Cellulose And Hemicellulose

The following sample essay on Cellulose And Hemicellulose deals with a framework of research-based facts, approaches and arguments concerning this theme. To see the essay’s introduction, body paragraphs and conclusion, read on.

When A.niger has been starved for 6 hours, CreA repression is alleviated and it releases a subset of scouting enzymes to find a carbon source to breakdown. From six to nine hours, the scouting enzymes (or degradative enzymes) look out for complex polysaccharides which will release inducing sugars (such as xylose).

The release of xylose triggers a subsequent induction of hydrolases by XInR and from nine hours plus, degradation of complex polysaccharides takes place. Other bacterial and fungal models suggest that the induction of these enzymes is from a basal level of expression of degradative enzymes.

Transcriptional changes are seen in A.niger in association with growth on wheat straw compared to growth on simple sugars (e.g. glucose). With growth on wheat straw, it was observed that levels of free glucose increased indicating that degradation had begun on wheat straw polysaccharide.

In addition, increases in xylose levels and arabinose in comparison to increase in glucose showed that hemicellulose degradation is the primary activity of A.niger at this point.

Degradation of Cellulose and Hemicellulose The plant cell wall is composed of polysaccharides; cellulose, hemicellulose, and pectin (in order of abundance) and lignin. The most famous and abundant polysaccharide is cellulose or β-1,4-glucan, in both primary and secondary cell walls. The cellulose content in cell walls vary but can be up to 45% in particular primary cell walls.

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It is a linear polymer made up β-1,4-linked D-glucose residues, existing in four polymorphic crystalline forms and is closely linked to xylan (a hemicellulose). In the cell wall, monomers are ordered to become fibres to give rigidity to the cell wall.

Additionally, there are two types of cellulose; the native type and regenerative type[18]. To breakdown cellulose through Trichoderma and Aspergillus species, three main classes of enzymes are involved: endoglucanases (EG), cellobiohydrolases (CBH) and β-glucosidases (β-GD). EG hydrolyses cellulose to glucooligosaccharides followed by CBH degrading crystalline cellulose to produce cellobiose.

Finally, β-glucosidase degrade the oligosaccharides to glucose. However, as a results of different enzyme studies, exoglucanases have been seen to release glucose from cellulose and glucooligosaccharides however, it is not a clear distinction from the role of cellobiohydrolases. Lastly, expression of cellulolytic genes in Aspergilli is observed in the presence of various monomeric and polymeric carbon sources.

Hemicelluloses are heterogeneous polysaccharides. They are used as a flexible cell wall support for plants and are able to bond to cellulose microfibrils through hydrogen bonding. However, the composition of this polysaccharide is different between plants and between species. There are many types of hemicelluloses but predominant types are xylan (in cereals) and xyloglucan (in onions). Xylan is a polymer consisting of a β-1,4-linked D-xylose backbone and a side group.

In wheat straw, the side group to the xylose backbone are D-glucopyranosyluronic acid at position 2 and L-arabinofuranosyl and D-xylopyranosyl groups linked at position 3 on the backbone. To degrade the xylan backbone, endoxylanses are used to cleave the backbone down to oligosaccharides and further degradation by β-xylosidases produces xylose. For the degradation of xyloglucan, EGs and β-GD are used.

Finally, pectins are a group of heteropolysaccharies. The backbone of pectin is compost of α-1,4-linked D-galacturonic acid residues. A non-polysaccharide component of the cell walls is lignin. In trees, lignin’s role in the cell wall is to support xylem cell and is covalently link to hemicellulose. However, in terms of advanced biofuel production, one of the key issues with the breakdown of wheat straw is lignin as it cannot be hydrolysed and monolignals are not involved to bioethanol production. Consequently, pre-treatment is a crucial step to allow access to cellulose and hemi-cellulose which can be thermal, chemical or fungal. A chemical pre-treatment is to degrade lignin in wheat straw, is using a mixture of acetic acid–nitric acid or using white rot fungi. An aim for future biofuels is to reduce the amount of pre-treatments required since stronger pre-treatments reduce the availability of polysaccharides.

Cellobiohydrolase (CBH) A and B are involved in the breakdown of β-1,4-glucan down to glucose, particularly the breakdown of the crystalline cellulose. These two enzymes are encoded by the genes cbhA and cbhB respectively and belong to the fungal family CBH 7. The modular structure of cbhB contains a cellulose binding domain or carbohydrate binding module (CBM) domain which is linked to a catalytic domain by a Pro/Ser/Thr-rich linker peptide but cbhA has just the catalytic domain. The modular structure of CbhB can be seen across a number of species and in T.reesei, over half of the protein secreted is cellobiohydrolase I (CBHI) which has the exact same structure is seen cbhB. It must be noted that the lack of a CBD in cbhA only affects its cellulase activities with insoluble cellulose but not with other soluble substrates.

The regulation of many cellulases is at the transcriptional level. Transcriptional repression of these genes can be seen in T.reesei in the presence of glucose by CreA whereas transcriptional activation is induced by XInR. Furthermore, the expression of cbhA and cbhB are activated by XInR, xylanolytic transcriptional activator in the presence of D-xylose. Carbohydrate-Binding Module (CBM) Domain The CBM domain catalyses the inefficient attack on glycosidic bonds of polysaccharides by GHs.

Glycosidic bonds do not always fit in the active site of GHs therefore to overcome this, many GHS use catalytic and non-catalytic CBMs to promote their association with their substrate. CBMs are mainly involved in to the hydrolysis of plant structure polysaccharides such as cellulose and hemicellulose. They also contain protein domains within a carbohydrate-active enzyme which is separate from a catalytic domain with carbohydrate binding activity[32]. A.niger has a subtype of CBM called the starch binding domain (SBD) which is found at the many amylolytic enzymes.

The CBM domain was previously defined as cellulose binding domain (CBD) because initial studies on these domains found these modules bound to cellulose. However, it can now be seen that modules appear to be bound to other carbohydrates. CBDs, in particular in T.reesei, are essential in cellulases performing the beginning steps of cellulose degradation as most of the substrate is still insoluble. However, not all cellobiohydrolase contain a CBD, such as cbhA in A.niger.

Expression of Xylanolytic Enzymes

Xylanolytic enzymes breakdown xylan into xylose and are produced on xylose (monosaccharide), xylan (polysaccharide) or substrates containing these sugars[20]. However, xylanolytic enzymes are not induced by other monomeric or polymeric substrates such as glucose and cellulose. Some are cellulases are induced by xylose suggesting the presence of xylose may activate transcription genes encoding cellulases.

There is a separate regulatory control of synthesis of cellulases and beta-xylanases. In certain species such as A.terreus, xylanolytic enzyme, β-xylanase was induced by cellobiose and cellulose (which are structurally related to xylobiose and xylan) as well as a heterodissachride of glucose and xylose. Studies of the genes encoding xylanolytic enzymes (from A.niger and A.tubingensis) have shown that these enzymes are expressed in the presence of D-xylose, xylobiose, or xylan by XInR (transcriptional activator) however, when xylose concentration is too high or glucose is present, thee genes are repressed by CreA(catabolite repressor protein).