Gribble enzyme might be holy grail for biofuel

Limnoria – the wood-eating gribble

Limnoria – the wood-eating gribble CREDIT: Laura Michie (Portsmouth PhD student) with thanks for access to equipment to Alex Ball, Electron Microscope Unit Manager, Imaging and Analysis Centre at the Natural History Museum

Scientists at the Universities of Portsmouth and York have discovered a new enzyme that could prove vital in the quest to turn waste paper, wood and straw into liquid fuel.

Dr Simon Cragg and Dr John McGeehan, from Portsmouth, and colleagues made their discovery after examining the gut of gribbles – a tiny marine wood-borer which destroys seaside piers.

Using advanced biochemical analysis and x-ray imaging, researchers in Portsmouth, York and at the National Renewable Energy Laboratory in the US, have identified the enzyme that allows gribbles to digest enormous quantities of wood and produced the first three dimensional image of it which reveals how it works.

The findings, published in PNAS (Proceedings of the National Academy of Sciences of the USA), will help the researchers to reproduce the enzyme’s effects on an industrial scale which is likely to lead to the generation of liquid biofuels from sustainable resources.

2.The 3D X-ray structure allows scientists to see inside the enzyme and reveals how it binds and digests cellulose chains

2. The 3D X-ray structure allows scientists to see inside the enzyme and reveals how it binds and digests cellulose chains

Dr McGeehan said: “This is a truly collaborative and exciting breakthrough. To create liquid fuel from wood and straw, the polysaccharides – sugar polymers – that make up the bulk of these materials have to be broken down into simple sugars. These are then fermented to produce liquid biofuels, but it is a difficult and expensive process.

“When Simon Cragg first looked at gribbles through an electron microscope he was able to see their gut was completely sterile, which means nature had got there before us. What is happening inside a gribble’s gut is nature’s own bioreactor and now we have the blueprint of this enzyme, we just need to copy it.”

3.The 3D structure shows the tunnel where the enzyme feeds in the cellulose chains for digestion. The red colour represents the highly acidic surface that allows it to be stable and active in very high salt condition

3. The 3D structure shows the tunnel where the enzyme feeds in the cellulose chains for digestion. The red colour represents the highly acidic surface that allows it to be stable and active in very high salt condition

The researchers have transferred the genetic blueprint of this enzyme to an industrial microbe that can produce it in large quantities, in the same way that enzymes for biological washing detergents are made. By doing this they hope to cut the costs of turning woody materials into biofuels.

Enzymes are proteins that serve as catalysts, in this case one that degrades cellulose. Their function is determined by their three-dimensional shape, but these are tiny entities that cannot be seen with high power microscopes. Instead, the researchers made crystals of the proteins, where millions of copies of the protein are arrayed in the same orientation.

Dr McGeehan said: “The major breakthrough came when colleagues Dr Simon Streeter and Richard Martin successfully made crystals of the enzyme in the Portsmouth Crystallography Facility.  We then transported them under liquid nitrogen to the Diamond Light Source, the UK’s national synchrotron science facility. Rather than magnify the enzyme with a lens as in a standard microscope, we fired an intense beam of X-rays at the crystals to generate a series of images that can be transformed into a 3D model.

“The Diamond synchrotron produced such good data that we could visualise the position of every single atom in the enzyme. Our US colleagues then used powerful supercomputers to model the enzyme in action. Together these results help to reveal how the cellulose chains are digested into glucose.”

This information will help the researchers to design more robust enzymes for industrial applications. While similar cellulases have been found in wood-degrading fungi, the enzyme from gribble shows some important differences. In particular, the gribble cellulase is extremely resistant to aggressive chemical environments and can work in conditions seven times saltier than sea water. Being robust in difficult environments means that the enzymes can last much longer when working under industrial conditions and so less enzyme will be needed.

Dr Cragg said: “Enzymes of this type are common in fungi, but this is the first animal enzyme to be explored and it has much to teach us.”.

Dr McGeehan said: “The 3D structure has revealed that although the skeleton of the gribble enzyme is very similar to the equivalent enzymes in fungi, the surface is unrecognisable. It appears that the consequence of evolution in a marine environment is an acidic coat that protects the enzyme from high concentrations of salt. This unusual salt tolerance and stability represent exciting properties with great potential benefit to industry.”

“The robust nature of the enzymes makes it compatible for use in conjunction with sea water, which would lower the costs of processing. Lowering the cost of enzymes is seen as critical for making biofuels from woody materials cost effective. Its robustness would also give the enzymes a longer working life and allow it to be recovered and re-used during processing.”

The work is part of the BBSRC Sustainable Bioenergy Centre, a £24m investment that brings together six world-class research programmes to develop the UK’s bioenergy research capacity.

Douglas Kell, BBSRC’s chief executive, said: “This is an exciting step in realising the potential of these important enzymes. If we can harness them effectively, waste materials could be used to make sustainable fuels. It’s a double bonus; avoiding competition with land for food production as well as utilising unused materials from timber and agricultural industries.”

 

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