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1.4.5. Link reaction and Krebs cycle
Starter
 Complete Glycolysis summary sheet
Learning Objectives
 Describe what the link
reaction is
 Describe what happens
during the Krebs cycle
 Explain the role of
Success Criteria
 Outline the link reaction, with
reference to decarboxylation of
pyruvate to acetate and the
reduction of NAD (Grade D-E)
 Explain that coenzyme A carries
acetate from the link reaction to
Krebs cycle (Grade C)
hydrogen carrier molecules
 Outline the Krebs cycle,
in the Krebs cycle
including the roles of NAD and
FAD, and substrate-level
phosphorylation (Grade A-B)
Respiration
The process of respiration can be split
into 4 stages.
1.
2.
3.
4.
Glycolysis
Link reaction
Krebs Cycle
Electron Transport Chain
Products of Glycolysis
 2 reduced NAD (NADH + H+)
 2 Pyruvate
 2 ATP
The Fate of Pyruvate?
THIS DEPENDS ON THE
AVAILABILITY OF OXYGEN!!!
The fate of pyruvate
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Aerobic Respiration
 If O2 is present 3C pyruvate passes into mitochondria.
 Here it is completely oxidised forming CO2 and H2O.
 The second stage of aerobic respiration is the link
reaction.
The Link Reaction
Cytoplasm
Mitochondria
The Link reaction
Glycolysis
The Krebs Cycle
 Links Glycolysis to the Krebs Cycle.
 The end product of the link reaction can enter the
Krebs Cycle.
NAD+
NADH + H+
Acetate
(2C)
Pyruvate
(3C)
Coenzyme A
Acetyl coenzyme A
(2C)
CO2
Coenzyme
A now
combines
with the
The Pyruvate
is decarboxylated
(CO2) to form
Start
with
Pyruvate
produced
in Acetate
Glycolysis
This
produces
Acetate
Acetyl coenzyme
It is also dehydrogenated
(H+Aremoved)
The Acetyl coenzyme A now enters Krebs cycle
Link Reaction
 Pyruvate decarboxylase removes




a carboxyl group from pyruvate
Pyruvate(3C)
in the form of CO2.
Pyruvate dehydrogenase
removes H atoms from pyruvate
Decarboxylation
NAD+ collects hydrogen ions.
CO2 (1C)
This forms reduced NAD
+
(NADH + H+)
NAD
Oxidation
The remaining 2-carbon
Reduction
molecule combines with
NADH + H+
Coenzyme
coenzyme A to produce
acetylcoenzyme A (acetyl CoA).
(CoA)
No ATP is produced in this
reaction.
Acetyl CoA (2C)
Task
 Write down the equation for the link reaction
Overall equation for one link reaction and Kreb cycle:
Pyruvate + NAD + CoA
acetyl CoA + reduced NAD + CO2
The Products of the Link Reaction go to
the Krebs Cycle (takes place in matrix)
So for each glucose molecule:
 2 acetylcoenzyme A (go into the Krebs cycle)
 2 Carbon dioxide (released as a waste products)
 2 Reduced NAD (go to the electron transport chain)
Krebs cycle – metabolic cycle
Each step in the cycle
requires enzymes to
reduce the activation
energy. The reactions
all take place in the
matrix of the
mitochondria and are
usually represented as
a circular diagram.
Oxaloacetate
citrate
DeNa DeNa A Fa... Na
 This little tune will help you to remember the
Krebs Cycle.
 Remember, respiration is all about releasing
energy from your food.
 Oxidation releases energy.
 When a carbon compound is oxidised,
coenzymes are reduced.
 The coenzymes involved are: NAD and FAD.
 Carboxylation is the removal of CO2.
DENA DENA A FA... NA
 DENA: Decarboxylation and production of reduced NAD
 DENA: Decarboxylation and production of reduced NAD
 A: Production of ATP
 FA... : Production of reduced FAD (The ‘...’ means a gap)
 NA: Production of reduced NAD
Oxaloacetate
citrate
DENA
NA
FA
DENA
A
Krebs cycle
 Acetyl CoA joins with the C4(oxaloacetate)
group
 CoA is released to transport more pyruvate
into the matrix
 A C6 fragment is formed (citrate)




DENA
C6 (citrate) is oxidatively decarboxylated.
A C5 group is formed.
The carbon is given off as carbon dioxide
NAD+ is reduced to NADH + H+
Krebs cycle
 The C5 fragment is oxidised and
decarboxylated further to a C4 compound.
 Again the carbon removed forms carbon
dioxide.
 NAD+ is further reduced to NADH + H+.
DENA
Krebs cycle
 The final stage in the cycle has the C4
(oxaloacetate) acceptor regenerated.
 ADP is reduced to ATP
A
 FAD (Coenzyme)is reduced to FADH2 .

FA
 There is a reduction of NAD+ to NADH + H+.
NA
Oxaloacetate
citrate
DENA
NA
FA
DENA
A
(a) Pyruvate (3C)
(b) Link reaction
(c) C4 + C2= C6
(d) Recycling of CoA
(e) Decarboxylation C6 to
C5 and the reduction of
NAD
(f) Decarboxylation C5 to
C4 and the reduction of
NAD
(g) C4 to C4 with the
reduction of coenzymes
FAD and NAD. ATP is
made directly.
This cycle follows one acetyl group.
Each glucose that enters glycolysis will
produce 2 acetyl groups.
(h) C4 to C4 acceptor
Krebs Cycle Summary
Each Acetyl CoA entering the cycle results in:
1.
2 CO2 molecules
2.
1 ATP molecules (S.L.P)
3.
8 pairs of H atoms (Later used in the E.T.C to produce ATP)
Used to reduce NAD and FAD. Three reduced NAD are produced and 1 reduced
FAD per cycle.
NAD = Nicotinamide adenine dinucleotide
FAD = Flavine adenine dinucleotide
Importance of the Krebs cycle
1) Breaks down macromolecules into smaller ones
2) Produces H atoms that are carried by NAD to ETC
for oxidative phosphorylation, to produce ATP.
3) Regenerates 4 –carbon molecule that combines with
acetylcoenzyme A, which would otherwise
accumulate.
4) Source of intermediate compounds used by cells in
the manufactures of other important substances
such as fatty acids, amino acids and chlorophyll
Counting carbons - Plenary
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Learning Objectives
 Describe what the link
reaction is
 Describe what happens
during the Krebs cycle
 Explain the role of
hydrogen carrier
molecules in the Krebs
cycle
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Success Criteria
• Outline the link reaction, with
reference to decarboxylation of
pyruvate to acetate and the
reduction of NAD (Grade D-E)
• Explain that coenzyme A
carries acetate from the link
reaction to Krebs cycle (Grade
C)
• Outline the Krebs cycle,
including the roles of NAD and
FAD, and substrate-level
phosphorylation (Grade A-B)
© Boardworks Ltd 2009