Each monomer of Photosystem I consists of a dozen proteins and over a hundred cofactors
such as (chlorophyll, bright green) and carotenoids (orange).
Photosystem I contains 12 polypeptides, 96 chlorophylls, 2 phylloquinones, three [4Fe-4S] clusters, 22
carotenoids, four lipids and a Ca2+ molecule.
PsaA and PsaB (red and blue), PsaF (yellow), PsaL (grey), PsaM (pink) and three stromal proteins [PsaC
(magenta), PsaD (blue) and PsaE (cyan)]. Photosystem I exists in the membrane of cyanobacteria as a
Photosystem I: Protein components
PsaA and PsaB heterodimer: location of primary electron transfer chain.
peripheral PsaC protein: peripheral, similar to a small, dicluster bacterial ferredoxins.
PsaD and PsaE: peripheral, assist in docking ferredoxin, regulate cyclic electron transfer.
PsaF: plastocyanin docking.
PsaG, PsaH and PsaK: stabilization of the light harvesting complexes.
PsaI and PsaJ: structural organization of the PSI complex.
PsaL: trimerization of PSI.
Photosystem I: harvesting light
These antenna molecules each absorb light and transfer energy to their neighbors. Rapidly, all
of the energy funnels into the three reaction centers, where is captured to create activated
The electron transfer chain
The heart of photosystem I is an electron transfer chain, a chain of chlorophyll (green),
phylloquinone (orange) and three iron-sulfur clusters (yellow and red).
The electron transfer cofactors from P700 to FX are embedded within the membrane phase
and thereby shielded from the solvent.
The electron transfer cofactors include a pair of chlorophyll a molecules as the primary
electron donor, a chlorophyll a monomer as the primary electron acceptor and a phylloquinone
as a secondary electron acceptor. Two molecules of phylloquinone exist per reaction center.
The differences with Type II reaction centers
exist primarily on the electron acceptor side.
Photosystem I utilizes a [4Fe-4S] cluster that,
unlike the non-heme iron in the bacterial
reaction center, functions in electron transfer.
Two additional [4Fe-4S] clusters, termed FA
and FB, participate in this process by
providing a pathway for electrons to leave the
Why electrons are transferred to ferredoxin than to plastocyanin?
The electrons are picked up by the soluble [2Fe-2S] protein, ferredoxin, a one-electron carrier
protein, which can in turn form a complex with ferredoxin:NADP+ oxidoreductase to reduce
NADP+ to NADPH.
Plant-type ferredoxins: 1-8; Halophilic ferredoxins: 9; Vertebrate
The transfer of electrons from reduced ferredoxin to NADP+ is catalyzed by ferredoxin-
This complex contains a tightly bound FAD which accepts the electrons one at a time from
ferredoxin. The FADH2 then transfers a hydride to NADP+ to form NADPH.
Ferredoxin is a strong reductant
but can only function in one
electron reductions. NADP+ can
accept two electrons in the form
of a hydride. Thus, an
intermediary is needed to
facilitate the electron transfer.
Respiration is the major process by which aeorbic organisms derive energy and involving a
series of electron carriers resulting in the reduction of dioxygen to water.
The inner mitochondrial membrane is
involved in energy transduction with
protein complexes transferring
electrons in steps coupled to the
generation of proton gradient.
In eukaryotes this process is confined
to the mitochondrion.
In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with
transfer of H+ ions across chloroplast membranes.
In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to
fumarate that are required to generate the proton gradient.
Photosynthesis vs Respiration
Production of ATP Yes Yes (~ 30-32 ATP molecules per glucose)
Reactants 6CO2 and 12H2O and light energy C6H12O6 and 6O2
Requirement of sunlight Yes No
Chemical reaction 6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H20 6O2 + C6H12O6 --> 6CO2 +6H2O + energy
Process The production of organic carbon (glucose and
starch) from inorganic carbon (carbon dioxide)
The production of ATP from the oxidation of
organic sugar compounds
Fate of oxygen and carbon
Carbon dioxide is absorbed and oxygen is released Oxygen is absorbed and carbon dioxide is
What powers ATP synthase H+ gradient across thylakoid membrane into stroma H+ gradient across the inner mitochondria
membrane into matrix
What pumps protons across
Electron transport chain Electrochemical gradient created energy that the
protons use to flow passively synthesizing ATP
Final electron receptor NADP+ (forms NADPH ) O2 (Oxygen gas)
Organisms Occurs in plants, protista (algae) and some bacteria. Occurs in all living organisms
Electron source Oxidation H2O at PSII Glucose, NADH + , FADH2
Catalyst Chlorophyll No catalyst
High electron potential
From light photons From breaking bonds
Mitochondrial redox carriers
NADH Complex I Q Complex III Cytochrome C Complex IV O2
The inner membrane contain four macromolecular complexes that catalyze the oxidation of
substrates such NADH/FADH2 through the action of metallo-proteins such as cytochromes
and iron sulfur proteins.
Complex I: NADH dehydrogenase
In mammals, there are 44 separate polypeptide chains, a
FMN and eight iron-sulfur clusters (FeS).
The structure of the 536 kDa complex comprises 16 different subunits with 64 transmembrane
helices and 9 Fe-S clusters.
There 14 ‘core’ subunits highly conserved from bacteria
Electron transfer mechanism
NADH is oxidized to NAD+, by reducing FMN to FMNH2 in one two-electron step. FMNH2
is then oxidized in two one-electron steps, through a semiquinone intermediate.
Each electron thus transfers from the FMNH2 to an Fe-S cluster to ubiquinone (Q). Transfer of
the first electron results in the free-radical Q* (semiquinone) and transfer of the second
electron reduces the Q* to QH2 (ubiquinol).
During this process, four protons are translocated from the mitochondrial matrix to the
The transfer of two electrons from NADH to oxygen, through complexes I, III (bc1) and IV
(cytochrome c oxidase) results in the translocation of 10 protons across the membrane,
creating the proton-motive force (pmf) for the synthesis of ATP by ATP synthase.
NADH + H+ + CoQ + 4H+
in → NAD+ + CoQH2 + 4H+
It catalyses the transfer of two electrons from NADH to ubiquinone, coupled to the
translocation of four protons across the bacterial or inner mitochondrial membrane
Complex I is a reversible machine able to utilize pmf and ubiquinol to reduce NAD+.
Complex II: Succinate dehydrogenase
Complex II consists of four protein subunits: SdhA, SdhB, SdhC and SdhD.
It is the only enzyme that participates in both the TCA and the ETC chain by catalyzing the
oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol.
Complex III: Cytochrome bc1 complex
Most of the primitive members of this family contain a b-type cytochrome, a c-type
cytochrome and an iron sulfur protein (ISP).
Core 1 Protein
Core 2 Protein
Isolated cytochrome bc1 complexes
from eukaryotic organisms contain
10/11 subunits including a b-type
cytochrome with two heme centres, an
iron-sulfur protein (Reiske protein) and
a mono heme c-type cytochrome.
The complex oxidizes quinols and transfers electrons to soluble acceptors such as cytochrome
QH2 + 2 cytochrome c (FeIII) + 2 H+
in → Q + 2 cytochrome c (FeII) + 4 H+
Complex IV: cytochrome c oxidase
Cytochrome c oxidase is the final complex of the respiratory chain catalyzing dioxygen
reduction to water. The complex contains several metal prosthetic sites and 14 protein
subunits in mammals. Isolation of cytochrome oxidase has two heme groups (a and a3)
together with two Cu centers (CuA and CuB).
Cyt c CuA heme a heme a3-CuB
The overall reaction:
4 Fe2+-cyt c + O2 + 8H+
in 4 Fe3+-
Cyt c + 2H2O + 4H+