Fates of Pyruvate

Question 1

The fate of pyruvate depends on two things. What are they?

Answer 1

1. The type of organism
2. The availability of oxygen

Question 2

What happens to pyruvate when oxygen is unavailable?

Answer 2

Fermentation, an anaerobic biological reaction process

Question 3

What are fermentation processes?

Answer 3

Energy-producing pathways that occur in the absence of oxygen

Question 4

What two fermentation pathways are available to pyruvate in the absence of oxygen?

Answer 4

1. Lactic acid fermentation
2. Alcoholic fermentation

Question 5

Where does lactic acid fermentation primarily occur?

Answer 5

Muscle

Question 6

Lactic acid fermentation occurs in both muscle (under excretion) and red blood cells. Why does it occur in red blood cells?

Answer 6

Red blood cells lack mitochondria and therefore cannot undergo the Krebs Cycle or oxidative phosphorylation

Question 7

Lactic acid fermentation is a type of _________ reaction.

Answer 7

Redox

Question 8

In lactic acid fermentation, pyruvate is in what form?

Answer 8

Its oxidized form

Question 9

In lactic acid fermentation, NADH is in what form?

Answer 9

Its reduced form

Question 10

Pyruvate is ___________ to lactic acid and NADH is _________ to NAD+ in lactic acid fermentation.

Answer 10

Reduced

Oxidized

Question 11

Lactate is considered to be a “dead end” metabolite because it is either exported from the cell or converted back into pyruvate. What is the metabolic reason then for lactic acid fermentation?

Answer 11

To regenerate NAD+ for continued glycolysis

Question 12

Why does lactic acid fermentation cause muscle soreness?

Answer 12

It produces protons and thereby decreases the pH of the tissue

Question 13

What is the net reaction for lactic acid fermentation?

Answer 13

Glucose + 2 ADP + 2 Pi –> 2 lactate + 2 ATP + 2 H20 + 2 H+

Question 14

What is the mechanism of lactate dehydrogenase?

Answer 14

Redox

Question 15

In the reduction of pyruvate to lactic acid, what functions as the nucleophile?

Answer 15

A hydride ion from NADH

Question 16

What cycle describes the pathway of lactate from active muscle to the liver (and back)?

Answer 16

The Cori Cycle

Question 17

What is the limiting reagent in glycolysis?

Answer 17

NAD+

Question 18

In the Cori cycle, where does gluconeogensis occur?

Answer 18

The liver

Question 19

In the Cori cycle, where does glycolysis occur?

Answer 19

The muscle

Question 20

Describe the overall process of the Cori cycle.

Answer 20

Glucose is broken down to pyruvate and then reduced further to lactate in the muscle; lactate moves through the bloodstream and enters the liver; once in the liver, lactate is converted back into pyruvate and used to regenerate glucose; this glucose is released into the bloodstream and used by the muscle

Question 21

In the Cori cycle, which is the fast step and slow step?

Answer 21

Glycolysis is the fast step

Gluconeogenesis is the slow step

Question 22

In the Cori cycle, glycolysis occurs under ________________ conditions while gluconeogenesis occurs under _______________ conditions.

Answer 22

Anaerobic

Aerobic

Question 23

Alcoholic fermentation occurs in two types of organisms. What are they?

Answer 23

Yeast (primarily) and some plants

Question 24

Alcoholic fermentation requires two steps. What are they?

Answer 24

1. Decarboxylation of pyruvate to acetaldehyde and carbon dioxide
2. Reduction of acetaldehyde to ethanol

Question 25

In alcoholic fermentation, pyruvate must first be decarboxylated into acetaldehyde and carbon dioxide by what enzyme?

Answer 25

Pyruvate decarboxylase

Question 26

What coenzyme is required for the functioning of pyruvate decarboxylase?

Answer 26

Thiamine pyrophosphate or TPP

Question 27

What part of the coenzyme thiamine pyrophosphate or TPP is the "business end" or the chemically reactive part regarding the decarboxylation of pyruvate?

Answer 27

The thiazolium ring

Question 28

Thiamine is also known as ____________ \_\_\_.

Answer 28

Vitamin B1

Question 29

Does the non-enzymatic decarboxylation of an alpha-keto acid work? Why or why not?

Answer 29

No, it does not work; non-enzymatic decarboxylation will only occur with a beta-keto acid as the products of decarboxylation of an alpha-keto acid are unstable

Question 30

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ _______________ or TPP is used for the decarboxylation of any \_\_\_\_-keto acid.

Answer 30

Thiamine pyrophosphate Alpha-keto acid

Question 31

What does thiamine pyrophosphate serve as in the enzyme-catalyzed decarboxylation of pyruvate in alcoholic fermentation?

Answer 31

An electron sink to delocalize and destablize the carbanion that forms following deprotonation of the thiazolium ring

Question 32

Thiamine pyrophosphate is a tightly bound coenzyme in the active site of pyruvate decarboxylase. True or false?

Answer 32

True

Question 33

What enzyme reduces acetaldehyde to ethanol in alcoholic fermentation?

Answer 33

Alcohol dehydrogenase

Question 34

What position does alcohol dehydrogenase prefer?

Answer 34

The pro-R position

Question 35

What does the reduction of acetaldehyde to ethanol by alcohol dehydrogenase require?

Answer 35

NADH, which serves as the hydride source

Question 36

What metal ion is required for alcoholic fermentation?

Answer 36

Zinc is required in the reduction of acetaldehyde to ethanol by alcohol dehydrogenase

Question 37

What is the mechanism for pyruvate decarboxylase?

Answer 37

1. The thiazolium ring is deprotonated 2. The carbanion functions as a nucleophile and attacks the carbonyl carbon of pyruvate, reducing the carbonyl to a hydroxyl grou 3. The remaining oxygen atom "kicks back," releases carbon dioxide, and forms a C=C double bond, ultimately causing electrons to move on to the nitrogen atom of thiazolium ring 4. The lone pair of electrons "kick back" and the double bond between the thiazolium ring and carbon is protonated, leaving behind a hydroxyl group 5. The hydroxyl group loses its hydrogen, "kicks back" and reforms the carbonyl, and expels the enzyme, leaving behind acetylaldehyde

Question 38

What is the mechanism of alcohol dehydrogenase?

Answer 38

Question 39

How does the body metabolize ingested alcohol?

Answer 39

Via alcohol dehydrogenase in the liver, first forming acetylaldehyde, which is responsible for the hangover, and and then acetic acid via oxidation

Question 40

What is the point of alcoholic fermentation?

Answer 40

To regenerate NAD+ for glycolysis

Question 41

Alcoholic fermentation occrs in ____________ and some plants.

Answer 41

Yeast