Aerobic Glycolysis

Aerobic fermentation, also known as aerobic glycolysis, converts sugars to energy for cells by fermentation in oxygen and suppressing cell metabolism. Cells in tumours undergo this process due to the Warburg effect, also known as the Crabtree effect in yeast. 

Aerobic fermentation prevents excessive catabolic oxidation of these nutrients into carbon dioxide. It can thus effectively convert glucose and glutamine into biomass, despite not readily producing ATP.

• As glucose is broken down into pyruvate by aerobic organisms through the aerobic glycolysis metabolic pathway, it becomes pyruvate. 
• Oxidation-reduction reactions are necessary to achieve this. Besides humans, aerobic organisms use aerobic glycolysis to produce energy. Many bacteria utilise aerobic glycolysis as well.
• Glycolysis converts glucose into pyruvate. The process is carried out by fermenting glucose into glyceraldehyde-3-phosphate (G3P) and then dihydroxyacetone phosphate (DHAP). 
• The next step in the production of ATP is the conversion of G3P into 1,3-bisphosphoglycerate (BPG). DHAP is converted to acetaldehyde and then to ethanol under aerobic conditions when oxygen is present. 
• The conversion of DHAP to BPG is impossible under anaerobic conditions because no oxygen is present.
• The conversion instead produces lactic acid. Lactic acid is toxic to the mitochondria of the cells and can damage them.

Glycolysis in Yeast

There is the independent evolution of aerobic glycolysis in three types of yeast (Saccharomyces, Dekkera, and Schizosaccharomyces). Plant pollen harbours the gene and trypanosomatids, mutated E. coli, and cancer cells. 

• When crabtree-positive yeasts are grown on glucose or carbohydrates other than glucose, they respire. 
• According to the Crabtree effect, fermentation is responsible for repressing respiration except under low sugar conditions.
• The fermentation pathway is expressed fully by Saccharomyces cerevisiae even if sugar availability is below the threshold. But it only expresses the respiration pathway in relation to sugar availability. 
• In contrast, oxygen inhibits fermentation in the Pasteur effect.
• To fully understand the genomic basis of aerobic glycolysis, research is still necessary to fully understand the evolution of copy number variation (CNV) and differential expression in metabolic genes.
• In the industrial sector, yeast species with Crabtree-positive C1 and C2 genes are employed in the production of wine, beer, sake, bread, and bioethanol because they can glycolyse. 
• These yeast species possibly evolved to better fit their environment through domestication, often through artificial selection. 
• Strains have evolved through interspecific hybridisation, horizontal gene transfer (HGT), gene duplication, pseudogenisation, and gene loss.

Aerobic Glycolysis in Other Non-Yeast Species

• Plants

A plant’s anaerobic glycolysis largely depends on alcohol glycolysis. The latter produces ATP and regenerates NAD+ to ensure glycolysis continues. 

• Anaerobic conditions are necessary for most plant tissues to ferment, but a few exceptions exist. 
• There is a significant amount of the fermentation enzyme ADH in both maise (Zea mays) and tobacco (Nicotiana tabacum & Nicotiana plumbaginifolia) pollen regardless of oxygen levels. 
• Likewise, tobacco pollen exhibits high levels of PDC transcription and transcript levels unaffected by oxygen concentration. Pollen from tobacco, like Crabtree-positive yeast, is highly fermentable when supplied with sugar rather than when supplied with oxygen. 
• High sugar availability permits simultaneous respiration and alcoholic fermentation in these tissues. In pollen development, fermentation produces toxin-producing compounds such as acetaldehyde and ethanol.
• Acetaldehyde is considered a cytoplasmic male sterility factor caused by acetaldehyde in pollen. 
• We can observe cytoplasmic male sterility in plants such as tobacco, maise, and other species which cannot produce pollen. 
• The presence of ADH and PDC genes a lot earlier than usual in pollen development might result in these traits because toxic aldehydes accumulate, and the fermentation genes express themselves earlier than they would normally.

• E. coli Mutants

Escherichia coli mutant strains are capable of fermenting glucose under aerobic conditions. 

Scientists created the ECOM3 strain (E. coli cytochrome oxidase mutant) by removing the three-terminal cytochrome oxidases (cydAB, cyoABCD, and cbbdAB) to reduce oxygen uptake. A mixed phenotype emerged from 60 days of experimental evolution on glucose media. 

Some populations’ fermentation produced lactate only under aerobic conditions, while others fermented in mixed acid conditions.

The Process of Aerobic Respiration

The process of glycolysis that occurs in the presence of oxygen is called aerobic glycolysis. 

• The presence of oxygen results in aerobic respiration. 
• Oxidative phosphorylation is the second step in aerobic glycolysis following the Krebs cycle.  
• Aerobic glycolysis is a three-phase process that consists of glycolysis, Krebs cycle, and electron transport chain.

The Glycolytic Process

Aerobic respiration begins with glycolysis, which occurs in the cytoplasm. Pyruvate is the product of glucose breakdown. During oxidative decarboxylation, pyruvate molecules convert to acetyl-CoA. This process yields two ATP molecules and two NADH molecules.

• Krebs Cycle

During the Krebs cycle, there is a production of two molecules of ATP and two molecules of NADH. Acetyl-CoA completely breaks down into carbon dioxide and regenerates into oxaloacetate in the Krebs cycle. 

When acetyl-CoA releases during the Krebs cycle, there is the formation of 2 GTP, 6 NADH, and 2 FADH2.

• Electron Transport Chain

NADH and FADH2 act as reducing agents during oxidative phosphorylation to produce ATP. The process occurs inside the mitochondrial membrane. 

Differences between Anaerobic Glycolysis and Aerobic Glycolysis 

The electron transport chain regenerates NAD+ in aerobic fermentation. In comparison, glycolysis regenerates NAD+ in anaerobic fermentation during glycolysis.

• Oxygen is the final electron acceptor in the electron transport chain in an oxygen-free environment.  
• Therefore, glycolysis occurs in an oxygen-free environment. 
• Therefore, aerobic glycolysis is more accurate than aerobic glycolysis. 
• Anaerobic glycolysis includes ethanol fermentation and lactic acid fermentation.

Similarities between Anaerobic Glycolysis and Aerobic Glycolysis 

Here are the similarities between the two processes.

• Anaerobic and aerobic glycolysis are two types of cellular respiration that provide cellular processes with energy.
• This glycolysis relies on glucose as a substrate and produces ATP as a byproduct.
• The byproduct of both processes is carbon dioxide.

Conclusion

The two forms of cellular respiration involved in the production of glucose energy are aerobic and anaerobic fermentation. Fermentation in anaerobic conditions requires oxygen, while fermentation in aerobic conditions does not. 

Anaerobic respiration involves the partial oxidation of pyruvate during the NAD+ regeneration step. In aerobic respiration, it occurs during the electron transport chain.