Week 10 Hitchcock Lecture 3

Question 1

What is fermentation?

• What is the organic electron acceptor used in fermentation?
• Is glycolysis in fermentation the same or different?
• What happens to fermentation products?
• What is the yield like for fermentation?

Answer 1

• Pyruvate (or a derivative) acts as the organic electron acceptor - formed from glucose (e.g. by glycolysis) with ATP produced by substrate-level phosphorylation
• Different, glycolysis is an incomplete catabolic pathway and the NADH generated must be re-oxidised in order to regenerate NAD+ (redox balancing) and keep glycolysis/ATP production going. In the absence of an electron acceptor/respiratory chain, NADH is re-oxidised to form fermentation products that are more reduced than the organic starting substrate.
• Most of the carbon substrate is routed to fermentation products, which are excreted as waste products in high yields. Therefore relatively little biomass is produced (about 5 times less compared to aerobic respiration)
• ATP yields typically very low - only 1-3 mol ATP per mol substrate (e.g.glucose)

Question 2

What is alcoholic (ethanol) fermentation?

-How does it work?

Answer 2

The conversion of sugars (e.g. glucose, sucrose) to ethanol and CO2
-ATP made by substrate level phosphorylation during glycolysis, producing pyruvate. Pyruvate decarboxylated to acetaldehyde by pyruvate decarboxylase, which in turn is reduced to ethanol by alcohol dehydrogenase, using NADH as electron donor, regenerating NAD+ to keep a redox balance. Overall yield per mol glucose = 2 mol ATP

Question 3

What is lactic acid homolactic fermentation?

-What kind of organism uses lactic acid fermentation?

Answer 3

redox balance is achieved by reducing pyruvate to lactate, which is catalyzed by the lactate dehydrogenase enzyme. Overall yield per mol glucose = 2 ATP
-lactic acid bacteria – gram positive, acid-tolerant anaerobes that lack cytochromes and do not respire

Question 4

What is lactic acid heterolactic fermentation?

-How does lactic acid heterolactic fermentation work?

Answer 4

Making lactate and ethanol from glucose.
-Use a different pathway called the pentose phosphate pathway to metabolise glucose. the pentose phosphoketolase pathway uses different enzymes to generate a 5 carbon sugar phosphate called ribusloe-5-P, is also an intermediate in the Calvin cycle. This is then converted to another 5 carbon molecule, xyulose-5-P, which is then cleaved into one molecule of G3P and one molecule of acetyl phosphate by phosphoketolase. The G3P is metabolized to pyruvate and then lactate as in the homfermentative process, while the Acetyl-P is reduced to ethanol via acetyl-CoA and acetylaldehyde intermediates. Overall yield per mol glucose = 1 ATP

Question 5

What is mixed acid fermentation?

-How does it work?

Answer 5

Conversion of glucose into a mixture of acteic acid, lactatic acid, and formic acid, in addition to ethanol, hydrogen and carbon dioxide
-Lactate is prodiced by lactate dehydrogenase (LDH), note that only some of the pyruvate is converted. Pyruvatee formate-lyase (PFL) converts pyruvate to formate and acetyl-CoA. Formate releases hydrogen and carbon dioxide while acetyl-CoA makes ethanol and acetate. Overall yield per mol glucose = 3 ATP

Question 6

What is acetone-butanol-ethanol (ABE) fermentation?

-What is the overall yield of ATP per mol glucose?

Answer 6

Produces solvents in a ratio of 3:6:1 acetone:butanol:ethanol
-Overall yield per mol glucose = 3.25 ATP

Question 7

What is malolactic fermentation?

-How does it work?

Answer 7

Take up malic acid and decarboxylate it to form lactic acid.
-Antiport of malate (2-) and lactic acid (1-) generates a membrane potential and decarboxylation of malate consumes a proton in the cytoplasm generating a pH gradient. Equivalent to one proton translocation per malate metabolised. ATP synthesis is chemiosmotic (rather than by substrate level phosphorylation) and no redox balancing