An international team has used solid state NMR spectroscopy to determine the structure of the chlorophyll molecules in green bacteria that are responsible for harvesting light energy. The discovery might ultimately lead to artificial photosynthetic systems.

Green bacteria are a group of organisms that commonly exist in extremely low-light environments, such as in light-deprived regions of hot springs and at depths of 100 metres in the Black Sea. The bacteria contain structures called chlorosomes, which contain up to 250,000 chlorophyll molecules. Chlorosomes are the most efficient light-harvesting antennae found in nature.

"The ability to capture light energy and rapidly deliver it to where it needs to go is essential to these bacteria, some of which see only a few photons of light per chlorophyll per day," explains lead researcher Donald Bryant of Pennsylvania State University. He adds that it is the orientation of chlorophylls in green bacteria that make them so highly efficient at harvesting light energy.

However, little else was known about the chlorosomes in green bacteria because they are so difficult to study. X-ray crystallography would be the usual analytical tool of choice for obtaining a three-dimensional molecular structure, but each chlorosome found in green bacteria has a unique organizational structure, says Bryant, which means they are not amenable to crystallography. Bryant offers a culinary simile to explain the problem.

"The chlorosomes are like little andouille sausages. When you take cross-sections of andouille sausages, you see different patterns of meat and fat; no two sausages are alike in size or content, although there is some structure inside, nevertheless. Chlorosomes in green bacteria are like andouille sausages, and the variability in their compositions had prevented scientists from using X-ray crystallography to characterize the internal structure," he says.

To side step this problem, Bryant and colleagues at Leiden Institute of Chemistry and the Groningen Biomolecular Sciences and Biotechnology Institute in the Netherlands, and the Max Planck Institute in Germany, first used genetic techniques to create a mutant green bacterium that has a more regular internal structure. This involved inactivating three genes that green bacteria acquired late in their evolution, which the team suspected are responsible for improving the bacteria's light-harvesting capabilities to produce an altogether simpler "model" organism.

They then used cryo-electron microscopy to identify the larger distance constraints for the chlorosome. The cryo-micrographs revealed that chlorophyll molecules within the wild type chlorosomes have a tubular shape on the nanoscale with one concentric tube fitting neatly inside the next, like nesting Russian Matryoshka dolls. "The mutant bacterium's chlorosomes contain only one set of tubes," says Bryant, which avoids the andouille sausage problem.

The team next used solid-state NMR spectroscopy to determine the structure of the chlorophyll molecules within the chlorosome.

"The NMR data revealed to us that the individual chlorophyll molecules in green bacteria are arranged in dimers, with their long hydrophobic, tails sticking out of either side," explains Bryant. "We also learned precisely how the chlorophyll molecules attach to one another, and we were able to measure the distance between chlorophyll molecules."

The NMR data also showed that the chlorophylls are arranged in helices. In the mutant bacteria, the chlorophyll molecules are almost perpendicular to the long axis of the nanotubes, whereas the angle is less steep in the wild-type organism. "It's the orientation of the chlorophyll molecules that is the most important thing here," adds Bryant.

Finally, computer modelling then allowed them to combine all of the information and to piece together a complete picture of the green bacterial chlorosome.

"At first it seems counterintuitive that green bacteria have managed to evolve a better light-harvesting system by increasing disorder in the chlorosome structure," says Bryant. Instinctively, you might assume that greater order would be better. However, more order would allow energy from absorbed photons to remain in an unusable state for longer, wandering from chlorophyll to chlorophyll.

In the wild-type form, the different domains containing chlorophyll molecules inhibits this rambling, which means the energy is passed on to the next staging post for conversion into chemical energy in photosynthesis much faster, which benefits the green bacterium in the wild.

Related links:

• Proc Natl Acad Sci USA, 2009, in press
• Bryant Group

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.