Table of Contents
- Chapter 9: Diversity theme continued: cell and tissue differentiation
- Chapter 10: Fungi in ecosystems
- Contributions of fungi to ecosystems
- Polysaccharide degradation: cellulose
- Degradation of polysaccharides: hemicellulose
- Degradation of polysaccharides: pectins
- Degradation of polysaccharides: chitin
- Breakdown of polysaccharides: starch and glycogen
- lignin degradation
- protein digestion
- lipases and esterases
- phosphatases and sulfatases
- The nutrient flow: transport and translocation
- Primary (intermediate) metabolism
- secondary metabolism
- Chapter 10 References and further reading
- Purchase a PDF of Chapter 10
- Chapter 11: Researching Mushrooms for Food
10.2 Decomposition of polysaccharide: cellulose
Polysaccharides are polymers of monosaccharides in which the sugar components are linked by glycosidic bonds. There is considerable diversity in polysaccharides because of the number and variety of sugars available and the variety of bonding possibilities between different carbon atoms of adjacent sugar residues. There are a variety of enzymes, hydrolases or glucosidases, that are capable of hydrolyzing this stretch of glycosidic bonds. Enzymes responsible for breaking down polymers (any polymer, not just polysaccharides) can employ one of two attack strategies. They can attack at random and effectively fragment the polymer molecule into different oligomers, these are theEndo-Enzyme, or they can approach terminally to digesting monomers or dimers thatExo-Enzyme.
celluloseit is the most abundant organic compound on earth, accounting for more than 50% of organic carbon; about 1011tons are synthesized each year. It is an unbranched glucose polymer in which neighboring sugar molecules are linked by β1→4 bonds (Fig. 1); there can be a few hundred to a few thousand sugar residues in the polymer molecule, corresponding to molecular weights from about 50,000 to about 1 million. The breakdown of cellulose is chemically easy, but its physical form makes it difficult. Mild acid hydrolysis of cellulose liberates soluble sugars but is not complete; Oligomers of 100-300 glucose residues remain. The easily hydrolyzable fraction is called amorphous cellulose, while the acid-resistant one is called crystalline cellulose. Since it affects chemical degradation, the conformation and three-dimensional structure of cellulose must affect cellulolytic enzyme activity.
Cowardly. 1. Structural formula of cellulose. There can be a few hundred to a few thousand sugar residues in the polymer molecule, corresponding to molecular weights from about 50,000 to about 1 million. Modified from Moore, 1998.
The cellulolytic enzyme (Cellulase) Basidiomycota complex of white rot likePhanerochaete chrysosporiumand Ascomycota asTrichoderma reeseiIt consists of a series of hydrolytic enzymes: endoglucanase, exoglucanase and cellobiase (a β-glucosidase) that work synergistically and are organized in both bacteria and fungi in an extracellular multi-enzyme complex called β-glucosidasecellulose(see next paragraph).EndoglucanasaIt randomly attacks cellulose and produces glucose, cellobiose (a disaccharide made up of two glucose molecules) and some cellotriose (a trisaccharide).exoglucanaseattacks the non-reducing end of the cellulose molecule and eliminates glucose units; it may also include cellobiohydrolase activity that produces cellobiose by attacking the non-reducing end of the polymer.of Zelobiais responsible for hydrolysisZellobioseto glucose. Glucose is therefore the easily metabolized end product of cellulose breakdown by enzymatic hydrolysis.
CellulosomenThey were first described in anaerobic cellulolytic bacteria and are also highly developed in fungi. These enzyme complexes are extracellular molecular machines ("nanomachines"). In addition to catalytic regions, cellulolytic enzymes contain domains that are not involved in catalysis but are involved in substrate binding, formation of multienzyme complexes (so-called "docking domains"), or binding to the cell surface.
Cellulosomes comprise a complex offramework, which is the structural subunit, and several enzymatic subunits. Interactions between subunits in these multienzyme complexes are made possible bycohesionjStauermodules. Cellulose-producing bacteria have been isolated from many different environments, suggesting that this microbial enzymatic strategy was important and ecologically successful early in evolution. However, the detailed structure of cellulosomes in any given species is variable, and there is considerable variation in cellulosome composition and configuration between species;Species-specific domains of dokerin support enzyme assembly into interspersed cohesin motifs in variable protein scaffolds(Artzand other. 2017).
Analogous structures in anaerobic fungi are also assembled using non-catalytic dockerin domains and scaffold proteins, but these haveno resemblance to their bacterial counterparts🇧🇷 Cellulosomes in anaerobic fungi contain several enzymatic activities not present in their bacterial counterparts. However, some of its catalytic domains are thought to derive from horizontal gene transfer from bacteria present in the common ancestral environment (like some aspects of secondary metabolism, seeSection 17.15🇧🇷 Genomic and proteomic analyzes of anaerobic fungi (rumen chytrids) suggest that the fungal cellulosomeIt is an evolutionarily chimeric structure consisting of a fungal complex that evolved independently and took over useful activities from neighboring bacteria.within the gut microbiome (Haitjemaand other., 2017).
Cellulosomes efficiently degrade crystalline cellulose and polysaccharides associated with the plant cell wall, adhere to the cell surface, and their adhesion to the insoluble substrate gives the individual microbial cell that produced them a competitive advantage in using soluble products. The enzymatic degradation of plant biomass has attracted the interest of researchers involved in the production of biofuels. Cellulosomes are said to have commercial potential in this biotechnology, in particular because of their efficient organization and hydrolytic activity. They are at the center of many efforts to use the versatile assembly mechanism of the cohesin-dokerin protein as a standard design principle for designing the ideal.design cellulosomes“ for the management of lignocellulosic waste from households and industry (Bayerand other., 2007; Frog and Frog, 2017).
When grown on cellulose, thewhite rot fungiI likePhanerochaete chrysosporiumproduce twoCelobiosa oxidoreductases🇧🇷 a cellobiose: quinone oxidoreductase (CBQ) and cellobiose oxidase (CBO). Cellobiose oxidase is able to oxidize cellobiose to δ-lactone, which can then be converted to cellobionic acid and then to glucose + gluconic acid; Cellobiose δ-lactone can also be formed by the cellobiose enzyme quinone oxidoreductase. Similar cellobiose-oxidizing enzymes, which can utilize a variety of electron acceptors, have been identified in many other fungi, although their function is uncertain. These enzymes are probably the most important in regulating cellobiose and glucose levels, the accumulation of which can inhibit endoglucanase activity. The role originally attributed to CBQ was that of a link between the breakdown of cellulose and lignin. Cellobiose oxidase also reduces Fe(III) and along withhydrogen peroxideGenerates hydroxyl radicals. These radicals can degrade both lignin and cellulose, suggesting that cellobiose oxidase plays a central role in the degradation of wood by rot fungi.
BraunfäuleFungi use an initial cellulolytic system that is very different from the hydrolytic base used by white rot. Black rot fungi are able to depolymerize cellulose quickly and almost completely. Even cellulose deep in walls and protected by lignin polymers is vulnerable to attack. The process appears to be dependent on hydrogen peroxide (H2Ö2) secreted by the fungus and ferrous ions (Fe2+) in the wood, leaving the sugar molecules in the polymer oxidized, fragmented and open to further attack by hydrolytic enzymes. Oddly enoughOxalatThe crystals lining so many fungal hyphae could be responsible for the reduction of ferric iron (Fe3+), normally found in wood, into ferrous ions (Fe2+), which supports the oxidative cleavage of cellulose.
This "oxidative-reductive cellulose degradation system" exists in parallel with the hydrolytic cellulase system already described. The two systems complement each other: copper oxidases attack the highly crystalline region of cellulose, while endoglucanases attack amorphous cellulose with cellobiohydrolases that break the ends of crystalline cellulose chains. Oxidative degradation of cellulose used.PPolysaccharidesMETROOh noÖoxygenateds(PMO),CpraiseDmiHHydrogenase (CDH)and members of what'family service activity 10', a family of polysaccharide monooxygenases known to cleave chitin and cellulose. Sequences specifying PMOs are widespread in the genomes of most Ascomycetes and Basidiomycetes (both white-rot and brown-rot fungi) and increase the efficiency of cellulose degradation through their action on the crystalline portion of cellulose, which breaks down tightly packed cellulose chains. making them susceptible to attack by hydrolytic cellulases (Dimarogoneand other. 2012).
Updated February 2020