In biochemical terms, pheophytin is a chlorophyll molecule lacking a central Mg2+ ion. It can be produced from chlorophyll by treatment with a weak acid, producing a dark bluish waxy pigment. The probable etymology comes from this description, with pheo meaning dusky and phyt meaning vegetation.
History and discovery
In the 1977s, scientists Klevanik, Klimov, Shuvalov performed a series of experiments to demonstrate that it is pheophytin and not plastoquinone that serves as the primary electron acceptor in photosystem II. Using several experiments, including electron paramagnetic resonance, they were able to show that pheophytin was reducible and, therefore, the primary electron acceptor between P680 and plastoquinone. This discovery was met with fierce opposition, since many believed pheophytin to only be a byproduct of chlorophyll degradation. Therefore, more experiments ensued to prove that pheophytin is indeed the primary electron acceptor of PSII, occurring between P680 and plastoquinone. The data that was obtained is as follows:
Photo-reduction of pheophytin has been observed in various mixtures containing PSII reaction centers.
The quantity of pheophytin is in direct proportion to the number of PSII reaction centers.
Photo-reduction of pheophytin occurs at temperatures as low as 100K, and is observed after the reduction of plastoquinone.
These observations are all characteristic of photo-conversions of reaction center components.
Reaction in purple bacteria
Pheophytin is the first electron carrier intermediate in the photoreaction center of purple bacteria. Its involvement in this system can be broken down into 5 basic steps. The first step is excitation of the bacteriochlorophylls2 or the special pair of chlorophylls. This can be seen in the following reaction.
2 + 1 photon → 2*
The second step involves the 2 passing an electron to pheophytin, producing a negatively charged radical and a positively charged radical, which results in a charge separation.
2* + Pheo → ·2+ + ·Pheo−
The third step is the rapidelectron movement to the tightlybound menaquinone, QA, which immediately donates the electrons to a second, loosely bound quinone. Two electron transfers convert QB to its reduced form.
2·Pheo− + 2H+ + QB → 2Pheo + QBH2
The fifth and final step involves the filling of the “hole” in the special pair by an electron from a heme in cytochromec. This regenerates the substrates and completes the cycle, allowing for subsequent reactions to take place.
Involvement in photosystem II
In photosystem II, pheophytin plays a very similar role. It again acts as the first electron carrier intermediate in the photosystem. After P680 becomes excited to P680*, it transfers an electron to pheophytin, which converts the molecule into a negatively charged radical. Two negatively charged pheophytin radicals quickly pass their extra electrons to two consecutive plastoquinone molecules. Eventually, the electrons pass through the cytochrome b6f molecule and leaves photosystem II. The reactions outlined above in the section concerning purple bacteria give a general illustration of the actual movement of the electrons through pheophytin and the photosystem. The overall scheme is:
