We’ll be presenting structural representations of polysaccharides throughout this site. The legend for the shapes and colors used is shown below.
Cell wall polysaccharides
The major carbohydrate components of the plant cell wall are cellulose, hemicellulose, and pectin. Cellulose is the most abundant biopolymer present in the world, with an estimated 10 trillion tons of cellulose present in the biosphere. The cellulose polymer is composed strictly of β-1,4 linked glucose residues, with a cellulose chain containing anywhere from several hundred glucose residues to a million glucose residues. Cellulose is not only produced by plants, it is also produced by a number of gram-negative bacterial species. The crystallinity of cellulose renders it insoluble in most solvents and highly refractory to enzymatic and chemical degradation. The refractory nature of cellulose is further magnified in the presence of other cellular components such as hemicellulose and lignin. Cellulose fibers are the key raw material for paper manufacture as well as a variety of chemically modified cellulose products.
The ubiquity and concentration of hemicellulose in plant cell walls make it the second most abundant polysaccharide in nature after cellulose. Hemicelluloses are classified into pentosans, such as xylans, which are polymers containing a backbone of five carbon sugars and glucans such as mannans, curdlans, xyloglucans and beta-glucans, which are polymers containing a backbone of six carbon sugars. Xylans are the most abundant form of hemicellulose. The defining feature of xylans is a backbone of beta-1,4-linked xylose residues. While cellulose is a homopolymer of beta-1,4-linked glucose, xylans are heteropolymers containing a range of species-specific modifications to the backbone chain. These modifications include the attachment of neutral sugars such as arabinose, galactose, and glucose, attachment of charged sugars such as glucuronic acid, and acetylation, giving rise to unsubstituted xylans, arabinoxylans, glucuronoxylans, and arabinoglucuronoxylans (these will all be collectively called xylan). The result of these modifications is a bewildering diversity in the chemical compositions and structures of xylans.
Other types of hemicellulose are also present. Mannans are major hemicellulose components in a number of plant seeds and softwoods such as pine and spruce. Mannans and galactomannans have a backbone of β-1,4 linked mannose with galactose sidechains possible at the 6 position. Glucomannans and galactoglucomannans molecules have a backbone of β-1,4 linked glucose and mannose; with galactose sidechains possible at the 6 position and acetate groups at the 2 and 3 positions. Xyloglucan is polymer composed strictly of a β-1,4 linked glucose backbone; it differs from cellulose in the presence of additional sugar groups linked at the 6 position.
Pectins are the third major category of cell wall polysaccharides in plants. Two major classes of pectins are present in plants, differing in the backbone structure of the pectin. Homogalacturonans are linear chains of α-(1–4)-linked D-galacturonate. D-Xylose or D-apiose may be linked to the backbone, and methanol is often esterified to the acid groups. The backbone of rhamnogalacturonan I is a linear chain of the disaccharide D-galacturonate linked α-(1–2) to L-rhamnose. Rhamnogalacturonan I accounts for 7% of the dry mass of sycamore suspension culture cell walls, making it the most abundant form of pectin found in the cell wall. High molecular weight arabinan is found as sidechains on the rhamnogalacturonan I backbone making up 3% to 30% of the weight of the rhamnogalacturonan I. Arabinans may be made up of 100 to 200 arabinofuranoside residues with a linear core composed of 50−80 residues; the remainder of the arabinose being in short side-chains off the linear core. Loss of these arabinan side chains in apple rhamnogalacturonan I results in loss of the apple’s firm texture, suggesting a structural function for the arabinan. A similar role may for arabinan may be found in potatoes, where the loss of arabinan side chains results in increased brittleness when subjected to compression. Rhamnogalacturonan II appears to have a linear backbone of α-(1–4)-linked D-galacturonate with at least four different carbohydrate side chains containing arabinose, xylose, galactose, fucose, apiose, and other sugars linked at the 2 and 3 positions. Rhamnogalacturonan II is of particular interest for its ability to form cross linked dimers using borate ions in vivo.
Starch is a complex polymer of alpha-(1,4) and alpha-(1,6)-linked glucose used by plants to store energy. Starch is typically found in plants as spherical granules, between 5 and 25 microns in size. The plant family, growth conditions, and genetics of the plant cultivar determine the exact granule shape and distribution of granule sizes. Starch granules are filled with two polymers, amylose and amylopectin. Amylose is a linear polymer of only alpha-(1,4)-linked glucose. When treated with iodine, this polymer gives the typical blue starch reaction. Amylopectin is a highly branched polymer of short and medium length alpha-(1,4)-linked glucose chains connected by single alpha-(1,6)-bonds, with an overall structure resembling a tree. When treated with iodine, amylopectin gives no reaction. The ratio of amylose to amylopectin depends upon the starch source. Corn, wheat and rice starches have 20% to 25% amylase. Waxy corn starch has essentially no amylose, while high amylose starches can have up to 75% amylose. Starches with higher amylose content are more difficult to degrade with enzymes than starches with lower amylose content. The amylose and amylopectin polymers are organized in the starch granules into alternating crystalline and amorphous rings, giving a highly compact structure. When heated in water above 60°C, the structure is disrupted and the individual polymer molecules are released; this is know as gelatinization.
Applications of enzymes
Hydrolysis of starch to glucose
Starch hydrolysis is carried out in two steps. The first step, liquefaction, converts the amylose and amylopectin in a gelatinized starch into soluble, short chain oligosaccharides containing one to twenty glucose residues (dextrins) using an alpha-amylase (AA). The second step, saccharification, converts these dextrins into glucose using an amyloglucosidase (GA, also known as glucoamylase).
Optimum starch hydrolysis for ethanol production requires the following (in order of importance):
1. Complete hydrolysis of both alpha-(1,4) and alpha-(1,6)-linked glucose residues. Incomplete hydrolysis results in formation of short oligosaccharides that are not converted by GA during the saccharification and fermentation steps. The incomplete hydrolysis results in lower ethanol yield.
2. Prevention of recrystallization of linear amylose chains. Recrystallization of amylose chains occurs rapidly when the temperature is lowered from 92°C to 60°C. The result is a white precipitate or haze that is not attacked by the GA during saccharification or fermentation, resulting in decreased ethanol yield.
3. Effective release from the plant cell matrix in which it is embedded. Starch granules can be trapped in the cellular matrix of the cassava root. These starch granules are less likely to swell and gelatinize, and are less likely to be hydrolyzed by AA and GA. The net result is more starch in the plant debris and less ethanol produced.
Paper production (Biobleaching)
The use of enzymes to reduce the use of chemicals in bleaching of kraft pulps has been studied extensively. The research has focused on two areas. The first area of study is oxidation of the lignin in the pulp using a variety of lignin degrading systems. Lignin degradation systems are found in only a limited number of fungal species, limiting the range of conditions the enzymes can be applied in. The enzyme systems also require oxygen, metals, and possibly chemical mediators. The second area of study is the use of biomass degrading enzymes to cleave the linkage of between xylan and lignin. The research in this area has centered on the use of xylanases from bacterial and fungal sources. Enzymes with a wide range of pH and temperature optimum have been tested for this application and have been shown to be somewhat, but not completely effective.
Treatment of kraft pulp with xylanase results in reduced usage of chemicals and increased brightness. The mechanism of this improvement is believed to be hydrolysis of xylan-lignin molecules that have been redeposited on the cellulose fibers. Research has shown that the xylanases tested in the literature were highly effective in hydrolyzing xylan isolated from kraft pulp, but much less effective on the same xylan when bound to cellulose fibers.
Limited work has been done on other biomass degrading enzymes, including cellulases and mannanases. Fungal cellulases were found to be detrimental to the cellulosic fibers. The enzymes used in this work were potent fungal proteins capable of degrading crystalline cellulose and this result is not unexpected. Use of a mannanase resulted in only slight improvements, but the mannanase may not have had the correct activity or CBM for effective action.