BIOC 4331 Lecture 30

What is the main purpose of fermentation?

To regenerate NAD+ so glycolysis can continue producing ATP in the absence of oxygen.

Why does glycolysis require fermentation under anaerobic conditions?

Glycolysis needs NAD+ at the glyceraldehyde-3-phosphate dehydrogenase step, and without oxygen there is no ETC to reoxidize NADH.

What happens to pyruvate in the absence of oxygen?

It is sacrificed in fermentation pathways to regenerate NAD+.

What is the unifying strategy of all fermentation pathways?

Use an organic molecule, usually derived from pyruvate, as an electron acceptor to reoxidize NADH to NAD+.

What is the major metabolic reason cells need NAD+ regenerated during hypoxia?

So glycolysis can keep running and continue making ATP.

Does fermentation increase the total ATP yield beyond glycolysis?

No. Fermentation itself does not add extra ATP; it allows glycolysis to continue.

In fermentation, is there a net change in oxidation state of carbon from glucose?

No net change overall.

What happens to the electrons from NADH during fermentation?

They are transferred to a pyruvate-derived intermediate.

What are the two fermentation pathways emphasized in these slides?

Lactate fermentation and ethanol fermentation.

What is pyruvate a major branch point for?

Catabolism: it can go to lactate, ethanol, or acetyl-CoA depending on conditions.

What is the end product of lactate fermentation?

Lactate.

What enzyme converts pyruvate to lactate?

Lactate dehydrogenase.

What cofactor is used in lactate fermentation?

NADH.

What happens to NADH during lactate fermentation?

It is oxidized to NAD+.

What happens to pyruvate during lactate fermentation?

It is reduced to lactate.

What reaction summarizes lactate fermentation?

Pyruvate + NADH + H+ -> Lactate + NAD+.

In what tissue was lactate fermentation highlighted in the slides?

Anaerobic muscle.

What is the net product from one glucose in lactate fermentation?

2 lactate.

What is the net ATP yield from one glucose in lactate fermentation?

2 ATP net.

Why is there no net NAD+ change in lactate fermentation overall?

Because NADH made during glycolysis is consumed when pyruvate is reduced to lactate.

Which glycolytic steps consume ATP in lactate fermentation?

Hexokinase and phosphofructokinase.

Which glycolytic steps produce ATP in lactate fermentation?

Phosphoglycerate kinase and pyruvate kinase.

How much ATP is invested during lactate fermentation per glucose?

2 ATP.

How much ATP is returned during lactate fermentation per glucose?

4 ATP.

What is the net ATP from lactate fermentation per glucose?

2 ATP.

What are the end products of ethanol fermentation?

2 ethanol + 2 CO2.

In what organism was ethanol fermentation emphasized in the slides?

Yeast.

What is the first step of ethanol fermentation from pyruvate?

Pyruvate is decarboxylated to acetaldehyde + CO2.

What enzyme converts pyruvate to acetaldehyde in ethanol fermentation?

Pyruvate decarboxylase.

What cofactor is required by pyruvate decarboxylase?

TPP (thiamine pyrophosphate).

What is the second step of ethanol fermentation?

Acetaldehyde is reduced to ethanol.

What enzyme converts acetaldehyde to ethanol?

Alcohol dehydrogenase.

What cofactor is used by alcohol dehydrogenase in ethanol fermentation?

NADH.

What happens to NADH during ethanol fermentation?

It is oxidized to NAD+.

What reaction summarizes the second step of ethanol fermentation?

Acetaldehyde + NADH + H+ -> Ethanol + NAD+.

What is the net ATP yield from ethanol fermentation per glucose?

2 ATP net.

Why is there no net NAD+ change in ethanol fermentation overall?

Because NADH generated in glycolysis is reoxidized during reduction of acetaldehyde to ethanol.

What gas is released during ethanol fermentation?

CO2.

What is the net product from one glucose in ethanol fermentation?

2 ethanol + 2 CO2.

What do lactate and ethanol fermentation have in common?

Both regenerate NAD+ so glycolysis can continue and both yield only the ATP from glycolysis.

What is the major difference between lactate and ethanol fermentation?

Lactate fermentation reduces pyruvate directly to lactate, while ethanol fermentation first converts pyruvate to acetaldehyde + CO2, then reduces acetaldehyde to ethanol.

Which fermentation pathway releases CO2?

Ethanol fermentation.

Which fermentation pathway requires TPP?

Ethanol fermentation.

Which fermentation pathway uses lactate dehydrogenase?

Lactate fermentation.

Which fermentation pathway uses alcohol dehydrogenase?

Ethanol fermentation.

Which fermentation pathway directly reduces pyruvate?

Lactate fermentation.

Which fermentation pathway proceeds through acetaldehyde?

Ethanol fermentation.

What cofactor is emphasized for lactate fermentation?

NADH.

What cofactors are emphasized for ethanol fermentation?

TPP and NADH.

What does TPP stand for?

Thiamine pyrophosphate.

What important biochemical feature of TPP was emphasized?

It forms a stabilized carbanion.

What kind of chemical center does the TPP carbanion like to attack?

Electrophilic carbonyls.

What proton on TPP is especially important?

The acidic proton at C2.

What role does TPP play in ethanol fermentation?

It assists pyruvate decarboxylase in converting pyruvate to acetaldehyde + CO2.

What transient intermediate idea was emphasized for TPP?

It can help form an active acetaldehyde equivalent.

What bacterium was associated with yogurt production?

Lactobacillus bulgaricus.

How does fermentation contribute to yogurt production?

It produces lactic acid, increasing acidity and precipitating milk proteins.

What bacterium was associated with Swiss cheese production?

Propionibacterium freudenreichii.

How does fermentation contribute to Swiss cheese holes?

Production of CO2 creates bubbles.

How does fermentation help preserve pickled foods?

Acidification acts as a preservative.

Name some non-beer uses of fermentation from the slides.

Yogurt, Swiss cheese, pickled foods, commodity chemicals, biofuels.

How can fructose enter glycolysis in muscle and kidney?

It is phosphorylated by hexokinase to fructose-6-phosphate.

What enzyme phosphorylates fructose in muscle and kidney?

Hexokinase.

What product is formed when hexokinase acts on fructose?

Fructose-6-phosphate.

How does fructose-6-phosphate enter glycolysis?

It enters directly as a standard glycolytic intermediate.

How does fructose enter glycolysis in the liver?

Fructose is converted to fructose-1-phosphate, then split into glyceraldehyde and DHAP.

What enzyme phosphorylates fructose in the liver?

Fructokinase.

What product is formed by fructokinase?

Fructose-1-phosphate.

What enzyme cleaves fructose-1-phosphate in the liver?

Fructose-1-phosphate aldolase.

What are the products of fructose-1-phosphate cleavage?

Glyceraldehyde and DHAP.

What happens to glyceraldehyde in hepatic fructose metabolism?

It is phosphorylated by triose kinase to glyceraldehyde-3-phosphate.

What enzyme converts glyceraldehyde to glyceraldehyde-3-phosphate?

Triose kinase.

Which fructose metabolite enters glycolysis directly after triose kinase acts?

Glyceraldehyde-3-phosphate.

Which fructose cleavage product already is a glycolytic intermediate?

DHAP.

What is the key difference between fructose metabolism in muscle/kidney vs liver?

Muscle/kidney: fructose -> F6P via hexokinase. Liver: fructose -> F1P via fructokinase, then to glyceraldehyde + DHAP.

What disaccharide yields galactose upon digestion?

Lactose.

What enzyme hydrolyzes lactose?

Lactase (beta-D-galactosidase).

What are the products of lactose hydrolysis?

D-galactose and D-glucose.

Galactose is an epimer of glucose at which carbon?

C4.

Why can’t free galactose be directly converted by a simple galactose 4-epimerase in humans?

Because we do not have an enzyme that epimerizes free galactose directly.

What epimerase do we have for galactose metabolism?

UDP-galactose 4-epimerase.

Does UDP-galactose 4-epimerase act on free galactose?

No, it acts only on UDP-conjugated galactose.

What is the first step in galactose entry into glycolysis?

Galactose is phosphorylated to galactose-1-phosphate.

What enzyme converts galactose to galactose-1-phosphate?

Galactokinase.

What cofactor/energy source does galactokinase use?

ATP.

What enzyme converts galactose-1-phosphate using UDP-glucose?

Galactose-1-phosphate uridylyltransferase.

What are the products of galactose-1-phosphate uridylyltransferase?

UDP-galactose and glucose-1-phosphate.

What enzyme converts UDP-galactose to UDP-glucose?

UDP-galactose 4-epimerase.

What cofactor is used by UDP-galactose 4-epimerase?

NAD+.

What glycolytic precursor is eventually formed from galactose?

Glucose-6-phosphate.

What enzyme converts glucose-1-phosphate to glucose-6-phosphate?

Phosphoglucomutase.

Why is UDP-glucose useful in galactose metabolism?

It serves as a sugar donor/intermediate and is already present for other pathways like glycogen synthesis.

What is the pathway of galactose into glycolysis, in order?

Galactose -> Galactose-1-phosphate -> Glucose-1-phosphate -> Glucose-6-phosphate -> glycolysis.

What is the role of UDP-galactose in galactose metabolism?

It is the conjugated form that can be epimerized to UDP-glucose.

Why is galactose metabolism more complicated than fructose entry in muscle?

Because galactose cannot simply be directly epimerized as a free sugar and must go through UDP-linked intermediates.

Why does the body use UDP-linked intermediates in galactose metabolism?

Because the epimerase acts on UDP-galactose, not free galactose.

What is the problem the galactose pathway solves?

Converting a C4 epimer of glucose into a form that can enter glycolysis.

What is the biochemical logic behind galactose metabolism?

Attach galactose to UDP, epimerize the UDP-sugar, then recover a glucose phosphate intermediate.

What is the biochemical logic behind liver fructose metabolism?

Convert fructose into triose phosphates that can feed into glycolysis.

Why is regeneration of NAD+ essential during fermentation?

Without NAD+, glycolysis stops and ATP production under anaerobic conditions ceases.