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Number 222
Coenzymes
This Factsheet summarises the role of coenzymes in photosynthesis and respiration and illustrates the types of exam questions which
feature coenzymes.
Coenzymes are small, organic, non-protein molecules that carry e.g. electrons and protons between enzymes. They are a type of cofactor
– a substance that is necessary for an enzyme-controlled reaction to take place.
They often bind to the enzyme’s active site just before, or at the same time as the substrate. By carrying essential substances between
enzymes, coenzymes are important in linking together many separate enzymatic reactions e.g. in metabolic processes such as respiration.
Coenzymes in photosynthesis
Photosynthesis occurs in two stages which occur in different parts of the chloroplast: the Light-Dependent Reactions (LDR) occur in the
thylakoid membranes (grana) the Light-Independent Reactions (LIR) occur in the stroma (Fig 1).
Fig 1. LDR and LIR
The LDR occurs in the grana
Products: Oxygen, ATP, reduced NADP.
CO2 diffuses in
O2 diffuses out
one grana (stack of disk-like
thylakoids)
sugars diffuse out
Starch stored temporarily in starch grain
Light Independent Reaction occurs in the stroma
Products; sugar - which can be converted into fats, amino acids etc.
The coenzyme nicotinamide adenine dinucleotide phosphate
(NADP) links these two sets of reactions together, as follows:
The three products of the LDR are oxygen , ATP, and reduced NADP.
Both the ATP and the reduced NADP are crucial for the second
stage, the LIR.
In the LDR:
In the LIR:
1. Chlorophyll molecules absorb light energy. Electrons within the
chlorophyll become excited and are emitted
2. The excited electrons are passed along a series of carriers on
the thylakoid membranes and some of their energy is used to
create ATP
3. Water molecules are split using light energy (photolysis). This
releases hydrogen ions (protons) and oxygen gas
4. The hydrogen ions and some of the electrons are picked up by
the coenzyme NADP, which is thus reduced
1. Carbon dioxide combines with the 5C sugar ribulose
bisphosphate (RuBP) to form two molecules of glycerate- 3phosphate (GP)
2. The GP is reduced to glyceraldehyde phosphate (GALP) using
the reduced NADP and the ATP formed in the LDR
3. Some of the GALP is used to resynthesise RuBP and some is
used to synthesise sugars, fatty acids and proteins that the
plant requires
Summary: The coenzyme NADP carries electrons and hydrogen
ions from the LDR (thylakoid membranes) to the stroma, where
the LIR creates organic compounds needed by the plant (and
the animal kingdom !)
Remember: a substance is reduced if it gains hydrogen ions or
electrons or loses oxygen
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222 Coenzymes
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Coenzymes in respiration
Just as in photosynthesis, the process of respiration involves a series of reactions that occur in different places. Glycolysis occurs in the
cell cytoplasm, Krebs Cycle occurs in the matrix of the mitochondria and electron transport occurs on the crista (Fig.2)
Fig 2. The structure of a mitochondrion
Cut-away view
Longitudinal section
outer
membrane
inner membrane
O2 and pyruvic
acid diffuse in
crista
inner membrane
crista
granules (70S ribosomes or phosphate granules)
outer membrane
CO2 and ATP
diffuse out
matrix
Link reaction, Krebs cycle
occur in the matrix
stalked particles on the walls of the crista
Oxidative phosphorylation occurs in spherical ends
of stalked particles
Electron transport chain occurs in bases of stalked
particles
Just as in photosynthesis, these spatially-separated reactions are
linked by coenzymes that transfer substrates from one region to
another.
In the Krebs cycle
1. The acetate is offloaded from acetyl coenzyme A and combines
with oxaloacetate (4C) to form citrate (6C)
In Glycolysis
2. In a series of reactions, both hydrogen atoms and carbon dioxide
are removed from the citrate and other intermediates
Glucose is converted into pyruvate, as follows
1. Glucose is phosphorylated using ATP, which makes the glucose
more reactive
2. Each glucose molecule is converted into triose phosphate
3. Two hydrogen atoms and two electrons are removed from each
TP. This requires a dehydrogenase enzyme and this enzyme
requires the coenzyme nicotinamide adenine dinucleotide to
accept the hydrogen atoms. Two molecules of NAD are thus
reduced
4. Glycolysis has a net yield of 2ATP
2ATP used
2NAD
3. The hydrogen atoms are passed to the coenzymes NAD and
FAD (flavine adenine dinucleotide), which are thus reduced
4. ATP is produced
5. Oxaloacetate is eventually regenerated, ready to react with more
acetate from acetyl coenzyme A
Fig 3 Krebs cycle
4ATP
2H+
Glucose
HB (6C)
2TP (3C)
2PA (3C)
2PA (3C)
NAD+
CO2
HB = Hexose bisphophate
TP = Triose phosphate
PA = Pyruvic acid
NADH
AcCoA (2C)
CoA
The pyruvate is transported across the membranes of the
mitochondria into the matrix. It then enters the Link Reaction.
Oxaloacetate (4C)
citrate (6C)
In the Link Reaction
1. Both hydrogen and carbon dioxide are removed from the pyruvate
to form acetate (using dehydrogenase and decarboxylase enzymes)
2. The hydrogen atoms are passed to the coenzyme NAD
3. The acetate is passed to coenzyme A and forms acetyl coenzyme A
CO2
ADP+iP
2H
ATP
NAD/FAD
The function of coenzyme A is to carry the acetate into the Krebs
cycle.
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222 Coenzymes
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The reduced coenzymes NAD and FAD carry electrons to the fold in the inner mitochondrial membrane (christae). This is where the
electron transport chain occurs.
Fig 4. Structure of inner membrane
Cofactor
Proton channel
matrix
ATP Synthase
enzyme
inner mitochondrial
membrane
intermembrane
space
outer membrane
Fig 5 Electron transport chain
ATP synthase
(4e- + 4H+ + O2 → 2H2O)
6 2H O
2
NAD
matrix
Reduced
NAD
2H from Krebs Cycle
2H -
2H 1
FAD
2
2e -
inner
mitochondrial
membrane
ATP
O2
2H+ 2H+
4e -
2H+
2e -
5
O2
ADP + P
2e -
2e -
2e -
2e -
2e -
2e -
3
2e -
3
H+
H+
3
4
H+
intermembrane space
1. The hydrogen atoms on the reduced NAD are split into protons (H+) and electrons (e-)
2. The electrons are passed a long a series of carriers (reoxidising the coenzyme)
3. Energy is released from the electrons and is used by coenzymes to pump protons across the intermembrane space, creating a proton
gradient
4. Protons are able to diffuse back across the inner membrane via ion channels
5. These channels contain the enzyme ATP synthase which generates ATP from ADP and P
6. Finally, the electrons and hydrogen atoms combine with oxygen to form water
Summary
Coenzyme
NAD
FAD
What it does
Accepts hydrogen atoms and electrons in e.g. glycolysis, the Link reaction and Krebs cycle
Carries electrons into the electron transfer chain
Coenzyme A Carries acetate into the Krebs cycle
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Bio Factsheet
222 Coenzymes
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Practice Questions
1. Coenzymes are vital in both photosynthesis and respiration.Complete the table (6)
Process
Coenzyme
Photosynthesis
Respiration
NADP
NAD
Stage of process in which the coenzyme is reduced
Glycolysis/Link Reaction/Krebs cycle
Region of organelle in which the coenzyme is reduced
Stage of process in which the coenzyme is oxidised
Light Independent Reaction
Region of organelle process in which the coenzyme is
oxidised
2. Coenzymes may link metabolic processes that occur in different parts of a cell i.e they allow cell compartmentalization. What are the
advantages of cell compartmentalization? (2)
3. Explain why oxygen is needed for the production of ATP on the cristae of the mitochondrion.
3. ATP formed as electrons pass along transport chain;
oxygen is terminal electron acceptor / accepts electrons from electron transport chain;
electrons cannot be passed along electron transport chain if no O2 to accept them;
forms H2O / accepts H+ from reduced NAD/FAD / oxidises reduced NAD/FAD;
2. Allows different reactions to occur in optimal conditions;
e.g. different pH;
Allows enzymes and substrates to be concentrated;
Faster/more efficient reactions;
Cristae
Stroma
Region of organelle process in which the coenzyme is
oxidised
Electron Transport Chain
Light independent Reaction
Stage of process in which the coenzyme is oxidised
Matrix
Thylakoids
Region of organelle in which the coenzyme is reduced
Glycolysis/Link Reaction/Krebs cycle
Light Dependent Reaction
Stage of process in which the coenzyme is reduced
NAD
NADP
Coenzyme
Respiration
Photosynthesis
Process
1.
Answers
Acknowledgements:
This Factsheet was researched and written by Ron Pickering & Kevin Byrne
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in
any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136
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