Bioscience Evidence 2024, Vol.14, No.5, 238-249 http://bioscipublisher.com/index.php/be 240 Figure 1 Contribution of hypoxia-inducible ADH and PDC genes to low oxygen conditions (Adopted from Bui et al., 2019) Image caption: (A) Phenotypic difference of plants treated with 21 h with anoxia. (B) Progressive scoring as arbitrary units of wild type (Col-0) plants and adh1 or pdc1pdc2 knock-out mutants subjected to an anoxic treatment of a variable duration. Comparison among different Arabidopsis genotypes was evaluated by the proportion of observations in categories applying a χ2 test. (C) Germination percentage of wild-type and adh1, adh2, and pdc1pdc2 seeds under variable oxic atmospheres after seven days. Statistical significance of the differences observed between genotypes at each time point was assessed by 1-way ANOVA followed by Holm-Sidak post-hoc test (n=5) (Adopted from Bui et al., 2019) 3.3 Conversion of acetaldehyde to ethanol The final step in ethanol fermentation involves the reduction of acetaldehyde to ethanol, a reaction catalyzed by alcohol dehydrogenase (ADH). This step also regenerates NAD+ from NADH, which is essential for the continuation of glycolysis. The bifunctional enzyme AdhE, found in bacteria, catalyzes both the conversion of acetyl-CoA to acetaldehyde and the subsequent reduction to ethanol, highlighting its importance in bacterial ethanol fermentation (Pony et al., 2020). 3.4 Role of enzymes in the fermentation process Enzymes play a critical role in the ethanol fermentation process. Pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) are the primary enzymes involved. In some organisms, such as Thermoanaerobacterium saccharolyticum, both pyruvate kinase (PYK) and pyruvate phosphate dikinase (PPDK) are involved in the conversion of phosphoenolpyruvate to pyruvate, indicating the complexity and redundancy of the metabolic pathways (Cui et al., 2020). Additionally, the filamentation of AdhE in bacteria is essential for its enzymatic activity and regulation, which is crucial for efficient ethanol production (Pony et al., 2020). 3.5 Energy yield and byproducts The energy yield of ethanol fermentation is relatively low compared to aerobic respiration, with a net gain of only two ATP molecules per glucose molecule. However, the process is advantageous under anaerobic conditions where oxygen is limited. Byproducts of ethanol fermentation include CO2, which is released during the decarboxylation of pyruvate, and other metabolites such as acetaldehyde. In some engineered strains, efforts have
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