Photosynthesis: Photoprotection of the Photosynthetic Apparatus

 

Photoprotection of the Photosynthetic Apparatus

Photoprotection is the process by which photosynthetic organisms protect themselves from the damaging effects of excess light energy, a condition known as photoinhibition. 

The photosynthetic membrane can be easily damaged if the large amount of energy absorbed by pigments cannot be stored through photochemistry. Photoprotection acts as a safety valve, dissipating this excess energy before it can cause harm.

The Role of Carotenoids

Carotenoids are essential accessory pigments that play a vital role in photoprotection in addition to their light-harvesting function. They protect the photosynthetic apparatus by a process called quenching, where they rapidly dissipate the excess energy from excited chlorophyll molecules.

If an excited chlorophyll molecule is not quickly quenched by energy transfer or photochemistry, it can react with molecular oxygen to form highly reactive singlet oxygen. Singlet oxygen can cause oxidative damage to various cellular components, particularly the lipids within the thylakoid membranes. 

Carotenoids prevent this by accepting the excess energy from excited chlorophyll and returning to their ground state by releasing the energy as harmless heat. The excited state of carotenoids does not have enough energy to form singlet oxygen, making them an effective shield against this damage.

Non-photochemical Quenching (NPQ) and the Xanthophyll Cycle

A major process of photoprotection is non-photochemical quenching (NPQ), which involves dissipating excess light energy as heat. It is often described as a "volume knob" that adjusts the flow of excitations to the Photosystem II (PSII) reaction center to a manageable level.

A specific group of carotenoids called xanthophylls are involved in NPQ. The xanthophyll cycle is a key regulatory mechanism that involves the interconversion of three xanthophylls: violaxanthin, antheraxanthin, and zeaxanthin. Under high light conditions, the enzyme violaxanthin de-epoxidase converts violaxanthin to zeaxanthin via antheraxanthin. The binding of protons and zeaxanthin to specific light-harvesting antenna proteins (like the PsbS protein) is believed to cause conformational changes that promote the quenching of chlorophyll and heat dissipation.

Xanthophyll cycle

Photoinhibition and Repair Mechanisms

Even with these protective mechanisms, excess light can lead to photoinhibition, which can inactivate and damage the PSII reaction center. When this happens, a multi-level defense system is activated:

1. Quenching: The first line of defense is the dissipation of excess energy as heat.

2. Scavenging: If quenching is insufficient, the formation of toxic photoproducts is followed by the action of various scavenging systems that eliminate these reactive molecules.

3. Repair: If the damage persists, the main target is the D1 protein of the PSII reaction center. The damaged D1 protein is removed from the membrane and a newly synthesized D1 protein is inserted to restore the function of the PSII reaction center. This ensures that the other undamaged components of the PSII complex are recycled.