Pyruvate Oxidationand the Krebs CycleCourtney, 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 Changes1. A CO2 portion is removed2. NAD+ is reduced by two Hydrogen atoms
obtained from food3. 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 CoAProducts: 2 acetyl-CoA, 2 NADH, 2 H+, 2 CO2
Converts Pyruvate/ pyruvic acid into Acetyl-CoAWhich 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 regulatedPyruvate 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 cycleoxaloacetate + acetyl-CoA + ADP + Pi + 3NAD+ + FAD
CoA + ATP + 3NADH + 3H+ + FADH2 + 2CO2 + oxaloacetate
The Krebs Cycle Diagram
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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 (5-C) 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 (4-
C). 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 molecules2 from glycolysis2 from Krebs cycle
○ 12 reduced coenzymes2 NADH from glycolysis2 NADH from pyruvate oxidation6 NADH from Krebs cycle2 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 α-
ketoglutarate 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● 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-an-
aerobic-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
(c) inner mitochondrial membrane(b) mitochondrial matrix
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?
Answer:Two
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