20.4: The Photochemical Reaction Center

Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic pigments present within a photosynthetic unit, only a few have photochemical activity. The reaction centers contain fewer pigment molecules that cannot lead to efficient photosynthesis. Therefore, an antenna complex, having a large number and variety of pigments, accumulates significant quantities of photons and transmits their energy to the reaction center. The photochemically active pigments of the reaction center then convert light energy into high-energy electrons that are utilized for downstream chemical reactions.

In oxygen liberating organisms such as cyanobacteria, green and red algae, and higher plants, the reaction center is found inside both photosystems I and II. It is also present in bacteria with a single photosystem. The reaction center of the bacterium- Rhodopseudomonas viridis is the first integral membrane-bound protein to be isolated and studied. The molecular architecture of the R. viridis reaction center resembles the PSII reaction center but lacks an oxygen-evolving complex and, therefore, the ability to produce oxygen. It comprises four protein subunits, called H, L, M, cytochrome, and 14 cofactors that include bacterial chlorophyll and carotenoids. The L and M subunits harbor two bacterial chlorophyll molecules that absorb a photon of 870nm wavelength, thus the name P870 reaction center. The R. viridis reaction center also comprises a bacterial pheophytin that acts as the primary electron acceptor, a mobile quinone as the terminal electron acceptor, a membrane-bound cytochromes bc1, and a periplasmic cytochrome bc2. Together, these form an integral component of the light-driven electron transfer cycle in R. viridis.

A photochemical reaction center is a pigment-protein complex located at the core of each photosystem.

The reaction centers are grouped into two categories depending on their terminal electron acceptors.

Type I reaction center has a ferredoxin and type II has a plastoquinone molecule as the terminal electron acceptor.

Each reaction center also has a special pair of chlorophyll a molecules that absorb specific wavelengths of light.

The PSI reaction center absorbs photons with a wavelength of 700 nanometers and is, therefore, called P700.

In contrast, the PSII reaction center absorbs a wavelength of 680 nanometers and is called P680.

Despite their differences, both PSI and PSII work on a very similar principle.

When the chlorophyll pair in a reaction center absorbs a photon, one of its ground state electrons attains an excited energy level.

Consequently, the chlorophyll molecule becomes excited and must lose the excess energy it possesses for stability. Thus, it emits the energized electron to a nearby electron acceptor.

The positively charged chlorophyll ion pair then withdraws electrons from a donor, like a water molecule, to return to its original state.