Cellulose isn't just cellulose

Researchers from KU-LIFE have recently discovered how one structure of cellulose, known as slip planes is important for the interaction between cellulose and enzymes.

The implications of this phenomenon have not yet been recognized or explored within the use of cellulose for fuels and chemicals, and may provide us with a new and more efficient tool for using biomass as a replacement of fossil fuels.

Cellulose is nature’s essential ‘LEGO brick’

Cellulose is by far the most abundant renewable material. Every year the photosynthesis produces more than 75 billion tons of cellulose on the basis of CO2 and water.

In every plant cellulose is a main structural element providing strength and structure to all plant organs, it is one of the essential "LEGO bricks" of nature. The structure of cellulose is quite unique, it is build up from the simple sugar glucose to form a linear polymer of glucan chains organised into microfibrils (see figure 1).

The organisation of the microfibrils provides cellulose with a crystal like structure, providing a tensile strength stronger than steel and making it insoluble in water and resistant to microbial degradation.

But there is much more to the structure of cellulose. Different structures and irregularities can be found all of which may have an impact on the properties of cellulose. Researchers from KU-LIFE have recently discovered how one such structure, known as slip planes are important for the interaction between cellulose and enzymes.

Cellulose for materials, fuels and chemicals

Being such an abundant material composed of sugar, there are a number of everyday applications for cellulose. The best known use of cellulose is for paper and textiles. The major part of newspaper, containers, office paper etc. is made from cellulose, just as cotton and viscose (regenerated cellulose) are used in many textiles.

But as cellulose is made from sugar it can also be processed to fuels and chemicals by microorganisms fermenting the sugar. Some examples are the conversion of cellulose to 2nd generation bioethanol by yeast or the production of bio-plastic based on lactic acid from bacteria.

The challenge here is not the conversion of sugar itself; it is about deconstructing the cellulose chains into individual cellulose molecules. For this purpose enzymes capable of hydrolysing the glucan chains are used. One example of this is the model shown in figure 2, where an enzyme protein is detaching the glucan chain and cleaving it.

The enzymes acting on cellulose are quite slow and much research is focused on understanding the interaction and role of the cellulose structure for enzyme hydrolysis.

New findings: Slip plane structures in cellulose

In a recent study we found that the irregular regions of the cellulose fibrils, known as slip planes does not have an amorphous structure as previously believed, but rather they have a crystal structure not that different from the remaining part of the cellulose structure. Using polarized light microscopy we found that slip planes are birefringent, which shows they have a crystalline organisation.

We also found that the slip planes are entry points for some of the most important enzymes for breaking cellulose down to sugars; the endoglucanases. Using a fluorescent labelled endoglucanase combined with confocal fluorescence microscopy, we found that the enzyme selectively binds to dislocations during the initial phase of the hydrolysis (see figure 3).

When we applied a full commercial cellulase mixture on hydrothermally treated wheat straw, the fibers were cut into segments corresponding to the sections between the slip planes initially present. Our findings show that not only does cellulose have a different organisation of the slip planes at the supramolecular level; these structures are apparently also the entry points for the initial enzymatic breakdown of cellulose.

The results indicate that dislocations are important during the initial part of enzymatic hydrolysis of cellulose.

The implications of this phenomenon have not yet been recognized or explored within the use of cellulose for fuels and chemicals, and may provide us with a new and more efficient tool for using biomass as a replacement of fossil fuels.

The study is published in an article called ‘Role of supramolecular cellulose structures in enzymatic hydrolysis of plant cell walls’ in Journal of Industrial Microbiology & Biotechnology.