Table of Contents
Photosystems are large protein–pigment complexes embedded in the thylakoid membrane of chloroplasts (and in some bacteria). In this chapter, the focus is on how these complexes are built and how their structure supports their function. The overall location of photosynthesis in chloroplasts and the general sequence of the light reactions are treated in other chapters.
Overview: Two Main Photosystems
In oxygenic photosynthesis of plants, algae, and cyanobacteria there are two types of photosystems working in series:
- Photosystem II (PSII) – often abbreviated as PSII, with a special reaction center chlorophyll called P680.
- Photosystem I (PSI) – abbreviated as PSI, with a special reaction center chlorophyll called P700.
Despite differences, both have a similar basic structural plan:
- A light-harvesting (antenna) complex
- A core complex with:
- A reaction center (special chlorophyll pair)
- Primary electron acceptors (and nearby redox cofactors)
- Surrounding accessory pigments and proteins that stabilize the complex and help with energy transfer.
Common Structural Principles
Antenna Complex (Light-Harvesting Complex, LHC)
The antenna complex captures light and funnels excitation energy to the reaction center.
Key elements:
- Pigments:
- Many chlorophyll a molecules
- Chlorophyll b (in plants) or other chlorophyll types in algae
- Carotenoids (e.g., β-carotene, xanthophylls)
- Proteins:
- Membrane proteins that bind pigments in precise positions
- This fixed arrangement optimizes energy transfer between pigments.
The pigments are arranged so that:
- Energy can move from pigments that absorb higher-energy (shorter wavelength) photons to those that absorb lower-energy (longer wavelength) photons.
- This movement occurs by resonance energy transfer (not electron transfer) until it reaches the reaction center chlorophylls.
Reaction Center
The reaction center is the “heart” of the photosystem.
It includes:
- A special pair of chlorophyll a molecules:
- In PSII: P680 (absorbs most efficiently around 680 nm)
- In PSI: P700 (absorbs most efficiently around 700 nm)
- Closely associated primary electron acceptors (e.g., pheophytin in PSII, special chlorophylls and phylloquinone in PSI).
- A series of redox cofactors (e.g., quinones, iron–sulfur clusters) that accept and pass on electrons.
The reaction center is built from core proteins embedded in the thylakoid membrane:
- Transmembrane helices anchor the complex.
- Specific amino acids coordinate metal ions (like Mg in chlorophyll, Mn in PSII) and hold cofactors in exact positions.
Pigment and Cofactor Arrangement
Both PSI and PSII have:
- Dozens to hundreds of pigment molecules arranged at precise distances and orientations.
- Electron carriers fixed in an ordered chain:
- Chlorophylls
- Pheophytin (in PSII)
- Quinones
- Iron–sulfur clusters (in PSI)
- Small, mobile carriers (plastoquinone, plastocyanin, ferredoxin) that connect to other complexes.
This fine-tuned geometry is essential for:
- Fast and directional energy transfer in the antenna.
- Fast and directional electron transfer in the reaction center.
Structure of Photosystem II (PSII)
Overall Organization
PSII is a large multiprotein–pigment complex located mainly in the grana thylakoid membranes in plants.
It can be divided into:
- Core complex (reaction center + inner antenna)
- Peripheral light-harvesting complexes (LHCII and minor LHCIIs)
- The oxygen-evolving complex (OEC), also called the water-splitting complex.
Core Proteins and Reaction Center
The core PSII complex includes:
- Two main transmembrane proteins: D1 and D2
- Each spans the membrane several times with α-helices.
- Together, they bind:
- The P680 special pair of chlorophyll a
- Nearby chlorophylls and pheophytin
- Two plastoquinone molecules (
Q_AandQ_B) as primary and secondary quinone acceptors. - Additional core antenna proteins: CP43 and CP47
- Bind extra chlorophyll a molecules surrounding D1/D2.
- Act as an inner light-harvesting antenna feeding energy to P680.
On the lumenal side (inside the thylakoid lumen):
- The oxygen-evolving complex (OEC) is attached to PSII.
- Several extrinsic proteins (e.g., PsbO, PsbP, PsbQ) stabilize the OEC.
On the stromal side:
- Plastoquinone binding sites are located (for
Q_AandQ_B). - The reducing side of PSII interfaces with the plastoquinone pool.
Pigments and Cofactors in PSII
Important bound components:
- Chlorophyll a:
- ~30–40 molecules in the core
- More in associated LHCII
- Arranged in a network for energy transfer to P680.
- Pheophytin:
- A chlorophyll derivative without central Mg.
- Acts as the first stable electron acceptor from P680.
- Plastoquinone:
Q_A: tightly bound, one-electron acceptor.Q_B: more loosely bound, accepts two electrons and two protons to form plastoquinol ($\mathrm{PQH_2}$), which then diffuses away.- Carotenoids:
- Protect chlorophylls from oxidative damage and participate in light harvesting.
- Non-heme iron:
- Located between
Q_AandQ_B, involved in electron transfer and stabilization.
Oxygen-Evolving Complex (Water-Splitting Complex)
Structurally unique to PSII is the OEC, a cluster on the lumenal side:
- Central Mn\(_4\)CaO\(_5\) cluster:
- 4 manganese ions
- 1 calcium ion
- Bridging oxygens
- Coordinated by amino acids from the D1 protein and surrounding proteins, plus water molecules.
This cluster:
- Cycles through several oxidation states (S-states) to extract electrons from water.
- Requires:
- 2 water molecules
- Provides:
- 4 electrons to refill P680 after its photo-oxidation events in sequence
- 4 protons to the lumen
- 1 molecule of O\(_2\).
Even though the detailed mechanism is part of the light-dependent reactions, structurally the OEC is a tightly bound inorganic–organic complex integrated into PSII’s protein environment.
Light-Harvesting Complex II (LHCII)
Around the PSII core there are multiple copies of LHCII trimers:
- Each LHCII monomer is a small membrane protein (3 transmembrane helices) binding:
- Several chlorophyll a
- Several chlorophyll b
- Carotenoids (e.g., lutein, neoxanthin)
- LHCII trimers form a peripheral antenna belt around the core complex.
Functionally, the arrangement:
- Increases the cross-sectional area for light capture.
- Allows flexible association and dissociation to adjust light harvesting (relevant for regulation, not detailed here).
- Transfers excitation to CP43/CP47 and then to P680.
Structure of Photosystem I (PSI)
Overall Organization
PSI is another large membrane protein complex, located mainly in the stroma lamellae and grana margins in plant thylakoids.
It consists of:
- A core complex (reaction center + core antenna)
- Peripherally bound light-harvesting complexes (LHCI)
- Binding site for the soluble electron carrier ferredoxin on the stromal side.
Core Proteins and Reaction Center
The PSI core includes several transmembrane proteins, mainly:
- PsaA and PsaB:
- Two large, homologous proteins.
- Form the central heterodimer.
- Bind the P700 special pair and the majority of core chlorophylls.
- Additional subunits (PsaC, PsaD, PsaE, etc.) located on the stromal side:
- Stabilize electron acceptors.
- Provide docking sites for ferredoxin and other interactors.
The reaction center region contains:
- P700:
- A special pair of chlorophyll a molecules (slightly modified) with an absorption maximum around 700 nm.
- Nearby accessory chlorophylls and pheophytin-like chlorophylls (though classical pheophytin plays a different role here compared with PSII).
- A chain of electron acceptors:
- Primary chlorophyll acceptor (often designated A\(_0\))
- Phylloquinone (vitamin K\(_1\), A\(_1\))
- Three iron–sulfur clusters:
- F\(_X\) (bound to PsaA/PsaB)
- F\(_A\) and F\(_B\) (bound to PsaC).
These cofactors are precisely positioned, with distances typically in the range of a few ångströms to nanometers, enabling very rapid electron transfer.
Pigments and Cofactors in PSI
PSI binds a large number of pigments:
- Chlorophylls:
- 100+ chlorophyll a molecules per PSI complex (core + LHCI).
- Many arranged in distinct “belt” or “cluster” patterns around the reaction center.
- Carotenoids:
- Distributed among core and LHCI, contributing to light harvesting and photoprotection.
- Phylloquinone:
- Acts as a one-electron carrier between early chlorophyll acceptors and iron–sulfur clusters.
- Iron–sulfur clusters:
- Contain Fe and S atoms in a cubane-like structure.
- Provide the final electron acceptor sites inside PSI before electrons move to soluble ferredoxin.
On the stromal side:
- Proteins PsaC, PsaD, and PsaE form a platform that:
- Holds the F\(_A\) and F\(_B\) clusters.
- Creates docking regions for ferredoxin and ferredoxin–NADP\(^+\) reductase (FNR).
Light-Harvesting Complex I (LHCI)
Adjacent to the PSI core are LHCI complexes, typically arranged as:
- A “half-moon” or crescent around one side of PSI in plants.
- Composed of several Lhca proteins, each:
- A membrane protein with 3 transmembrane helices.
- Binding multiple chlorophyll a and b and carotenoids.
Structurally, LHCI:
- Extends the antenna of PSI.
- Often has chlorophylls that absorb at slightly longer wavelengths (“red forms”), which:
- Capture far-red light.
- Transfer excitation to the P700 region with high efficiency.
Spatial Arrangement Within the Thylakoid Membrane
The thylakoid membrane is not uniform; its structural organization supports the function of both photosystems:
- Grana stacks (tight stacks of membranes):
- Enriched in PSII and LHCII.
- Favor high-density light harvesting and close packing of PSII–LHCII supercomplexes.
- Stroma lamellae and grana margins:
- Enriched in PSI and ATP synthase.
- Provide space for large stromal-side protein extensions (such as ferredoxin-binding surfaces in PSI).
This lateral separation:
- Minimizes physical crowding between large complexes.
- Helps organize linear electron flow from PSII (in grana) via the cytochrome b\(_6\)f complex to PSI (in stroma lamellae).
Supramolecular Organization: Supercomplexes
Both photosystems form supercomplexes by associating with their antenna complexes:
- PSII–LHCII supercomplexes:
- A PSII core dimer with multiple LHCII trimers and minor antenna complexes.
- Arranged in ordered patterns visible by electron microscopy.
- PSI–LHCI supercomplexes:
- A PSI core monomer (or trimer in some organisms) with attached LHCI complexes forming a distinct “crescent.”
These higher-order structures:
- Define the effective size and shape of each photosystem.
- Influence:
- How light is absorbed and shared.
- How excitations are transferred.
- How the photosystems respond structurally to changing light conditions (e.g., state transitions, discussed elsewhere).
Comparison of PSI and PSII Structure
Although both share a common design principle, there are key structural differences:
- Special Pair:
- PSII: P680, more oxidizing (stronger electron pull), enabling water splitting.
- PSI: P700, more reducing on the acceptor side, enabling reduction of ferredoxin.
- Unique Structural Feature:
- PSII: Mn\(_4\)CaO\(_5\) oxygen-evolving cluster on lumenal side.
- PSI: Iron–sulfur cluster chain (F\(_X\), F\(_A\), F\(_B\)) on stromal side.
- Antenna Size and Composition:
- PSII: Strong association with LHCII; large peripheral antenna, heavily stacked in grana.
- PSI: LHCI forms an asymmetric ring; often fewer chlorophyll b and a higher proportion of “red” chlorophyll forms.
- Localization:
- PSII: Mainly in stacked grana regions.
- PSI: Mainly in unstacked stroma lamellae and margins.
These structural distinctions match their roles at opposite ends of the light-dependent electron transport chain.
Why Structure Matters for Function
The structure of photosystems is not arbitrary; it is tightly coupled to their function:
- Precise distances and orientations between pigments allow rapid, directional energy transfer.
- Ordered redox cofactor chains ensure stepwise downhill movement of electrons with minimal energy loss.
- The integration of metal clusters (Mn\(_4\)CaO\(_5\), Fe–S) enables hard chemical tasks:
- Oxidation of water (PSII)
- Reduction of ferredoxin (PSI).
- Association with antenna complexes controls how much light each photosystem can capture and how they balance their activities.
Later chapters on the process of the light-dependent reactions build directly on this structural organization, showing how these complexes work together as an integrated system.