Abstract
Biological conversion of plant biomass into commercially valuable products is one of the highly studied subjects in the last two decades. Studies were continuously being conducted to understand and develop efficient enzymes for the breakdown and conversion of plant cell-wall components into valuable commercial products. Naturally, plant cell-wall components are differentially esterified to protect from the invading microorganisms. However, during the process of evolution, microorganisms have developed special set of enzymes to de-esterify the plant cell-wall components. Among the carbohydrate-active enzymes (CAZy), carbohydrate esterases stand first during the process of enzymatic conversion of plant biomass, as these enzymes de-esterify the plant biomass and make it accessible for the hydrolytic enzymes such as cellulases, hemicellulases, ligninolytic and pectinases. In this article, we have extensively discussed about the structural and functional properties of pectin methyl esterases, feruloyl, cinnamoyl and glucuronoyl esterases which are required for the de-esterification of pectin and lignin–carbohydrate complexes. Pectin esterases are classified among CE8, CE12, CE13 and CE15 carbohydrate esterase class of CAZy database. Whereas, lignin–carbohydrate complex de-esterifying enzymes are classified among CE1 (feruloyl esterase) and CE15 (glucuronoyl esterase) classes. Understanding the structural and functional abilities of pectin and lignin–carbohydrate esterases will significantly aid in developing efficient class of de-esterases for reducing the recalcitrant nature of plant biomass. These efficient de-esterases will have various applications including pretreatment of plant biomass, food, beverage, pulp and paper, textile, pharmaceutical and biofuel industries.
Highlights
Pectin is a natural heterogenous polysaccharide present in plant cell walls especially between the middle lamella, occurring as calcium and magnesium salts
This study has reported that following residues Asp199, Asp178, Gln177 are involved in reaction mechanism, though Arg267 was found to be conserved among all the pectin methyl esterases and is not directly involved in enzyme catalysis (Fries et al 2007)
These invading microorganisms have evolved over the course of time and developed an efficient enzyme system for the de-esterification of the plant cell-wall components
Summary
Pectin is a natural heterogenous polysaccharide present in plant cell walls especially between the middle lamella, occurring as calcium and magnesium salts. Homogalacturonan is one of the major constituents of the pectic polysaccharides It contains long chains of d-galacturonic acid units linked through α-(1 → 4) bonds, which are methyl or acetyl esterified at C-6 position with acetyl/methyl group on O-2 or O-3 positions. It is synthesized from the nucleotide sugars in the Golgi apparatus and are transported to cell wall in fully methylesterified forms (Sénéchal et al 2014). Previous studies have extensively reported about various endogenous pectinases secreted by plants (Sakai and Okushima 1982; Sakai et al 1993; Sakamoto et al 1994; Whitaker 1990) Based on their specific location of activity, protopectinases were classified as A-type (inner site/reacts at the polygalacturonic acid region) and B-type (outer site/polysaccharide chains connected to polygalacturonic acid chain). Polygalacturonases are divided into endo- (widely reported among fungi, bacteria and yeast, and were reported in higher plants and parasitic nematodes) (De Lorenzo et al 1987; Luh and Phaff 1951; Manachini et al 1987; Marcus et al 1986; Maria de Lourdes et al 1991; Sakai et al 1984) and exo-polygalacturonases
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