Fates of Pyruvate Flashcards
The fate of pyruvate depends on two things. What are they?
- The type of organism
- The availability of oxygen
What happens to pyruvate when oxygen is unavailable?
Fermentation, an anaerobic biological reaction process
What are fermentation processes?
Energy-producing pathways that occur in the absence of oxygen
What two fermentation pathways are available to pyruvate in the absence of oxygen?
- Lactic acid fermentation
- Alcoholic fermentation
Where does lactic acid fermentation primarily occur?
Muscle
Lactic acid fermentation occurs in both muscle (under excretion) and red blood cells. Why does it occur in red blood cells?
Red blood cells lack mitochondria and therefore cannot undergo the Krebs Cycle or oxidative phosphorylation
Lactic acid fermentation is a type of _________ reaction.
Redox
In lactic acid fermentation, pyruvate is in what form?
Its oxidized form
In lactic acid fermentation, NADH is in what form?
Its reduced form
Pyruvate is ___________ to lactic acid and NADH is _________ to NAD+ in lactic acid fermentation.
Reduced
Oxidized
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?
To regenerate NAD+ for continued glycolysis
Why does lactic acid fermentation cause muscle soreness?
It produces protons and thereby decreases the pH of the tissue
What is the net reaction for lactic acid fermentation?
Glucose + 2 ADP + 2 Pi –> 2 lactate + 2 ATP + 2 H20 + 2 H+
What is the mechanism of lactate dehydrogenase?
Redox
In the reduction of pyruvate to lactic acid, what functions as the nucleophile?
A hydride ion from NADH
What cycle describes the pathway of lactate from active muscle to the liver (and back)?
The Cori Cycle
What is the limiting reagent in glycolysis?
NAD+
In the Cori cycle, where does gluconeogensis occur?
The liver
In the Cori cycle, where does glycolysis occur?
The muscle
Describe the overall process of the Cori cycle.
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
In the Cori cycle, which is the fast step and slow step?
Glycolysis is the fast step
Gluconeogenesis is the slow step
In the Cori cycle, glycolysis occurs under ________________ conditions while gluconeogenesis occurs under _______________ conditions.
Anaerobic
Aerobic
Alcoholic fermentation occurs in two types of organisms. What are they?
Yeast (primarily) and some plants
Alcoholic fermentation requires two steps. What are they?
- Decarboxylation of pyruvate to acetaldehyde and carbon dioxide
- Reduction of acetaldehyde to ethanol
In alcoholic fermentation, pyruvate must first be decarboxylated into acetaldehyde and carbon dioxide by what enzyme?
Pyruvate decarboxylase
What coenzyme is required for the functioning of pyruvate decarboxylase?
Thiamine pyrophosphate or TPP
What part of the coenzyme thiamine pyrophosphate or TPP is the “business end” or the chemically reactive part regarding the decarboxylation of pyruvate?
The thiazolium ring
Thiamine is also known as ____________ ___.
Vitamin B1
Does the non-enzymatic decarboxylation of an alpha-keto acid work? Why or why not?
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
________________ _______________ or TPP is used for the decarboxylation of any ____-keto acid.
Thiamine pyrophosphate
Alpha-keto acid
What does thiamine pyrophosphate serve as in the enzyme-catalyzed decarboxylation of pyruvate in alcoholic fermentation?
An electron sink to delocalize and destablize the carbanion that forms following deprotonation of the thiazolium ring
Thiamine pyrophosphate is a tightly bound coenzyme in the active site of pyruvate decarboxylase. True or false?
True
What enzyme reduces acetaldehyde to ethanol in alcoholic fermentation?
Alcohol dehydrogenase
What position does alcohol dehydrogenase prefer?
The pro-R position
What does the reduction of acetaldehyde to ethanol by alcohol dehydrogenase require?
NADH, which serves as the hydride source
What metal ion is required for alcoholic fermentation?
Zinc is required in the reduction of acetaldehyde to ethanol by alcohol dehydrogenase
What is the mechanism for pyruvate decarboxylase?
- The thiazolium ring is deprotonated
- The carbanion functions as a nucleophile and attacks the carbonyl carbon of pyruvate, reducing the carbonyl to a hydroxyl grou
- 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
- The lone pair of electrons “kick back” and the double bond between the thiazolium ring and carbon is protonated, leaving behind a hydroxyl group
- The hydroxyl group loses its hydrogen, “kicks back” and reforms the carbonyl, and expels the enzyme, leaving behind acetylaldehyde
What is the mechanism of alcohol dehydrogenase?
How does the body metabolize ingested alcohol?
Via alcohol dehydrogenase in the liver, first forming acetylaldehyde, which is responsible for the hangover, and and then acetic acid via oxidation
What is the point of alcoholic fermentation?
To regenerate NAD+ for glycolysis
Alcoholic fermentation occrs in ____________ and some plants.
Yeast
