ORGANIC MATTER DECOMPOSITION AND BIOCONVERSION OF PECTIN

Pectin make up the substantial fraction of the organic matter. Soil microorganisms colonize and decompose the substrate by producing various enzyme. Pectin is present in the middle lamella of the cell wall and is also found in the primary cell wall of higher plants. It’s a heterogeneous polysaccharide rich in galacturonic acid linked through α-(1-4) glycosidic bond and the carboxylic acid group is partially esterified with a methyl group.  The demethylated molecule is known as pectic acid and the salt of de-esterified pectin is known as pectate. Pectin cross links cellulose to hemicellulose providing rigidity to cell wall. Extensin a protein similar to collagen make up 15% of the primary cell wall and cross-links lignin to the pectin/hemicellulose network (Lodish et al., 2000).

The pathogens, symbionts like rhizobia or mycorrhiza gain access to the middle lamella by degrading pectins.  Pectinase enzyme termed by Valleau (1915) was responsible for the destruction of the middle lamella. The enzyme which hydrolyses pectic substances are known as pectinases or pectinolytic enzymes (Blanco et al., 1999). Pectinases degrade the pectic substance through depolymerisation (hydrolases and lyases) and deesterification (esterases) reaction. Pectinolytic microorganisms produce range of pectin degrading enzymes. These enzymes play a role in pathogenesis of plant by attacking the tissue (Collmer and Keen 1986) and are also involved in the process of establishing symbiosis and decay of the vegetable residue (Hondal et al., 2002). Pectic enzymes are produced by bacteria, yeast, fungi and plants (Lang and Dorenenburg 2000).

Pectinases are classified according to their site of cleavage endo- (they cleave within) or exo-(cleave the end of the substrate chain), preferred substrate (pectin or pectate) and the mode by which they cleave the glycosidic bond (hydrolase or lyase) (Prade et al., 1999). Pectin degradation is facilitated by pectinases including pectin lyases, polygalactrunases and pectin methyl esterases (Abbott and Boraston 2008).

  1. POLYGALACTURONASES:

Hydrolytic enzyme polygalacturonases hydrolyzes the glycosidic bond between the galacturonic acid residue.

  1. PECTIN LYASE:

Enzyme pectin lyase cleaves the glycosidic bonds between the galacturonic residues by β-elimination

  1. PECTIN METHYL ESTERASES:

Pectin methyl esterases cleaves the methyl group from the galacturonic acid residue.

Pectinases such as endopectinases cleave the glycosidic bonds between galacturonic acid residues randomly within the polysaccharide. Whereas, exopectinases cleave bond and release subunits from the end of the polysaccharide (Suberkropp 2007).

The saprophytes when initiates colonization of the substrate, the first complex carbon source they come across is pectin and for the complete hydrolysis of pectin several enzymes are released. These microorganisms are dependent on the end product of prior substrate enzyme interaction so that they can use the pectin and their hydrolyzed product as a carbon source.

References:

Abbott, D. W. and Boraston, A. B. 2008 Structural Biology of Pectin Degradation by Enterobacteriaceae  Microbiol Mol. Biol. Rev. 72(2): 301–316

doi:  10.1128/MMBR.00038-07

Blanco, P., Sieiro, C. and Villa, T. G. 1999 Production of Pectic Enzymes in Yeasts. FEMS Microbiology Letters 175 (1):  1-9

Collmer, A. and Keen, N. T. 1986 The Role of Pectic Enzymes in Plant Pathogenesis Annual Review of Phytopathology 24:  383-409

Hoondal, G., Tiwari, R., Tewari, R., Dahiya, N. and Beg, Q. 2002 Microbial Alkaline Pectinases and Their Industrial Applications: A Review. Appl Microbiol. Biotechnol. 59: 409 – 418

doi:10.1007/s00253-002-1061-1

Lang, C. and Dörnenburg, H. 2000 Perspectives in the Biological Function and the Technological Application of Polygalacturonases. Applied Microbiology and Biotechnology 53 (4):  366-375

Lodish, H., Berk, A., Zipursky, L.S., Matsudaira, P., Baltimore, D. and Darnell, J. 2000 The Dynamic Plant Cell Wall in “Molecular Cell Biology” 4th edition Freeman W. H. & Co., New York Section 22.5

Prade, R. A., Zhan, D., Ayouby, P. and Mort, A. J. 1999 Pectins, Pectinases and Plant-Microbe Interactions. Biotechnology & Genetic Engineering Reviews 16(1): 361- 391

doi: 10.1080/02648725.1999.10647984

Suberkropp, K.  2007 Pectin Degrading Enzymes: Polygalacturonase and Pectin Lyase in “Methods to Study Litter Decomposition: A Practical Guide” edited by Graça, M. A. S., Bärlocher, F. and Gessner, M. O. Springer Chapter 36:  267-272

Valleau, W. D. 1915 Varietal Resistance of Plum to Brown Rot. Journal of Agricultural Research 5: 365-395

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