Synchrotron Sheds Light On Bacteria’s Solar Cell

Researchers based at the University of Glasgow, using X-ray data collected at the Synchrotron Radiation Source (SRS) at CCLRC Daresbury Laboratory, have made a major advance in our understanding of the process by which sunlight is converted to food energy, without which life on earth could not exist. The work is published this week (12 December 2003) in the journal Science.

Green plants convert the sun’s energy to a usable form in a process called photosynthesis, which ultimately gives us all the oxygen and food we need to survive. Photosynthetic bacteria have evolved to do all this efficiently in a single cell, so they make good model systems. The Glasgow team, led by Professors Richard Cogdell and Neil Isaacs, worked out the structure of the LH1 light-absorbing complex and Reaction Centre that lies at the heart of photosynthesis in the purple bacterium Rhodopseudomonas palustris.

They first isolated and crystallised the intact protein complex from the bacterial cell membrane, then recorded its X-ray diffraction pattern using X-rays generated at the Daresbury synchrotron.‘The highly focused and intense X-ray beam provided at Daresbury was essential for this data collection’, commented Professor Isaacs.

The X-ray data helped to solve a long-standing mystery about the structure of the LH1-RC. Solar energy absorbed by the light harvesting complex is used by the Reaction Centre to power the transfer of electrons across the cell membrane, using a shuttle molecule to carry the electrons. Researchers have been puzzled about how this shuttle molecule gets in and out of the Reaction Centre, which is surrounded by the ring of protein molecules that makes up the LH1. The structure shows that the LH1 ring has a molecular ‘gate’ to enable the shuttle molecule to move freely.

Since 1984 the structures of only 25 membrane proteins have been worked out, compared with around 15,000 soluble ones. ‘Membrane proteins are notoriously difficult to crystallise in the first instance,’ explained Miroslav Papiz, Head of the Biology and Medicine College at Daresbury, ‘and when crystals are obtained they nearly always diffract very weakly. This is why such an intense source of X-rays is needed to study them.’

This work is the third major breakthrough in this fundamental area of biological research to be based on X-ray crystallographic data collected at the SRS. In 1995 the teams of Richard Cogdell and Neil Isaacs at Glasgow, in collaboration with Miroslav Papiz and the Daresbury team, elucidated the structure of another key component of the light-harvesting machinery, the LH2 complex, from the purple bacterium Rhodopseudomonas acidophila. The LH2 complex funnels energy into the LH1 complex. This year the resolution of this structure has been further improved, helping to reveal more details about energy transfer within it.

Meanwhile, in 1997 John Walker from the Laboratory for Molecular Biology in Cambridge was awarded a Nobel Prize for his research on the enzyme responsible for formation of the energy-rich molecule ATP at the end of this energy transfer sequence, based on crystallographic studies done at the SRS in 1995.