Hemicellulose is a collection of polysaccharides with complex structures that together with cellulose and lignin constitute the major constituent of biomass (lignocellulose). The complex structure of hemicellulose makes it difficult for enzymes to degrade biomass, which is viewed as an important and CO2 neutral energy source if properly utilized.
The thesis first describes the synthesis of arabino- and glucuronoxylan fragments found in hemicellulose. These are of great interest as it is valuable to have access to pure and well-defined fragments of hemicellulose that can be used as substrates for hemicellulose-degrading enzymes such as xylanases, arabinosidases and glucuronidases. The synthesized fragments consist of composite xylose units to which either arabinose or glucuronic acid are bound. First, the xylose units have been assembled into either tetra- or pentasaccharides using a selective coupling method that allows the coupling of two thioglycosides with one another. Next, arabinose units have been loaded onto the tetrasaccharides, while the pentaxylan has been coupled to two different glucuronic acid units, giving a total of five synthetically made xylan fragments. Going forward, removal of the protecting groups will yield the desired fragments that can be used to characterize hemicellulose enzymes.
Another project in the dissertation was completed during an external stay at Wageningen University and Research. Here, work has been done on the characterization of ferulic acid esterases, which form an important part of the arsenal of hemicellulose-degrading enzymes, as these can cleave the ester bond between ferulic acid and polysaccharides. Thus, these enzymes may reduce some of the complexity of the hemicellulose structure. However, these enzymes show considerable differences in reactivity and specificity, which is why it has been investigated whether a previously published classification based on the genomes can be related to the specific reactivity. This was investigated via the release of ferulic acid and diferulic acids from several different types of natural biomass substrates. This showed that two classes of enzymes in particular were very active.
Heatmap for the release of FA and pCA from the natural substrates tested, the results are shown as percentages of bound content (0 – 100%) as measured by UHPLC-UV. CS: corn stover; WS: wheat straw.