Fermentation - (Alcohol + Lactic Acid)

Explain the biological purpose of fermentation

Regenerates NAD⁺ so glycolysis can continue in the absence of oxidative phosphorylation

Explain why fermentation is essential under anaerobic conditions

ETC cannot oxidize NADH without O₂; fermentation restores NAD⁺

Explain why fermentation does not produce additional ATP

No electron transport or proton gradient; only substrate-level phosphorylation from glycolysis

Explain the reaction catalyzed by lactate dehydrogenase (LDH)

Pyruvate + NADH + H⁺ → Lactate + NAD⁺

Explain the catalytic strategy of lactate dehydrogenase

Redox reaction via hydride transfer from NADH to carbonyl carbon of pyruvate

Identify key active-site features of lactate dehydrogenase

NADH binding site; His acts as proton donor/acceptor; Arg stabilizes carboxylate

Explain why lactate formation drives NAD⁺ regeneration

Reduction of pyruvate pulls NADH oxidation forward

Explain why lactate dehydrogenase reaction is near equilibrium

ΔG ≈ 0; direction depends on NADH/NAD⁺ ratio

Explain why lactic acid fermentation occurs in muscle and erythrocytes

High ATP demand or lack of mitochondria

Explain why alcohol fermentation requires two enzymatic steps

First removes CO₂, second regenerates NAD⁺

Explain the reaction catalyzed by pyruvate decarboxylase

Pyruvate → Acetaldehyde + CO₂

Explain the catalytic role of TPP in pyruvate decarboxylase

Stabilizes carbanion during decarboxylation

Explain the reaction catalyzed by alcohol dehydrogenase

Acetaldehyde + NADH + H⁺ → Ethanol + NAD⁺

Explain the catalytic strategy of alcohol dehydrogenase

Hydride transfer from NADH; Zn²⁺ stabilizes aldehyde

Identify key active-site features of alcohol dehydrogenase

Zn²⁺ ion; His/Glu for proton transfer; NADH binding pocket

Explain the role of carbonyl reduction in fermentation

Accepts hydride from NADH to regenerate NAD⁺

Explain why pyruvate is a suitable electron acceptor

Contains electrophilic carbonyl carbon

Explain how TPP stabilizes reaction intermediates

Thiazolium ring forms carbanion/enamine intermediate

Explain how Zn²⁺ contributes to alcohol dehydrogenase catalysis

Polarizes carbonyl group, facilitating hydride transfer

Explain the net ATP yield of fermentation per glucose

2 ATP (from glycolysis only)

Explain why fermentation is inefficient but sufficient for survival

Low ATP yield but rapid NAD⁺ regeneration

Explain how fermentation supports short-term energy needs

Allows glycolysis to continue at high rates

Explain how NADH/NAD⁺ ratio regulates fermentation

High NADH favors fermentation reactions

Explain why fermentation rate increases during intense exercise

O₂ limitation increases NADH accumulation

Explain why fermentation decreases when oxygen becomes available

ETC regenerates NAD⁺ more efficiently

Explain why fermentation is tightly linked to glycolytic flux

Fermentation rate depends on pyruvate and NADH availability

Explain how fermentation is coupled to glycolysis

Glycolysis produces NADH; fermentation consumes it

Explain why fermentation cannot replace oxidative phosphorylation long-term

ATP yield too low to meet cellular demands

Explain how lactate can be reused by the liver

Converted to glucose via gluconeogenesis (Cori cycle)

Predict what happens if lactate dehydrogenase is inhibited

NAD⁺ depletion; glycolysis halts

Predict the effect of impaired NAD⁺ regeneration on cellular ATP levels

ATP drops rapidly

Explain why cells lacking fermentation pathways cannot survive anaerobically

No mechanism to regenerate NAD⁺

Explain how ethanol accumulation affects cellular metabolism

Alters NADH/NAD⁺ ratio, inhibiting gluconeogenesis

Explain the carbon fate in lactic acid fermentation

3-carbon pyruvate → 3-carbon lactate

Explain the carbon fate in alcohol fermentation

3-carbon pyruvate → 2-carbon ethanol + CO₂

Explain why CO₂ release occurs only in alcohol fermentation

Decarboxylation step required to form acetaldehyde

Explain everything you know about fermentation

Anaerobic NAD⁺ regeneration pathway coupled to glycolysis; low ATP yield; essential without O₂

Explain fermentation in the context of whole-body metabolism

Supports muscle during hypoxia; lactate recycled by liver

Compare lactic acid and alcohol fermentation

Both regenerate NAD⁺; differ in end products and enzymes

Summarize fermentation in one sentence

Fermentation regenerates NAD⁺ from NADH under anaerobic conditions, allowing glycolysis to continue without producing additional AT