A first direct glimpse of photosynthesis in action
Researchers visualize the processes by which photosynthetic proteins split water into hydrogen and oxygen
An international team of researchers, including scientists from the Max Planck Institute for Medical Research in Heidelberg, has just a reported a major step in understanding photosynthesis, the process by which the Earth first gained and now maintains the oxygen in its atmosphere and which is therefore crucial for all higher forms of life on earth.
The researchers report the first direct visualization of a crucial event in the photosynthetic reaction, namely the step in which a specific protein complex, photosystem II, splits water into hydrogen and oxygen using energy provided by light. This is a catalytic process in which the molecules of photosystem II enable and promote the reaction without themselves being consumed. Given the very high sensitivity of photosystem II to radiation damage, the photosynthetic reaction cannot be followed by standard methods of structural investigation such as conventional time-resolved X-ray crystallography. It is, however, amenable to study using the very recently developed method of protein crystallography with free-electron lasers.
In this technique, exceedingly short but extremely intense pulses of X-rays are used to gather data from very small crystals. The pulses are so short, in fact, that they “outrun” most effects of radiation damage, including the complete annihilation of the sample that inevitably follows on much longer time scales. The technique is thus well suited for collecting data from highly sensitive systems such as this catalyzed splitting of water in photosynthesis. Crucial to the process is a special site within the photosystem-II-molecule that contains four manganese atoms and one calcium atom. The experimental measurements show large structural changes in this particular metal cluster, which elongates significantly.
The measurements were made at the SLAC National Accelerator Laboratory in Stanford, using the short and intense flashes from SLAC’s X-ray laser, the Linac Coherent Light Source (LCLS). The international team from 18 different institutions was led by Petra Fromme (Arizona State University) and included, in addition to the Heidelberg group, members from SLAC and the University of Hamburg. The Heidelberg team contributed expertise on injecting a thick slurry (suspension) of crystals as a micron-sized jet of particles to be intersected by the femtosecond X-ray pulses from the free electron laser. Crucial to this injection was the design, manufacture and operation of a temperature-controlled, anti-settling device to allow uninterrupted sample injection over the course of many hours.
This work is significant not only for its direct relevance to understanding photosynthesis, but also because it directly proves the feasibility of performing dynamic X-ray diffraction measurements at room temperature - in particular using free-electron lasers - to study mechanisms of the fast enzyme reactions that are characteristic of so many processes in living organisms.