Glycolysis- Occurrence, Stages, Regulations, Significance

Introduction:

Glycolysis is a cytoplasmic pathway of both prokaryotic and eukaryotic cells that breaks down glucose into two three-carbon compounds and generates energy. It is also named as Embden–Meyerhof–Parnas (EMP) pathway. Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism.  The process does not use oxygen and is therefore anaerobic (processes that use oxygen are called aerobic).

Occurrence:

Glycolysis takes place in all cells of the body. The enzymes of this pathway are present in the cytosomal fraction of the cell. Glycolysis occurs in the absence of oxygen (anaerobic) or in the presence of oxygen (aerobic). Lactate is the end product under anaerobic condition. In the aerobic condition, pyruvate is formed, which is then oxidized to CO2 and H2O. Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria, e.g., erythrocytes, cornea, lens etc. Glycolysis is very essential for brain which is dependent on glucose for energy.

Three fates of pyruvate:

• Aerobic conditions (conversion to acetyl CoA (pyruvate dehydrogenase) for use in TCA cycle and oxidative phosphorylation for ATP production)

• Anaerobic conditions
• (Lactate (animal muscles)
• Ethanol (yeast)

Source: sachabiochem0001

Different Stages of Glycolysis:

Stage 1: Investment stage (Preparatory phase)

Stage 2: Splitting stage

Stage 3: Payoff phase (Harvesting stage)

Preparatory phase

In preparatory phase glucose molecule is activated for breakdown and energy is invested in the process of phosphorylation of glucose.

The first three reactions constitute the preparatory phase.

Step I: Phosphorylation of glucose

Glucose is phosphorylated at –OH group of C6 in which one molecule ATP is consumed. The reaction is catalysed by the enzyme Hexokinase in the presence of Mg++ ion.

Step II: Isomerization of glucose-6 phosphate to fructose-6- Phosphate

Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase. This reaction is catalysed by the enzyme phosphoglucose isomerase.

Step III: Phosphorylation of fructose-6-phosphate

This reaction is catalysed by Phospho-fructo-kinase (PFK) in the presence of Magnesium ion, in which fructose-6-phosphate is converted into fructose-1,6-bisphosphate. One molecule of ATP is consumed.

Splitting Phase

Step IV: Cleavage of Fructose-1,6-bisphosphate

The enzyme Aldolase (fructose-1,6-diphosphate aldolase) cleave fructose-1,6-bisphosphate to yield two molecule glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate.

Source: Lehninger Principles of Biochemistr

Step V: Conversion of dihydroxy-acetone phosphate to glyceraldehyde-3-phosphate

Source: Lehninger Principles of Biochemistry

Payoff phase

In payoff phase oxidation of glucose releases energy in the form of ATP and NADH. The remaining five reactions constitutes payoff phase

Step VI: Oxidation of glyceraldehyde-3-phosphate:

The glyceraldehyde-3-phosphate is oxidized into 1,3-bisphospho-glycerate in the presence of enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). In this reaction one molecule of NADH is released.

Source: Lehninger Principles of Biochemistry

Step VII: Transfer of phosphoryl group from 1,3-bisphosphoglycerate to ADP

The enzyme phosphoglycerate kinase (PGK) transfer phosphoryl group from 1,3 bisphosphate glycerate to ADP forming ATP and 3-phospholycerate. This reaction is an example of substrate level phosphorylation in which phosphoryl group is transfer from substrate ie 1,3-bisphosphoglycerate to ADP to form ATP.

Source: Lehninger Principles of Biochemistry

Step VIII: Conversion of 3-phosphoglycerate to 2-phoshoglycerate

The enzyme phosphoglycerate mutase catalyses reversible shift of phosphoryl group between C2 and C3 of phosphoglycerate. Mg++ is essential for this reaction.

Step IX: Dehydration of 2-phosphoglycerate (Removal of H2O from 2-phosphoglycerate)

Enolase promote reversible removal of a molecule of water from 2-phosphoglycerate forming Phosphoenolpyruvate (PEP).

Step X: Transfer of phosphoryl group from PEP to ADP

This reaction is catalyzed by the enzyme pyruvate kinase in the presence of  K+ and Mg++ or Mn++ions. This is also a substrate level phosphorylation in which phosphoryl group is transferred from PEP to ADP forming ATP and Pyruvate. In this substrate level phosphorylation, the product pyruvate first appears in its enol form which then tautomerize rapidly and non-enzymatically to its keto form.

Source: Lehninger Principles of Biochemistry

Regulation of glycolysis:

The reaction catalyzed by Phosphofructose kinase is the rate limiting step or control point of glycolysis. However, glycolysis is regulated by two mechanisms.

Allosteric regulation:

ATP and citrate are allosteric inhibitor of phosphfructo kinase. Therefore, glycolysis stops in cell having large amount of ATP and citrate (High energy condition). AMP and ADP are allosteric activator and they get accumulated in cell when energy content is depleted.

Reciprocal regulation:

Fructose-2,6 bisphosphate is potent activator of phosphofructose kinase while Fructose-1,6-bisphosphate is inhibitor of phosphofructose kinase. Increased concentration of fructose-1,6-bisphosphate favours formation of glucose from pyruvate (gluconeogenesis).

Fig: Steps of Glycolysis

Significance of glycolysis:

• Glycolysis is the only pathway that is taking place in all the cells of the body.
• Glycolysis is the only source of energy in erythrocytes.
• In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.
• The glycolytic pathway may be considered as the preliminary step before complete oxidation.
• The glycolytic pathway provides carbon skeletons for synthesis of non-essential amino acids as well as glycerol part of fat.
• Most of the reactions of the glycolytic pathway are reversible, which are also used for gluconeogenesis.

Glycolysis in disease

Glycolytic pathway defects are autosomal recessive red blood cell metabolic disorders that cause hemolytic anemia. The glycolytic pathway is one of the body’s important metabolic pathways. It involves a sequence of enzymatic reactions that break down glucose (glycolysis) into pyruvate, creating the energy sources adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH). Various inherited defects in enzymes of the pathway may occur.

The most common defect is

• Pyruvate kinase deficiency

Other defects that cause hemolytic anemia include deficiencies of

• Erythrocyte hexokinase
• Glucose phosphate isomerase
• Phosphofructokinase

Hemolytic anemia occurs only in persons who are homozygous for all of these pathway abnormalities. Hemolysis’ specific mechanism is uncertain. Jaundice and splenomegaly are two symptoms that are related to the degree of anemia. Small numbers of irregularly shaped cells (echinocytes) may be present in the absence of spherocytes.

References:

• Lehninger, A.L., Nelson, D.L., Cox, M.M. and Cox, M.M., 2005. Lehninger principles of biochemistry. Macmillan.
• Chandel, N.S., 2021. Glycolysis. Cold Spring Harbor Perspectives in Biology, 13(5), p.a040535.
• Satyanarayana, U., 2021. Biochemistry, 6e-E-book. Elsevier Health Sciences.
• Voet, D. and Voet, J.G., 2010. Biochemistry. John Wiley & Sons.
• Voet, D., Voet, J.G. and Pratt, C.W., 2002. Fundamentals of biochemistry (No. QD415 V63). New York: Wiley.
• Kumari, A., 2017. Sweet biochemistry: Remembering structures, cycles, and pathways by mnemonics. Academic Press.