Pyruvate Oxidation
and the Krebs Cycle
Courtney, Chelsey, Morgan,
GSchilbeG, Tessa
Pyruvate Oxidation
Pyruvate Oxidation
● Pyruvate is a glucose molecule cut in half
● Pyruvate oxidation is the second stage of
cellular respiration
● One step process occurring in
mitochondrial matrix
● This is the end product of glycolysis
Pyruvate Oxidation (cont.)
● Can be derived from lactate taken up from
the environment or multicellular
organisms from other cells
● Produced from a variety of amino acids
● Can be converted to acetyl coenzyme A
● This conversion is irreversible
● There are three changes to pyruvate
The Changes
1. A CO2 portion is removed
2. NAD+ is reduced by two Hydrogen atoms
obtained from food
3. Coenzyme A is attached to the remaining
acetic acid portion
Activation
The pyruvate oxidation process starts off
because a multienzyme complex catalyzes
the following three changes.
Reactants and Products
Equation: 2 pyruvate + 2 NAD+ + 2 CoA -> 2 acetyl-CoA + 2 NADH + 2 H+ +2 CO2
Reactants: 2 pyruvate, 2 NAD+, 2 CoA
Products: 2 acetyl-CoA, 2 NADH, 2 H+, 2 CO2
Converts Pyruvate/ pyruvic acid into Acetyl-CoA
Which takes places two times for every glucose molecule
Acetyl - CoA
● Used for lipid synthesis, animals cannot
use to synthesize amino acids or
carbohydrates
● This means that this conversion is an
important step
● Removes the fully oxidized carbon while
extracting some energy
● Prepares molecule for the remaining
process
Reactions of the Pyruvate
Dehydrogenase Complex
● First step: an oxidative decarboxylation
reaction
● This is carried out by a very large enzyme
complex called the pyruvate
dehydrogenase complex (located in the
mitochondrial matrix)
● This is irreversible and tightly regulated
Pyruvate Dehydrogenase:
2 pyruvate + 2 NAD+ + 2 CoA
2 acetyl-CoA + 2 NADH + 2 H+ +2 CO2
The Krebs Cycle
● step 3 in cellular respiration
● 8 step, cyclic process
● each step is catalyzed by a specific
enzyme
● overall chemical equation for Krebs cycle
oxaloacetate + acetyl-CoA + ADP + Pi + 3NAD+ + FAD
CoA + ATP + 3NADH + 3H + + FADH2 + 2CO2 + oxaloacetate
The Krebs Cycle Diagram
Page 102
Krebs Cycle Steps
● step 1: Acetic acid subunit of acetyl CoA is combined
with oxaloacetate to form molecule of citrate
● step 2:Citrate(6-C) is rearranged to isocitrate(6-C).
● step 3:Isocitrate (6-C) is converted to -ketoglutarate (5C) by losing a CO2 and two hydrogen atoms that
reduce NAD+to NADH.
● step 4:-ketoglutarate (5-C) is converted to succinyl-CoA
(4-C). A CO2 is removed, coenzyme A is added, and
two hydrogen atoms reduce NAD+ to NADH.
Krebs Cycle Steps
● step 5:Succinyl CoA(4-C) is converted to succinate (4C). ATP is formed by substrate level phosphorylation,
and coenzyme A is released.
● step 6: Succinate (4-C) is converted to fumarate (4-C).
Two hydrogen atoms reduce FAD to FADH2.
● step 7: Fumarate (4-C) is converted to malate (4-C).
● step 8: Malate (4-C) is converted to oxaloacetate (4-C).
Two hydrogen atoms reduce NAD+ to NADH.
Krebs Cycle
● original glucose molecule is entirely consumed
● 6 carbon atoms leave process as 6 low-energy
CO2 molecules, which are disposed of as waste
● original glucose molecule reduced to energy:
○ 4 ATP molecules
2 from glycolysis
2 from Krebs cycle
○ 12 reduced coenzymes
2 NADH from glycolysis
2 NADH from pyruvate oxidation
6 NADH from Krebs cycle
2 FADH2 from Krebs cycle
Reactants and Products
oxaloacetate + acetyl-CoA + ADP + Pi 3NAD+ + FAD
CoA + ATP + 3NADH + 3H+ FADH2 + 2CO2 + oxaloacetate
Reactants: Acetyl Co-A, oxaloacetate, Citrate,
Alpha ketoglutarate
Products: 6 NADH + H+, 2 FADH2, Carbon Dioxide, ATP
Krebs Cycle Simplified
Activation
● The Krebs cycle has to be carefully monitored to
remain efficient
● Too fast: energy would be wasted producing ATP
and reduced coenzymes
● Too slow: not enough energy released to support
cell function
Activation Methods
●
The Krebs cycle requires the products of pyruvate
oxidation to start, so its rate can't surpass that stage's
(substrate availability)
●
Feedback inhibition is used for different stages
○
NADH inhibits citrate synthase, isocitrate dehydrogenase and αketoglutarate dehydrogenase
○
succinyl-CoA competes with acetyl-CoA and inhibits α-ketoglutarate
dehydrogenase
Aerobic or Anaerobic
● Neither process uses O2 to function
● Both are still aerobic because their
products (NADH and FADH2) move to the
ETC which eventually uses O2(aerobic)
Sources
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http://www.incolor.com/mcanaday/Krebs%20Phases.htm
http://wiki.pingry.org/u/ap-biology/index.php/Pyruvate_Oxidation_%
26_The_Citric_Acid_Cycle
http://course1.winona.edu/sberg/241f00/Lec-note/Respira.htm
http://www.livestrong.com/article/406081-why-is-the-krebs-cycle-anaerobic-process/
http://www.tamu.edu/faculty/bmiles/lectures/regulationtca.pdf
http://biochem.siu.
edu/bmb_courses/mbmb451b/lectures/mbmb451b_tcacycle.pdf
Nelson Biology 12, Chapter 2
Quiz
1. Pyruvate oxidation occurs in which of the
following locations?
(a) cytoplasm
(b) mitochondrial matrix
(c) inner mitochondrial membrane
Quiz
2. True or false CoA stands for Coenzyme
A?
Answer: True
Quiz
3. How many pyruvate molecules does one
glucose molecule yield after pyruvate
oxidation?
Two
Answer:
Quiz
4. What compound is both a reactant and a
product in the Krebs cycle that makes it
cyclic?
Answer: oxaloacetate
Quiz
5. Would you consider pyruvate oxidation
aerobic or anaerobic?
Answer: aerobic
Quiz
6. What are all of glucose's carbons
eventually released as during both
processes?
Answer: CO2
Quiz
7. How many ATP are produced from the
Krebs cycle?
Answer: 2
Quiz
8. Why is it important that the Krebs cycle
doesn't proceed too slowly?
Answer: Not enough energy would be released
to support cell function