Download 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.
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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 2 ATP that is
formed and the reduced high
energy electron carriers, 6
NADH (3 per acetyl-CoA) and 2
FADH2 (1 per acetyl-CoA) which
go onto the Electron Transport
Chain.