Abstract
Pectin methylesterase (PME) is the first enzyme acting on pectin, a major component of plant cell wall. PME catalyzes reactions according to the double-displacement mechanism. In plants, PMEs can be classified on the basis of presence or absence of the PRO domain in pectin methylesterase into Type 1 and Type II. Type1 contains one to three PRO domains and two or three introns, type II PMEs are without PRO domain and with five or six introns. Once PMEs are secreted into the cell wall, mature PMEs exhibit three different modes of action: single chain mechanism, multiple chain mechanism and multiple attack mechanism. For bacterial and plant PMEs, single chain and multiple chain mechanism is used and for fungal PMEs only multiple chain mechanism has been proposed. PME activity is regulated by differential expression both spatially and temporally.
Highlights
Pectinolytic enzymes or pectinases are a heterogeneous group of enzymes that hydrolyze the pectic substances present in plants
Pectin methylesterase catalyzes reactions according to the double-displacement mechanisms, de-esterification through transferring the C6 carboxyl groups in the pectin-PME complexes to water molecules altering the degree and pattern of methyl esterification and transacylation through transferring the C6 carboxyl groups to the hydroxyl groups of another pectin molecules and resulting in the formation of high molecular weight pectins with new non-methoxy ester linkages [3,4,5,6,7,8]
It has been reported that PME activity is partly regulated by PMEI (Pectin methylestearse inhibitor) [14,15] For example, in flax (Linum usitatissimum) Lu PMEs gene family, most highly conserved residues were catalytic residues, while in Lu PMEIs gene family, cysteine’s forming disulphide bridges between helices α2 and α3 were found to be conserved residues [16]
Summary
Pectinolytic enzymes or pectinases are a heterogeneous group of enzymes that hydrolyze the pectic substances present in plants. They include polygalacturonases, pectin lyase, and PME that hydrolyze the glycosidic bonds of pectic substances. Pectin methylesterase (E.C. 3.1.1.11), a methyl ester group hydrolytic enzyme is produced by plants, pathogenic fungi, and bacteria constantly being used in the wine, juice, and other food industries [1]. Pectin methylesterase catalyzes reactions according to the double-displacement mechanisms, de-esterification through transferring the C6 carboxyl groups in the pectin-PME complexes to water molecules altering the degree and pattern of methyl esterification and transacylation through transferring the C6 carboxyl groups to the hydroxyl groups of another pectin molecules and resulting in the formation of high molecular weight pectins with new non-methoxy ester linkages [3,4,5,6,7,8]. It has been recently confirmed that PME has a definite role in plant defence system by virus induced gene silencing of PME gene in Nicotiana benthamiana [17]
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