Pyruvate Oxidation and the Krebs Cycle
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Transcript Pyruvate Oxidation and the Krebs Cycle
Reminder
From glycolysis, 2 ATP net
were produced, along with
2 NADH and 2 pyruvate
molecules.
If oxygen is present,
pyruvate will move on with
aerobic cellular respiration.
If oxygen is not present,
pyruvate will undergo
anaerobic cellular
respiration (a.k.a.
fermentation).
Pyruvate Oxidation
The two pyruvate molecules from glycolysis go from the
cytoplasm to the matrix of the mitochondrion.
Pyruvate Oxidation
There a multi-enzyme
catalyzes the following 3
reactions:
1) A carboxyl group that
has lost its hydrogen
atom is taken away as
CO2. (pyruvate
decarboxylase)
Pyruvate Oxidation
2) A redox reaction
occurs. NAD+ is
reduced to NADH plus
H and pyruvate is
oxidized to acetate (an
acetic acid group).
This transfers potential
energy to NAD+.
Pyruvate Oxidation
3)
A sulfur containing enzyme
called Coenzyme-A
attaches to the acetate
forming acetyl-CoA. The
C-S bond is unstable which
prepares acetyl-CoA to be
easily oxidized in the Krebs
Cycle that follows.
Acetyl-CoA enters the
Krebs cycle while the two
NADH (one per pyruvate)
go to the electron transport
chain.
Carbon dioxide leaves as a
waste product.
Krebs Cycle
Discovered by Hans Krebs (Nobel Prize in 1953)
It is an 8-step process with each step catalyzed by a specific enzyme.
Occurs in the matrix of the mitochondrion.
Starts and ends with oxaloacetate, showing that this is indeed a
cyclic process.
The Reactions of the Krebs Cycle
Reaction #1
Acetyl-CoA joins with oxaloacetate to form citrate.
The coenzyme complex leaves the acetyl group and can
go back and oxidize another pyruvate molecule.
Enzyme is citrate synthase.
Reaction #2
Citrate isomerizes into isocitrate.
The enzyme is aconitase.
Reaction #3
Isocitrate loses a carbon dioxide molecule and two
hydrogen atoms.
The hydrogen atoms reduce NAD+ to NADH.
The new 5C molecule is called -ketoglutarate. Notice the
ketone group!
The enzyme is isocitrate dehydrogenase.
Reaction #4
-ketoglutarate is converted to succinyl-CoA as carbon
dioxide is given off and the Coenzyme returns.
2 hydrogen atoms reduce NAD+ to NADH.
The enzyme is -ketoglutarate dehydrogenase.
Reaction #5
The coenzyme leaves again creating succinate and ATP
is formed by substrate level phosphorylation.
The enzyme is succinyl CoA synthetase.
Reaction #6
Succinate is converted into fumerate as two hydrogen
atoms leave to reduce FAD to FADH2.
FADH2 is like NADH, but holds lower energy electrons.
The enzyme is succinic dehydrogenase.
Reaction #7
Water is added to fumerate to create malate by breaking
the double bond between the second and third carbon.
The enzyme is fumerase.
Reaction #8
Malate converts into oxaloacetate as two hydrogen
atoms are used to reduce NAD+ to NADH.
The enzyme is malate dehydrogenase.
The end…
The cycle then restarts with
the other acetyl-CoA
molecule.
At the end of the Krebs cycle,
all of the original carbons
from the glucose molecule
have been removed as
carbon dioxide.
All that is left is the ATP that
is formed and the reduced
high energy electron carriers,
6 NADH (3 per acetyl-CoA)
and 2 FADH2 (1 per acetylCoA) which go onto the
Electron Transport Chain.