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#fermentation

48 public questions tagged with this topic.

Basic Applied Biology

Enzyme converting pyruvate to acetaldehyde in ethanol fermentation is:

Alcoholic fermentation in Saccharomyces cerevisiae and Zymomonas mobilis proceeds through an expanded glycolytic branch after pyruvate formation. Pyruvate decarboxylase, a thiamine pyrophosphate-dependent homotetramer encoded by PDC1, PDC5, PDC6, binds Mg2+ and performs non-oxidative decarboxylation of pyruvate to acetaldehyde and carbon dioxide. Mechanistically, TPP ylide attacks carbonyl carbon forming lactyl-TPP, decarboxylates to hydroxyethyl-TPP carbanion, then releases acetaldehyde and regenerates ylide. This irreversible, highly exergonic step commits carbon toward ethanol and prevents entry into tricarboxylic acid cycle. Subsequently alcohol dehydrogenase ADH1, a zinc-containing NADH-dependent dimeric enzyme, reduces acetaldehyde to ethanol while oxidizing NADH to NAD+, restoring redox balance essential for continued ATP production under anaerobiosis. Pyruvate dehydrogenase complex, in contrast, produces acetyl-CoA for aerobic respiration, while lactate dehydrogenase produces lactate in animal tissues. Regulation by substrate concentration and TPP availability fine tunes flux. Structural studies show TPP bound in V-conformation essential for carbanion stabilization, with pH optimum 6.0, and engineered PDC variants improve ethanol tolerance and flux for industrial biofuel production, making this step rate-limiting in yeast ethanol plants worldwide.

Ref: Lehninger Principles of Biochemistry 8th ed. Ch 14; Nature Scientific Reports 2022 PDC mechanism https://www.nature.com/articles/s41598-022-08568-4

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Basic Applied Biology

True fermentation strictly occurs under:

Fermentat+I2:I530ion represents an ancient energy-yielding strategy where organic molecules serve as both electron donors and acceptors without involvement of external terminal acceptors like oxygen or nitrate. Glycolysis converts glucose to pyruvate with net production of two ATP via substrate-level phosphorylation, mediated by phosphoglycerate kinase and pyruvate kinase, reducing NAD+ to NADH at glyceraldehyde-3-phosphate dehydrogenase step. Under strictly anaerobic conditions, cells cannot operate electron transport chain, so NADH must be reoxidized by transferring electrons to pyruvate or its derivatives, regenerating NAD+ to sustain glycolytic flux. This metabolic logic, first described by Pasteur and elucidated enzymatically by Buchner and Harden, distinguishes true fermentation from anaerobic respiration which employs alternative inorganic acceptors. Yeasts, lactic acid bacteria, Clostridia, and contracting skeletal muscle under oxygen debt all employ this low-yield pathway producing only two ATP per glucose but permitting survival when oxidative phosphorylation is impossible. This recycling ensures continuous NADH oxidation, stabilizes intracellular pH via organic acid production, and illustrates fundamental bioenergetics taught in NEET and GATE as simplest form of anaerobic energy transduction requiring no membrane gradients.

Ref: Berg et al., Biochemistry 9th ed., Chapter 16 Glycolysis and Fermentation; NCBI Bookshelf NBK22356.

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Microbial Physiology

Which phase of bacterial growth is best suited for industrial fermentation?

Industrial fermentation aims to maximize volumetric productivity of biomass, recombinant proteins or primary metabolites in minimal time with reproducible quality. Log phase cells are physiologically most uniform population with high ribosome content, active glycolysis and TCA cycle, abundant ATP and NADPH and low stringent response. Enzyme systems for substrate uptake like PTS and product synthesis operate at peak specific activity providing fastest conversion per cell per hour. Transferring mid-exponential inoculum into production vessel shortens nonproductive lag phase improving batch turnaround and consistency. Lag cells are still adapting and synthesize transporters, stationary cells undergo sigmaS mediated stringent response shifting to maintenance and secondary metabolism which while useful for antibiotic production not ideal for growth-associated products where synthesis coupled to growth, death phase suffers lysis releasing proteases that degrade product. Therefore optimal harvest and inoculum selection for probiotics, starter cultures and recombinant protein processes coincides with mid to late exponential phase where viability and catalytic productivity peak together giving maximum yield and process efficiency.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 12: Log Phase for Industrial Fermentation and Biomass Production.

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