Hemicellulose structure and importance.pptx

Hemicellulose is a complex carbohydrate, a heteropolymer found in plant cell walls alongside cellulose and lignin, that plays a crucial role in plant structure and function. It is composed of various sugar units like xylose, glucose, mannose, and galactose. While cellulose provides strength, hemicellulose forms a matrix that binds cellulose and lignin together, contributing to the cell wall's flexibility and overall structure.
Here's a more detailed look at hemicellulose:
Composition:
Hemicellulose is a heteropolymer, meaning it's made of different types of sugar units linked together.
Common sugar units include pentoses (like xylose and arabinose) and hexoses (like glucose, galactose, and mannose).
Uronic acids (like glucuronic acid) are also found in some hemicellulose structures.
Different types of hemicellulose exist, including xylans (main in hardwoods), glucomannans (main in softwoods), and arabinoxylans.
Structure and Function:
Hemicellulose is a branched polymer, unlike the linear structure of cellulose.
It forms a matrix that surrounds cellulose microfibrils, contributing to the cell wall's flexibility and structure.
Hemicellulose also interacts with lignin, helping to bind the cell wall components together.
It can be hydrolyzed (broken down) into its component sugars, making it a potential feedstock for biofuel production.
Key Characteristics:
Second Most Abundant:
Hemicellulose is the second most abundant carbohydrate in nature, after cellulose.
Solubility:
It is insoluble in water but soluble in alkaline solutions.
Fermentation:
Microorganisms in the colon can partially ferment hemicellulose, producing volatile fatty acids.
Dietary Fiber:
Hemicellulose is a type of dietary fiber that can affect intestinal transit time and starch digestion.
In summary, hemicellulose is a vital component of plant cell walls, contributing to their structural integrity and serving as a source of energy and raw materials.

1. Hemicellulose

2. Hemicellulose is a complex carbohydrate that is found in plant cell walls. It is a heterogeneous mixture of polysaccharides comprising, among others, xyloglucans , xylans , and glucomannans . In contrast to cellulose, which consists of long chains of glucose molecules , hemicellulose is composed of a variety of sugar molecules, including xylose , arabinose , mannose, galactose , and glucuronic acid. Hemicellulose is essential to the structural integrity of plant cell walls, as well as a source of energy for certain animals capable of degrading its complex polysaccharides.

3. A hemicellulose (also known as polyose ) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans , found in practically all terrestrial plant cell walls alongside cellulose .( Cellulose is crystalline, durable, and hydrolysis-resistant .) Hemicelluloses are branching, shorter than cellulose, and crystallize easily . They can be hydrolyzed by dilute acid or base, in addition to a multitude of hemicellulase enzymes. There are numerous types of hemicelluloses recognized. Xylan , glucuronoxylan , arabinoxylan , glucomannan , and xyloglucan are notable examples. Hemicelluloses are polysaccharides frequently associated with cellulose, but with unique structures and chemical compositions.

4. Chemical structure of (a) cellulose, (b) hemicellulose , and (c) lignin.

5. In contrast to cellulose, hemicelluloses are made up of a variety of sugars, including the five-carbon sugars xylose and arabinose , the six-carbon sugars glucose, mannose, and galactose , and the six-carbon deoxy sugar rhamnose . Hemicelluloses consist primarily of D-pentose sugars, with occasional trace amounts of L-sugars . In most situations, xylose is the most abundant sugar monomer, however mannose may be the most abundant sugar in softwoods. In addition to ordinary sugars, hemicellulose may also contain their acidified counterparts, such as glucuronic acid and galacturonic acid. Together with cellulose and lignin, hemicellulose is a complex carbohydrate present in the cell walls of plants.

6. It is a heterogeneous mixture of polysaccharides, which means it is composed of a variety of sugars and carbs . Hemicellulose gives the plant cell wall structural support, contributing to its strength and stiffness. Unlike cellulose, which is exclusively made of glucose, hemicellulose contains a range of sugars, including xylose , arabinose , mannose, and galactose . Hemicellulose is more easily destroyed by enzymes than cellulose, making it a significant source of energy for organisms capable of breaking down its complex carbohydrates.

7. Unlike cellulose, hemicelluloses are composed of shorter chains, ranging from 500 to 3,000 sugar units. In comparison, each cellulose polymer contains between 7,000 and 15,000 glucose molecules. Moreover, hemicelluloses can be branched polymers whereas cellulose is unbranched . Hemicelluloses are embedded in the cell walls of plants, sometimes in chains that form a ‘ground’ — they attach to cellulose and pectin to form a network of cross-linked fibres. Structure of hemicellulose

8. On the basis of structural differences, such as backbone linkages and side groups, as well as other criteria, such as abundance and distribution in plants, hemicelluloses can be classified into the four following classes: 1) xylans , 2) mannans 3) mixed linkage β- glucans 4) xyloglucans .

9. 1. Xylans Xylans are typically composed of β-(1→4)- linked xylose residues and can be further classified as homoxylans or heteroxylans . Homoxylans contain a backbone composed of D- xylopyranose residues connected by β(1→3)  or mixed, β(1→3, 1→4)- glycosidic bonds. Homoxylans mostly provide structural purposes. Heteroxylans , including glucuronoxylans , glucuronoarabinoxylans , and complex heteroxylans , possess a D- xylopyranosyl backbone and short carbohydrate branches. Glucuronoxylan , for instance, has substitutions involving α-(1→2)- linked glucuronosyl and 4-O-methyl glucuronosyl residues. Arabinose residues are linked to the backbones of arabinoxylans and glucuronoarabinoxylans .

10. 2. Mannans The mannan -type hemicellulose can be divided into two categories, galactomannans and glucomannans , based on the difference in their main chain. Galactomannans contain solely linear chains of β-(1→4)  linked D- mannopyranose residues. In the major chains, glucomannans include both β-(1→4)  linked D- mannopyranose and β-(1→4)  linked D- glucopyranose residues. D- galactopyranose residues tend to be 6-linked to both types of side chains as single side chains in varying amounts. 3. Mixed linkage β- glucans Typically, the conformation of mixed-linkage glucan chains consists of blocks of β-(1→4)  D- Glucopyranose separated by a single β-(1→3) D- Glucopyranose . The populations of β-(1→4) and β-(1→3)  are roughly 70% and 30%, respectively. These glucans are predominantly composed of random segments of cellotriosyl (C18H32O16) and cellotraosyl (C24H42O21).

11.   Xyloglucans Xyloglucans have a cellulose-like backbone with -D- xylopyranose residues at position 6. Each form of side chain is denoted by a single letter code to facilitate a more precise description. G — unbranched Glc residue; X — α- d- Xyl -(1→6)- Glc . L — β- Gal , S — α- l- Araf , F– α- l- Fuc . These are the most commonly encountered side chains.  XXXG and XXGG are the two most frequent forms of xyloglucans found in plant cell walls.

12. Hemicellulases Hemicellulases are a type of enzyme that can break down hemicellulose , which is a complex carbohydrate found in the cell walls of plants. Hemicellulases are produced by microorganisms such as bacteria and fungi, as well as by some animals and insects that consume plant material. There are several different types of hemicellulases , each with a specific function and substrate specificity. For example, xylanases are hemicellulases that break down xylan , a type of hemicellulose found in some plant tissues. Similarly, mannanases can break down mannans , another type of hemicellulose . Hemicellulases play an important role in the biodegradation of plant material in natural environments.

13. In biotechnology, hemicellulases are often used in combination with other enzymes such as cellulases to break down lignocellulosic biomass into its constituent sugars, which can then be used as a feedstock for the production of biofuels or other chemicals. The development of efficient and cost-effective hemicellulase enzymes is therefore an important area of research in the field of biotechnology. Hemicellulases are enzymes that can break down hemicellulose , which is a complex carbohydrate found in the cell walls of plants.

14. There are several different types of hemicellulases , including xylanases , mannanases , and xyloglucanases , among others. Hemicellulases are produced by a variety of microorganisms, including bacteria and fungi, as well as by some animals and insects that consume plant material. Hemicellulases have a specific substrate specificity, meaning that they are only able to break down certain types of hemicellulose .

15. Hemicellulases work by breaking the bonds between the individual sugars in hemicellulose , releasing the constituent sugars for use by the organism. Hemicellulases are often used in industrial processes, such as the production of biofuels and paper, to break down plant material into its constituent sugars. Hemicellulases can also be used in animal feed to improve the digestibility of plant-based feed ingredients

16. Microorganisms involved in hemicellulose degradation Bacteria : Many bacteria are able to produce hemicellulases that can break down hemicellulose into its constituent sugars. Some examples of bacteria that are known to produce hemicellulases include Bacillus subtilis , Clostridium thermocellum , and Cellvibrio japonicus . Fungi : Fungi are also important decomposers of plant material, including hemicellulose . Some fungi, such as Aspergillus niger and Trichoderma reesei , are known to produce hemicellulases that can break down hemicellulose . Actinomycetes : Actinomycetes are a group of bacteria that are commonly found in soil and are known for their ability to produce a wide range of enzymes, including hemicellulases . Protozoa : Some protozoa, such as ciliates, are able to break down hemicellulose in the digestive tracts of animals that consume plant material.

17. Microorganisms involved in hemicellulose degradation 1. Hemicellulolytic fungi Fungi are among the most active agents of decomposition of organic matter in general, and thus, these microorganisms are the most important group of hemicellulolytic microorganisms. Aerobic fungi, such as the fungi  Trichoderma  and  Aspergillus , secrete at high concentrations a large variety of hemicellulases that work synergistically. The source of hemicellulases also depends on the type of hemicellulases produced by the microorganisms. Besides, other saprophytic fungi like  Alternaria solani , Botryosphaeria ribis , Botrytis allii , Corticium centrifugum , Monilia fructigena , Neurospora , Penicillium digitatum , Rhizopus nigricans , Sclerotinia fructigena , etc. are known to produce L- arabinanases and D- mannanases .

18. Similarly, fungi like  Gibberella saubinetti , Helminthosporium oryzae , Phytophthora infestans , Trametes gibbosa , etc. produce D- galactanases . Fungal species found in the marine environments, including  Aspergillus sojae , Chaetomium globosum , Agaricus bisporus , Diplodia viticola , Oxiporus , etc. are also considered important sources of hemicellulases . Most of the hemicellulases are extracellular with some intracellular hemicellulases .

19. 2. Hemicellulolytic bacteria Bacteria also produce different types of hemicellulases either as a singular enzyme or as a multi-enzyme system. Most of the bacterial cellulolytic enzymes are reported from  Clostridium felsineum , Bacillus subtilis , Acetenobacter mannanolyticus , Bacillus aroideae , Sporocytophaga myxococcoides , etc. Besides, rumen bacteria found in various ruminant mammals are also known to produce hemicellulases . Some examples include  Streptococcus sp., Butyrivibrio fibrisolvens , Ruminococcus albus , Bacteroides ruminicola , etc.

20. Enzymes involved in the degradation of hemicellulose There are several types of enzymes involved in the degradation of hemicellulose , including: 1.  Xylanases 2.  Mannanases 3.  Arabinanases 4.  Galactanases 5. Acetyl xylan esterases

21. 1. L- arabinanases L- Arabinanases are hydrolytic enzymes capable of degrading L- arabinans , including both of the α-(1→3)-linked L- arabinofuranosyl appendages of the L- arabinan , and the α-(1→5)-linked L- arabinofuranosyl residues of the “linear” chain. L- Arabinanases have been reported to be produced by the bacterium  Clostridium felsineum  var. sikokianum , saprophytic and phytopathogenic fungi, the snail, and plants. Rumen bacteria and protozoa and caecal bacteria also produce enzymes capable of liberating L- arabinose from arabinoxylans and are probably a-L- arabinofuranosidases . Most L- arabinanases of fungal origin are usually secreted extracellularly into the medium in which the organism is grown, but intracellular L- arabinanases have also been found to exist. Several phytopathogenic fungi are known to produce L- arabinanases by induction when grown on media supplemented with L- arabinan , and constitutively when these organisms were grown on D-glucose as the sole carbon source

22. 2. D- galactanases D- galactanases are hydrolytic enzymes capable of degrading D- galactans and L- arabino -D- galactans . Two distinct types of D- galactanases are known that are specific for (13) and (14)-β-D- galactopyranosyl linkages. Both enzymes are able to degrade D- galactan randomly, to afford D- galactose and D- galactooligosaccharides , and they are therefore endo -D- galactanases . D- Galactanases have been reported to be produced by Bacillus subtilis , by a rumen anaerobic bacterium, by fungi, and by plants. D- Galactanases are inductive, and those of microbial origin are usually produced extracellularly in response to the carbon source of the culture medium.

23. 3. D- Mannanases D- mannanases [( 14)- β- D- mannan mannanohydrolases , endo -D mannanases ) are hydrolytic enzymes capable of hydrolyzing the (14)- β- D- mannopyranosyl linkages of D- mannans and D- galacto -D- mannans . These enzymes are capable of degrading the D- gluco -D- mannans , D-glucose, D-mannose, and a series of manno - and glucomanno -oligosaccharides. D- Mannanases have been reported to be produced by various species of bacteria, including bacteria from human intestines and the rumen. Other sources include the rumen protozoa, various fungi (including saprophytic, phytopathogenic , and mycorrhiza fungi), marine algae, germinating terrestrial plant-seeds, and various invertebrates.

24. 4. β- D- xylanases β- D- xylanases are hydrolyzing enzyme that degrades β- D- xylans into D- xylose or D- xylose and D- xylo -oligosaccharides. Xylanases are of two types that degrade either the (13) linkages and (14) linkages. D- Xylanases are known to occur in bacteria from marine and terrestrial environments, fungi (saprophytes, phytopathogens , and mycorrhiza ), rumen bacteria and protozoa, ruminant caecal bacteria, insects, snails, crustaceans, marine algae, and germinating seeds of terrestrial plants.

25. Factors affecting hemicellulose degradation 1. Temperature and pH Temperature and pH both affect the rate of hydrolysis of hemicelluloses. The highest rate of hydrolysis is achieved at pH 6 and 40°C. The increase of reaction temperature above 40°C has a negative impact on total xylose production as the temperature probably affects the stability of hemicellulolytic enzymes. 2. Organic matter The presence of organic matter also increases the rate of hemicellulose degradation as much of the organic matter act as a substrate. However, if hemicellulose is the only component of the matter, the rate of hydrolysis decreases. The rate of degradation increases with the addition of a small amount of readily decomposable organic matter as it allows the growth of microorganisms.

26. 3. Dosage of enzymes The increase in enzyme concentration also increases the rate of hemicellulose degradation. The increase in enzyme concentration increases the number of active sites which subsequently increases the product concentration. 4. Substrate conversion The xylose and arabinose production rates decrease rapidly after a few hours of the enzymatic hydrolysis reaction. The percentage of arabinose content released during hydrolysis remains significantly lower than the percentage of xylose produced at the same time. The sudden fall of the hydrolysis rate is attributed rather to the increasingly limited structural accessibility of the cell wall matrix to enzymes as hydrolysis proceeds than to enzymes’ properties.

27. Mechanisms of microbial degradation of hemicellulose The mode of action or the mechanism of microbial degradation can only be explained when the enzyme preparations used are homogenous, i.e. a single protein component. The mechanism of microbial degradation is different with different hemicellulases .

28. 1. Xylanases D- xylanases of the endo enzyme type is the only xylanases that have been properly characterized. These xylanases hydrolyze the 1, 4- β- D- xylopyranosyl linkages of D- glycans such as L- arabino -D- xylans , L- arabino -D- glucoro -D- xylans , and D- glucorono -D- xylans . Some of these enzymes might even hydrolyze the (13)- α- L- arabinofuranosyl branch points of arabinoxylan . Another group of endo xylanases degrades arabinoxylan and other D- xylans to D- xylose , D- xylooligosaccharides , and in some cases, oligosaccharides containing both L- arabinose and D- xylose . Example Bacteria xylanases Bacterial xylanases are produced by bacteria like  Bacillus  and  Streptomyces . The xylanase preparation from the alkalophile Bacillus degrades arabinoxylan to xylobiose and xylotriose as major end products with smaller amounts of higher xylooligosaccharides .

29. 2. Mannanases Mannanases of both the exo and endotypes have been characterized that hydrolyze 1,4- β- D- mannopyranosyl linkages of branched mannans , copolymer mannans , and linear D- mannans . The endo - β- mannanases degrade β- D- mannans to D-mannose and a series of mannose oligosaccharides. On acid hydrolysis, the enzymic degradation with a β- D- mannosidase yields D-mannose as the only hydrolysis product. The preferential attack of endomannanases is on the D-mannose chain at the 3rd and 4th linkages from the nonreducing end of the molecule. Example Fungal mannanases D- Mannanases of fungal origin have been known to degrade D- mannans in a random manner, and to be of the endotype . Mixed oligosaccharides resulting from enzymatic hydrolysis of galactoglucomannans are likely to contain D- galactose in addition to D-glucose and D-mannose.

30. 3. Galactanases Galactanases are hydrolytic enzymes that degrade D- galactans and L- arabino -D- galactans . Two distinct types of endogalactonases are recognized with a single exo type. Endogalactanases degrade the 1,4- β- D- galactosyl linkages of D- galactans randomly to produce D- galactose and galactose oligosaccharides, some of which might contain L- arabinose residues. Example Fungal galactonases D- galactanases produced by Rhizopus sp . do not act on galactobiose as it is specific only to (13)- β- D- galactopyranosylinkages . The enzymes are also capable of removing L- arabinofuranose from arabinogalactosides but do not liberate any L- arabinose from oligosaccharides.

31. 4. Arabinanases The action of the L- arabinanae as an endoenzyme yields L- arabinose and an L- arabinose oligosaccharide as major products of hydrolysis. Besides, smaller proportions of L- arabinose disaccharides may even be formed. L- arabinan -degrading enzymes of the exo -type degrade L- arabinan completely to L- arabinose . These enzymes hydrolyze both the (13) and (15)-α-L- arabinofuranosyl residues of L- arabinan . Arabinanases hydrolyze both types of linkage of L- arabinan at one active site, and the substrate is attacked from the nonreducing end by a multi-chain mechanism. In its attack on L- arabinan , it hydrolyzes the substrate rapidly to the extent of 30%; thereafter, the attack is slow. This initial, rapid hydrolysis of L- arabinan corresponds to the favored attack on the (α-L-(13)-linked L- arabinofuranosyl residues, leaving a mainly linear (15)-α-L- arabinan which is slowly, and eventually, completely, hydrolyzed to L- arabinose .