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Pectinesterase (EC 3.1.1.11; systematic name pectin pectylhydrolase) is a ubiquitous cell-wall-associated enzyme that presents several isoforms that facilitate plant cell wall modification and subsequent breakdown. It catalyzes the following reaction:
It is found in all higher plants as well as in some bacteria and fungi. Pectinesterase functions primarily by altering the localised pH of the cell wall resulting in alterations in cell wall integrity.
Pectinesterase catalyses the deesterification of pectin into pectate and methanol. Pectin is one of the main components of the plant cell wall. In plants, pectinesterase plays an important role in cell wall metabolism during fruit ripening. In plant bacterial pathogens such as Erwinia carotovora and in fungal pathogens such as Aspergillus niger, pectinesterase is involved in maceration and soft-rotting of plant tissue. Plant pectinesterases are regulated by pectinesterase inhibitors, which are ineffective against microbial enzymes.
Pectinesterase action on the components of the plant cell wall can produce two diametrically opposite effects. The first being a contribution to the stiffening of the cell wall by producing blocks of unesterified carboxyl groups that can interact with calcium ions forming a pectate gel. The other being that proton release may stimulate the activity of cell wall hydrolases contributing to cell wall loosening.
Homogalacturonan is highly methyl-esterified when exported into cell walls and is subsequently de-esterified by the action of pectinesterase and other pectic enzymes. Pectinesterase catalyses the de-esterification of methyl-esterified D-galactosiduronic acid units in pectic compounds yielding substrates for depolymerising enzymes, particularly acidic pectins and methanol.
Most of the purified plant pectinesterases have neutral or alkaline isoelectric points and are bound to the cell wall via electrostatic interactions. Pectinesterases can however display acidic isoelectric points as detected in soluble fractions of plant tissues. Until recently, it was generally assumed that plant pectinesterases remove methyl esters in a progressive block-wise fashion, giving rise to long contiguous stretches of un-esterified GalA residues in homogalacturonan domains of pectin. Alternatively it was thought that fungal pectinesterases had a random activity resulting in the de-esterification of single GalA residues per enzyme/substrate interactions. It has now been shown that some plant pectinesterase isoforms may exhibit both mechanisms and that such mechanisms are driven by alterations in pH. The optimal pH of higher plants is usually between pH 7 and pH 8 although the pH of pectinesterase from fungi and bacteria is usually much lower than this.
Spatial and temporal regulation of pectinesterase activity during plant development is based on a large family of isoforms. Recently, the systematic sequencing of the Arabidopsis thaliana genome has led to the identification of 66 open reading frames that are annotated as pectinesterases, most of which are encoded as large pre-proproteins. The signal peptide pre-region is required for targeting the enzyme to the endoplasmic reticulum and consists of about 25 amino acid residues. These N-terminal regions contain several glycosylation sites and it is thought that these sites also play a role in targeting.
Pectinesterase is thought to be secreted to the apoplasm with highly methylated pectin although at some point along this secretory pathway the N-terminal pro-peptide is cleaved off. Currently, the role of the pro-region is unknown although it has been hypothesised that it may act as an intramolecular chaperone, ensuring correct folding or deactivating activity until PE insertion in the cell wall is complete.
Recently, particular attention has been devoted to molecular studies of pectinesterase leading to the characterisation of several related isoforms in various higher plant species. Some of these pectinesterases were shown to be ubiquitously expressed, whereas others are specifically expressed during fruit ripening, germination of the pollen grain, or stem elongation. Such data suggests that pectinesterases are encoded by a family of genes that are differentially regulated in cell type in response to specific developmental or environmental cues.
Prokaryotic and eukaryotic pectinesterases share a few regions of sequence similarity. The crystal structure of pectinesterase from Erwinia chrysanthemi revealed a beta-helix structure similar to that found in pectinolytic enzymes, though it is different from most structures of esterases.; The putative catalytic residues are in a similar location to those of the active site and substrate-binding cleft of pectate lyase.
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