Glycolysis – Glucose Catabolic Pathway
Glycolysis literally means “Splitting Sugars“. In glycolysis, glucose (a six carbon sugar) is split into two molecules of a three-carbon sugar. Glycolysis yields two molecules of ATP (free energy containing molecule), two molecules of Pyruvic acid and two “high energy” electron carrying molecules of NADH.
Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called fermentation.
Glucose as fuel:
The major function of carbohydrates in metabolism is as a fuel to be oxidized and provide energy for other metabolic processes. The carbohydrate is utilized by cells mainly as glucose. The three principal monosaccharide resulting from digestive processes are glucose, fructose and galactose. Much of the glucose is derived from starch which accounts for ever half of the fuel in the diets of most humans.
Glucose is also produced from other dietary components by the liver and, to a lesser extent, by the kidneys. Fructose results on large intake of sucrose while Galactose is produced when lactose is the principal carbohydrate of the diet.
Both fructose and Galactose are easily converted to glucose by the liver. It is thus apparent that glucose is the major fuel of most organisms and that it can be quickly metabolized from glycogen stores when there arises a sudden need for energy.
Glucose transporters are Glucose transporting proteins. Usually glucose is transported into the cells by sodium-glucose pump. In addition to symport pump, most of the cells have another type of transport proteins called Glucose Transporters (GLUT).
So far, six types of GLUT are identified (GLUT 1-5 and 7). Among these, GLUT-4 is insulin sensitive and it is located in cytoplasmic vesicles. It is present in large numbers in muscle fibers and adipose cells.
When insulin-receptor complex is formed in the membrane of such cells, the vesicles containing GLUT-4 are attracted towards the membrane and GLUT-4 is released into the membrane. Now, GLUT-4 starts transporting the glucose molecules from ECF into the cell. The advantage of GLUT-4 is that it transports glucose at a faster rate.
Glycolysis – Glucose Catabolic Pathway:
Glycolysis is divided into two phases
- Preparative phase (Step 1 to 5)
- Pay off Phase (Step 6 to 10)
This is the first phase of Glycolysis. In this Glucose is converted into Glyceraldehyde-3-Phosphate and DHAP (Dihydroxy Acetone Phosphate). This phase contains 5 Steps.
Step 1: Phosphorylation :
The enzyme Hexokinase phosphorylates (adds a phosphate group to) glucose in the cell’s cytoplasm. In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate.
Glucose (C6H12O6) + hexokinase + ATP → ADP + Glucose 6-phosphate (C6H11O6P1)
Step 2: Isomerization :
The enzyme phosphoglucoisomerase converts glucose 6-phosphate into its isomer fructose 6-phosphate. Isomers have the same molecular formula, but the atoms of each molecule are arranged differently.
Glucose 6-phosphate (C6H11O6P1) + Phosphoglucoisomerase
→ Fructose 6-phosphate (C6H11O6P1)
Step 3: Phosphorylation:
The enzyme phosphofructokinase uses another ATP molecule to transfer a phosphate group to Fructose-6-phosphate to form fructose-1,6-bisphosphate.
Fructose 6-phosphate (C6H11O6P1) + Phosphofructokinase + ATP
→ ADP + Fructose 1, 6-bisphosphate (C6H10O6P2)
Step 4: Condensation:
The enzyme aldolase splits Fructose-1,6-bisphosphate into two sugars that are isomers of each other. These two sugars are Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3- phosphate.
Fructose 1, 6-bisphosphate (C6H10O6P2) + aldolase →
Dihydroxyacetone phosphate (C3H5O3P1) + Glyceraldehyde phosphate (C3H5O3P1)
Step 5 :Isomerization:
The enzyme triose phosphate isomerase rapidly inter-converts the molecules dihydroxyacetone phosphate (DHAP) into glyceraldehyde-3-phosphate. Glyceraldehyde phosphate is removed as soon as it is formed to be used in the next step of glycolysis.
Dihydroxyacetone phosphate (C3H5O3P1)
→ Glyceraldehyde phosphate (C3H5O3P1)
Net result for steps 4 and 5:
Fructose 1, 6-bisphosphate (C6H10O6P2)
↔ 2 molecules of Glyceraldehyde phosphate (C3H5O3P1)
This is the second phase of Glycolysis. In this phase Glyceraldehyde-3-Phosphate is converted into Pyruvate molecules. This phase have 5 steps.
Step 6: Dehydrogenation:
The enzyme triose phosphate dehydrogenase serves two functions in this step. First the enzyme transfers a hydrogen (H–) from Glyceraldehyde-3-phosphate to the oxidizing agent Nicotinamide Adenine Dinucleotide (NAD+) to form NADH. Next Triose phosphate dehydrogenase adds a phosphate (P) from the cytosol to the oxidized glyceraldehyde-3-phosphate to form 1, 3-bisphosphoglycerate. This occurs for both molecules of glyceraldehyde-3-phosphate produced in step 5.
A. Triose phosphate dehydrogenase + 2 H– + 2 NAD+ → 2 NADH + 2 H+
B. Triose phosphate dehydrogenase + 2 P + 2 glyceraldehyde phosphate (C3H5O3P1)
→ 2 molecules of 1,3-bisphosphoglycerate (C3H4O4P2)
Step 7: Substrate level Phosphorylation:
The enzyme phosphoglycerokinase transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP to form ATP. This happens for each molecule of 1,3-bisphosphoglycerate. The process yields two 3-phosphoglycerate molecules and two ATP molecules.
2 molecules of 1,3-bisphoshoglycerate (C3H4O4P2) + phosphoglycerokinase + 2 ADP
→ 2 molecules of 3-phosphoglycerate (C3H5O4P1) + 2 ATP
Step 8: Dehydration (Intramolecular rearrangement):
The enzyme phosphoglyceromutase relocates the Phosphate from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate.
2 molecules of 3-Phosphoglycerate (C3H5O4P1) + phosphoglyceromutase
→ 2 molecules of 2-Phosphoglycerate (C3H5O4P1)
Step 9 : Enolyzation :
The enzyme Enolase removes a molecule of water from 2-phosphoglycerate to form Phospho Enol Pyruvic acid (PEP). This happens for each molecule of 2-phosphoglycerate.
2 molecules of 2-Phosphoglycerate (C3H5O4P1) + enolase
→ 2 molecules of phosphoenolpyruvic acid (PEP) (C3H3O3P1)
Step 10: Substrate level Phosphorylation:
The enzyme Pyruvate kinase (PK) transfers a Phosphate from PEP to ADP to form Pyruvic acid and ATP. This happens for each molecule of PEP. This reaction yields 2 molecules of Pyruvic acid and 2 ATP molecules. Pyruvate Kinase is a Potassium Containing Enzyme
2 molecules of PEP (C3H3O3P1) + pyruvate kinase + 2 ADP
→ 2 molecules of pyruvic acid (C3H4O3) + 2 ATP
Glucose + 2ATP +4ADP+ 2 NAD++ H3PO4 –> 2 Pyruvate + 2 ADP + 4 ATP + 2 NADH + H+ + H2O
Net ATP Calculation:[table id=1 /]
Regulation of Glycolysis:
- Insulin activates the key glycolytic enzymes – Gluco Kinase (GK), Phospho Fructo Kinase (PFK) and Pyruvate Kinase (PK)
- Glucocorticoids inhibits Glycolysis.
In summary, a single glucose molecule in glycolysis produces a total of 2 molecules of Pyruvic acid, 2 molecules of ATP, 2 molecules of NADH and 2 molecules of water.
- Citric acid cycle : Central metabolic cycle and its Significance
- Glycogenolysis : How Glycogen is Utilizing in Animals
Although 2 ATP molecules are used in steps 1-3, 2 ATP molecules are generated in step 7 and 2 more in step 10. This gives a total of 4 ATP molecules produced. If you subtract the 2 ATP molecules used in steps 1-3 from the 4 generated at the end of step 10, you end up with a net total of 2 ATP molecules produced.