Plant Photosynthetic Reaction Centers

In plants, algae and cyanobacteria:

Oxygenic and anoxygenic photosynthesis

2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2

carbon dioxide + water + light energy → carbohydrate + oxygen

Today, the average rate of

energy capture by

photosynthesis globally is

~130 terawatts which is about

six times larger than the current

power consumption of human

civilization.

However, there are some types of bacteria that carry out anoxygenic photosynthesis, which

consumes carbon dioxide but does not release oxygen.

CO2 + (AsO33–) + photons → (AsO4

3–) + CO

carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other

compounds in subsequent reactions)

Examples of organisms belonging to anoxygenic photosynthesis are the filamentous green

bacteria, the green sulfur bacteria, the purple bacteria and the heliobacteria.

The basic difference between oxygenic and bacterial photosynthesis is that bacteria have only

one type of reaction center, whereas oxygenic species have two (Photosystem I and II).

These photosynthetic bacteria can use light energy to extract electrons from molecules other

than water such as sulfur compounds or simple organic molecules.

Green plants and algae have two different types of reaction centers that are part of larger

supercomplexes known as photosystem I (P700) and photosystem II (P680).

Transforming light energy into charge separation

The purple bacterial reaction center shares a striking similarity to Photosystem II of plants and

cyanobacteria.

Photosystem II

The overall reaction catalyzed by photosystem II is:

The colors of photosynthesis

The light-absorbing molecules include green chlorophylls and orange carotenoids.

Photosystem II: the reaction center

Photosystem II: harvesting light

Photosystems have large antennas of light-absorbing molecules that harvest light and transfer

their energy inwards to the reaction center.

The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino

acid, which then delivers it to the chlorophyll, making it ready to absorb another photon.

It contains cluster of manganese ions

(magenta), calcium (cyan), chloride ion

and oxygen atoms (red). It grips two water

molecules and removes four electrons,

forming oxygen gas and four hydrogen

ions.

It is surrounded by histidines, aspartates

and glutamates.

Photosystem II: oxygen-evolving center

Photosystem II: Synechococcus elongatus structure

The lumenal side reaction center proteins (D1 & D2): A through E. The peripheral antenna proteins

(CP43 & CP47): six membrane spanning α-helices. The PsbO protein: the water splitting enzyme of

photosynthesis. PsbV (cytochrome c-550): plays role in the oxygen-evolving complex of photosystem II.

Because of the oxidation of water and the release of dioxygen, the Photosystem II reaction

center does not operate in a cycle.

Bacterial RC vs Photosystem II

The released protons from water oxidation are deposited on one side of the thylakoid

membrane to help build the electrochemical gradient.

In the bacterial reaction centre, the electron is obtained from a reduced compound haem group

in a cytochrome subunit or from a water-soluble cytochrome-c protein.

In addition, Photosystem II differs from the bacterial reaction centre in that it has many

additional subunits that bind additional chlorophylls to increase efficiency.

Cytochrome b6f complex

It catalyzing the transfer of electrons from plastoquinol to plastocyanin.

Crystal structure from C. reinhardtii

Each monomer consists of

four large subunits: cytochrome f (32

kDa), a cytochrome b6 (25 kDa), a 19

kDa Rieske iron-sulfur protein (19 kDa),

subunit IV (17 kDa);

four small subunits (3-4 kDa): PetG,

PetL, PetM, and PetN.

The total molecular weight is ~217 kDa.

Reaction mechanism of cytochrome b6f complex

The cytochrome b6f complex is responsible for ‘non-cyclic’ and ‘cyclic’ electron transfer

between two mobile redox carriers, plastoquinone (QH2) and plastocyanin

H2OPhotosystem II (1) QH2 Cyt b6f Pc Photosystem I NADPH

(2) QH2 Cyt b6f Pc Photosystem I Q

Plastocyanin

Plastocyanin (Cu2+Pc) is reduced by cytochrome f

according to the following reaction:

Cu2+Pc + e- → Cu+Pc

After dissociation, Cu+Pc diffuses through the

lumen until recognition/binding occurs with P700+

which oxidizes Cu+Pc according to:

Cu+Pc → Cu2+Pc + e-