Abstract
Based on the current model of its structure and function, photosystem II (PSII) seems to have evolved from an ancestor that was homodimeric in terms of its protein core and contained a special pair of chlorophylls as the photo-oxidizable cofactor. It is proposed that the key event in the evolution of PSII was a mutation that resulted in the separation of the two pigments that made up the special chlorophyll pair, making them into two chlorophylls that were neither special nor paired. These ordinary chlorophylls, along with the two adjacent monomeric chlorophylls, were very oxidizing: a property proposed to be intrinsic to monomeric chlorophylls in the environment provided by reaction centre (RC) proteins. It seems likely that other (mainly electrostatic) changes in the environments of the pigments probably tuned their redox potentials further but these changes would have been minor compared with the redox jump imposed by splitting of the special pair. This sudden increase in redox potential allowed the development of oxygen evolution. The highly oxidizing homodimeric RC would probably have been not only inefficient in terms of photochemistry and charge storage but also wasteful in terms of protein or pigments undergoing damage due to the oxidative chemistry. These problems would have constituted selective pressures in favour of the lop-sided, heterodimeric system that exists as PSII today, in which the highly oxidized species are limited to only one side of the heterodimer: the sacrificial, rapidly turned-over D1 protein. It is also suggested that one reason for maintaining an oxidizable tyrosine, TyrD, on the D2 side of the RC, is that the proton associated with its tyrosyl radical, has an electrostatic role in confining P(+) to the expendable D1 side.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
More From: Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.