Enzymatic conversion of cellulose
The ethanol project
Ethanol can be used as fuel for vehicles. Ethanol (produced from plants) is also very good for reduction of the "greenhouse effect". In Lund we are working on the development of a process for producing ethanol from wood, the ethanol project.
- The wood is pretreated to facilitate the hydrolysis. Parts of the hemicellulose are hydrolysed during this step.
- The cellulose is hydrolysed to glucose by enzymes, cellulases.
- The glucose is fermented to ethanol by yeast.
At the department of Biochemistry we are studying the cellulose hydrolysing enzymes, cellulases. We use enzymes produced by the fungus Trichoderma reesei. This is a well studied micro-organism which produces a set of enzymes known to hydrolyse cellulose efficiently.
Cellulose hydrolysing enzymes, cellulases
The cellulases can be divided in two classes, endoglucanases (EG) and cellobiohydrolases (CBH) (figure).Cellobiohydrolases hydrolyse the cellulose chain from one end whereas endoglucanases hydrolyse randomly along the cellulose chain. T. reesei produces two cellobiohydrolases and at least five endoglucanases. Cellobiohydrolases and endo-glucanases have been shown to hydrolyse cellulose cooperatively, i.e. they act in synergy. This is not surprising since the product from an endo-glucanase is a substrate for a cellobiohydrolase. However, several aspects of the synergism between cellulases are still not understood. Cellulose is an insoluble compound. Therefore, the enzymes first have to adsorb to the substrate before they can start the hydrolysis. All T. reesei cellulases (except EGIII) have a common two domain structure to facilitate this, a cellulose binding domain (CDB) and a catalytic domain. The CBD is very similar for the different cellulases. The three dimensional structure has been determined for the catalytic core of CBHI, CBHII and EGI as well as for the CBD of CBHI.
The main goals for our research
- To understand why the enzymatic hydrolysis of cellulose is slow. The initial hydrolysis of cellulose is relatively rapid but the hydrolysis-rate decreases during the process. Increased knowledge on the hydrolysis mechanism for the cellulases alone and in different combinations, could enable us to speed up the hydrolysis rate.
- To analyse (and control) the adsorption of the cellulases to the substrate. Cellulases have the ability to adsorb to lignin as well as to cellulose. The cellulases adsorbed on lignin will not hydrolyse cellulose, they will thus be nonproductively bound. When we know more about the adsorption mechanism we might be able to control the adsorption to the productive sites. Furthermore, we want to recycle the enzymes by adsorbing the cellulases on fresh substrate when the hydrolysis is completed. This would decrease the enzyme consumption in the process and, thus, decrease the costs in the ethanol process.
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Last updated: 2007-04-12