BT631-26-Membrane_proteins

Plant Photosynthetic Reaction Centers

Photosystem I

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

trimer.

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

electrons.

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

reaction center.

Electron Transfer Rates

Why electrons are transferred to ferredoxin than to plastocyanin?

Ferredoxin

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

ferredoxins: 10-11

Ferredoxin-NADP+ Reductase

2 reduced ferredoxin + NADP+ + H+ ↔ 2 oxidized

ferredoxin + NADPH

NADP-binding site

The transfer of electrons from reduced ferredoxin to NADP+ is catalyzed by ferredoxin-

NADP+-reductase.

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.

Glu312

Ser38

Type I and Type II Reaction Centers

A summary of the five distinct photosynthetic reaction centers known

Respiratory systems

Photosynthetic systems

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.

Respiratory complexes

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

Photosynthesis 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

dioxide

Carbon dioxide is absorbed and oxygen is released Oxygen is absorbed and carbon dioxide is

released

What powers ATP synthase H+ gradient across thylakoid membrane into stroma H+ gradient across the inner mitochondria

membrane into matrix

What pumps protons across

the membrane

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

energy

From light photons From breaking bonds

Mitochondrial redox carriers

NADH Complex I Q Complex III Cytochrome C Complex IV O2

Complex II

FADH

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

to humans.

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

intermembrane space.

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+

out

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

Overall reaction

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.

Electron transport

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).

Cytochrome b

Rieske Protein

Core 1 Protein

Core 2 Protein

Matrix side

Cytosolic side

Transmembrane region

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

c.

QH2 + 2 cytochrome c (FeIII) + 2 H+in → Q + 2 cytochrome c (FeII) + 4 H+

out

Electron transport

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).

Electron transport

Cyt c CuA heme a heme a3-CuB

The overall reaction:

4 Fe2+-cyt c + O2 + 8H+in 4 Fe3+-

Cyt c + 2H2O + 4H+out