules of ATP are generated.
- Phosphorylation of Glucose: In this step, D-Glucose is phosphorylated with the help of the enzyme hexokinase. The hexokinase enzyme is a class of transferase enzyme. This enzyme transfers the phosphoryl group from ATP to the OH group of C6 carbon in Glucose and form Glucose 6 phosphate. In this step, Mg2+ is required.
- Conversation of Glucose 6 Phosphate to Fructose 6 Phosphate: In this step, the enzyme Phosphohexoisomerase catalyze the conversation reaction of Glucose 6 Phosphate to Fructose 6 Phosphate. This enzyme triggers the isomerization of Glucose 6 phosphate. In this step, Mg2+ is required.
- Phosphorylation of Fructose 6 Phosphate: In this step, enzyme phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation reaction of Fructose 6 Phosphate and forms Fructose 1,6 Bisphosphate.
- Cleavage of Fructose 1,6 Bisphosphate: Enzyme aldolase or fructose 1,6-bisphosphate aldolase cleave the Fructose 1,6 Bisphosphate into two triose phosphates such as Glyceraldehyde 3 phosphate (aldose) and Dihydroxyacetone phosphate (Ketose). In this step, Mg2+ is required.
- Interconversion of triose phosphates: The enzyme triose phosphate isomerase converts the Dihydroxyacetone phosphate into Glyceraldehyde 3 phosphate.
- Oxidation of Glyceraldehyde 3 phosphate: The enzyme Glyceraldehyde 3 phosphate dehydrogenase converts the Glyceraldehyde 3 phosphate into 1,3-Bisphosphoglycerate with the help of 2 molecule of Pi and 2 molecules of NAD+.
- Conversation of 1,3-Bisphosphoglycerate to 3 phosphoglycerate: The enzyme phosphoglycerate kinase transfer one molecule of a phosphoryl group from 1,3-Bisphosphoglycerate to ADP and forms 3 phosphoglycerate and ATP. In this step, Mg2+ is required.
- Conversation of 3 phosphoglycerate to 2 phosphoglycerate: The enzyme phosphoglycerate mutase catalyzes the reversible shift of phosphoryl group between C-2 and C-3 of glycerate and forms 2 phosphoglycerate from 3 phosphoglycerate. In this step, Mg2+ is required.
- Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate: The enzyme enolase removes one molecule of H2O from 2-Phosphoglycerate and forms Phosphoenolpyruvate.
- Transfer of the Phosphoryl Group from Phosphoenolpyruvate to ADP: The enzyme pyruvate kinase transfers the phosphoryl group from Phosphoenolpyruvate to ADP to yield pyruvate and ATP. In this step K+ and either Mg2+ or Mn2+ si required.
The overall equation of glycolysis is;
Glucose + 2NADP+ + 2Pi = 2 Pyruvate + 2ATP + 2NADPH + 2H+ + 2H2O
- When more than one enzyme catalyzes the same reaction but is encoded by different genes is known as isozymes.
- In glycolysis, 2 molecules of ATP and 2 Molecules of NADPH is generated.