P700
P700, or photosystem I primary donor, is a molecular dimer of chlorophyll a associated with the reaction center of photosystem I in plants, algae, and cyanobacteria.
Etymology
Its name is derived from the word pigment and the presence of a major bleaching band centered around 695–700 nm in the flash-induced absorbance difference spectra of P700/P700+•.Components
The structure of P700 consists of a heterodimer with two distinct chlorophyll molecules, specifically chlorophyll a and chlorophyll a, giving it the additional name of special pair. The pair behaves as if it were just one molecule. This dimer is vital to photosystem I, due to its ability to absorb light energy with a wavelength approximately between 430 nm-700 nm, and transfer high-energy electrons to a series of electron acceptors that are situated near it, like the iron–sulfur protein, ferredoxin, which has a higher redox potential, i.e. a greater affinity for electrons.Action and functions
Photosystem I produces NADPH, the reduced form of NADP, at the end of the photosynthetic reaction through electron transfer, and provides energy to a proton pump and eventually ATP, for instance in cyclic electron transport.Excitation
When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll. The resulting P700 with an excited electron is designated as P700*, which is a strong reducing agent due to its very negative redox potential of -1.2 V.Electron transport chain
Following the excitation of P700, one of its electrons is passed on to an electron acceptor, A, triggering charge separation and producing an anionic A and cationic P700. Subsequently, electron transfer continues from A to a phylloquinone molecule known as A, and then to three iron–sulfur clusters.Type I photosystems use iron–sulfur proteins as terminal electron acceptors. Thus, the electron is transferred from F to another iron–sulfur cluster, F, and then passed on to the last iron–sulfur cluster serving as an electron acceptor, F. Eventually, the electron is transferred to ferredoxin, causing it to transform into its reduced form, which finalizes the process by reducing NADP to NADPH.