What is Glycolysis? Explain the Phases of Glycose oxidative pathway at cellular levels?

Glycolysis, meaning “splitting of sugar,” is the metabolic pathway in which glucose metabolism occurs, breaking down glucose, a six-carbon sugar, into two three-carbon molecules of pyruvate. This process generates ATP as an immediate energy source, which serves as an immediate energy source for cellular activities. Glycolysis occurs in the cytoplasm of all living cells and can function in both the presence and absence of oxygen, making it a fundamental step in cellular respiration.

What is Glycolysis?

Glycolysis is also known as the Embden-Meyerhof-Parnas (EMP) pathway or the glucose oxidative pathway. It plays a crucial role in energy production, providing ATP through substrate-level phosphorylation. The glycolysis pathway is an anaerobic process, meaning it does not require oxygen. However, when oxygen is present, glycolysis is the first step in complete oxidation of glucose in the mitochondria, an essential step in carbohydrate metabolism.

Glycolysis Location in the Cell

• Occurs in the cytoplasm of prokaryotic and eukaryotic cells.
• Functions as a universal metabolic pathway across different organisms.
• Produces ATP even under anaerobic conditions, supporting anaerobic respiration.

Phases of Glycolysis

This phase consumes 2 ATP molecules to prepare glucose for breakdown, a crucial step in carbohydrate metabolism.

1. Preparative Phase (Energy Investment Phase)

This phase consumes 2 ATP molecules to prepare glucose for breakdown, a crucial step in glucose catabolism.

Step 1: Phosphorylation of Glucose

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.

• Reaction: Glucose (C₆H₁₂O₆) + ATP → ADP + Glucose 6-phosphate (C₆H₁₁O₆P₁)
• Enzyme: Hexokinase

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.

• Reaction: Glucose 6-phosphate (C₆H₁₁O₆P₁) → Fructose 6-phosphate (C₆H₁₁O₆P₁)
• Enzyme: Phosphoglucoisomerase

Step 3: Second Phosphorylation

The enzyme phosphofructokinase uses another ATP molecule to transfer a phosphate group to fructose 6-phosphate to form fructose 1, 6-bisphosphate.

• Reaction: Fructose 6-phosphate (C₆H₁₁O₆P₁) + phosphofructokinase + ATP → ADP + Fructose 1, 6-bisphosphate (C₆H₁₀O₆P₂)
• Enzyme: Phosphofructokinase (PFK)

Step 4: Cleavage of Fru-1,6-BP

The enzyme aldolase splits fructose-1,6-bisphosphate into two sugars that are isomers of each other. These two sugars are dihydroxyacetone phosphate and glyceraldehyde phosphate.

• Reaction: Fructose 1, 6-bisphosphate (C₆H₁₀O₆P₂) + aldolase → Dihydroxyacetone phosphate (C₃H₅O₃P₁) + Glyceraldehyde phosphate (C₃H₅O₃P₁)
• Enzyme: Aldolase

Step 5: Isomerization of DHAP

The enzyme triose phosphate isomerase rapidly inter-converts the molecules dihydroxyacetone phosphate and glyceraldehyde phosphate. Glyceraldehyde phosphate is removed as soon as it is formed to be used in the next step of glycolysis.

• Reaction: Dihydroxyacetone phosphate (C₃H₅O₃P₁)   →     Glyceraldehyde phosphate (C₃H₅O₃P₁)
• Enzyme: Triose phosphate isomerase

The net result for steps 4 and 5:

Fructose 1, 6-bisphosphate (C₆H₁₀O₆P₂)   ↔ 2 molecules of Glyceraldehyde phosphate (C₃H₅O₃P₁)

2. Payoff Phase (Energy Generation Phase)

This phase generates 4 ATP molecules and 2 NADH molecules, ensuring energy balance within the cell.

Step 6: Oxidative phosphorylation

The enzyme triose phosphate dehydrogenase serves two functions in this step. First, the enzyme transfers hydrogen (H^–) from glyceraldehyde 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 phosphate to form 1, 3-bisphosphoglycerate. This occurs for both molecules of glyceraldehyde phosphate produced in step 5.

• Reaction: G3P + NAD+ + Pi → 1,3-Bisphosphoglycerate + NADH + H+
• Enzyme: Glyceraldehyde-3-phosphate dehydrogenase

Step 7: ATP Generation (Dephosphorylation)

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.

• Reaction: 2 molecules of 1,3-bisphoshoglycerate (C₃H₄O₄P₂) + phosphoglycerokinase + 2 ADP                                               → 2 molecules of 3-phosphoglycerate (C₃H₅O₄P₁) + 2 ATP Generation
• Enzyme: Phosphoglycerate kinase

Step 8: Isomerization

The enzyme phosphoglyceromutase relocates the ~P from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate.

• Reaction: 2 molecules of 3-Phosphoglycerate (C₃H₅O₄P₁) + phosphoglyceromutase  → 2 molecules of 2-Phosphoglycerate (C₃H₅O₄P₁)
• Enzyme: Phosphoglyceromutase

Step 9: Dehydration

The enzyme enolase removes a molecule of water from 2-phosphoglycerate to form a Phosphoenolpyruvic acid (PEP). This happens for each molecule of 2-phosphoglycerate.

• Reaction: 2 molecules of 2-Phosphoglycerate (C₃H₅O₄P₁) + enolase → 2 molecules of phosphoenolpyruvic acid (PEP) (C₃H₃O₃P₁)
• Enzyme: Enolase

Step 10: Phosphorylation (ATP Production)

The enzyme pyruvate kinase transfers a ~P 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.

• Reaction: 2 molecules of PEP (C₃H₃O₃P₁) + pyruvate kinase + 2 ADP → 2 molecules of pyruvic acid (C₃H₄O₃) + 2 ATP
• Enzyme: Pyruvate kinase

Net ATP Yield of 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.

Aerobic vs. Anaerobic Glycolysis

1. Aerobic Glycolysis
   • Occurs in oxygen-rich conditions.
   • Pyruvate enters the mitochondria for further oxidation in the Krebs cycle.
   • Produces 38 ATP molecules per glucose (including oxidative phosphorylation), optimizing ATP synthesis.
   • Supports efficient energy production and sustained metabolic activities. (energy balance within the cell)
   • Plays a crucial role in the function of highly active tissues such as the brain, heart, and muscles.
2. Anaerobic Glycolysis
   • Occurs in oxygen-deficient conditions.
   • Pyruvate is converted into lactic acid (animals) or ethanol (yeasts).
   • Produces only 2 ATP molecules per glucose, making it crucial for anaerobic metabolism.
   • Provides quick but short-lived energy bursts, essential for activities like sprinting.
   • Leads to muscle fatigue due to the accumulation of lactic acid, which must be metabolized later when oxygen is available.
   • Ensures that glycolysis is the first step in aerobic respiration.

Importance of Glycolysis

• Provides Energy: Generates ATP for essential cellular functions, enabling muscle contraction, nerve impulse transmission, and biosynthesis of macromolecules.
• Forms Precursors: Produces intermediates essential for biosynthesis, including ribose sugars for nucleotide synthesis, amino acid precursors, and fatty acid metabolism.
• Supports Anaerobic Conditions: Functions when oxygen levels are low, making it crucial for muscle metabolism during intense exercise and for organisms that thrive in low-oxygen environments.
• Critical for Cancer Cells: Many cancer cells rely on glycolysis (Warburg Effect) for rapid growth and survival, even in oxygen-rich environments, by shifting metabolism towards increased glucose uptake and lactate production.
• Contributes to Metabolic Flexibility: Helps cells switch between energy sources, supporting survival in fluctuating environmental conditions.
• Regulated by Enzymatic Steps: Understanding the enzymatic steps in glycolysis helps in metabolic studies.
• Interacts with Other Pathways: Glucose metabolism connects with lipid and protein metabolism, affecting energy balance.
• Involvement in Disease and Disorders: Dysregulation of glycolysis is linked to metabolic disorders such as diabetes and plays a role in neurodegenerative diseases and ischemic conditions.

Conclusion

Glycolysis is an essential metabolic pathway that plays a key role in energy production, making it a fundamental process for all living organisms. Whether under aerobic or anaerobic conditions, it ensures ATP synthesis and provides metabolic intermediates for other biochemical pathways, including gluconeogenesis and lipid metabolism.

Additionally, glycolysis plays a crucial role in various physiological and pathological conditions. In muscle cells, it supports energy production during strenuous activity, while in cancer cells, it is upregulated to sustain rapid growth (Warburg effect). The regulation of glycolysis is vital in metabolic diseases such as diabetes, where glucose utilization is impaired.

Understanding glycolysis is crucial for medical, biological, and biochemical applications, particularly in cellular energy production, metabolic disorders, and therapeutic interventions targeting glycolytic enzymes.

Summary of Glycolysis

• Universal Pathway: Occurs in all living cells, regardless of oxygen availability.
• Energy Production: Generates ATP for cellular activities through substrate-level phosphorylation.
• Metabolic Intermediates: Provides key molecules for biosynthetic pathways.
• Role in Disease: Dysregulation is linked to cancer metabolism, diabetes, and other metabolic disorders.
• Therapeutic Target: Certain glycolytic enzymes serve as potential drug targets in cancer and infectious diseases.