Pectinesterase (PE) (EC number|184.108.40.206) is a ubiquitous cell wall associated
enzymethat present several isoformsthat facilitate plant cell wall modification and subsequent breakdown. It is found in all higher plants as well as in some bacteriaand fungi. pectinesterase functions primarily, by altering the localised pHof the cell wall resulting in alterations in cell wall integrity.
Effects of pectinesterase
Recent studies have shown that the manipulation of pectinesterase
expressioncan influence numerous physiological processes. In plants, pectinesterase plays a role in the modulation of cell wall mechanical stability during fruit ripening, cell wall extension during pollen germination and pollen tubegrowth, abscission, stem elongation, tuberyield and root development. Pectinesterase has also been shown to play a role in a plants response to pathogenattack. A cell wall-associated pectinesterase of "Nicotiana tabacum"is involved in host cell receptor recognition for the tobacco mosaic virusmovement protein and it has been shown that this interaction is required for cell-to-cell translocationof the virus.
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
carboxylgroups that can interact with calcium ionsforming a pectate gel. The other being that proton release may stimulate the activity of cell wall hydrolasescontributing to cell wall loosening.
Esterification of pectin
Pectinsform approximately 35% of the dry weight of dicotcell walls. They are polymerised in the cis Golgi, methylesterified in the medial Golgi and substituted with side chains in the trans Golgi cisternae Pectin biochemistry can be rather complicated but put simply, the pectin backbone comprises 3 types of polymer: homogalactruonan (HGA); rhamnogalacturan I (RGI); rhamnogalacturan II (RGII).
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
Most of the purified plant pectinesterases have neutral or alkaline isoelectric points and are bound to the cell wall via
electrostaticinteractions. 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 isoformsmay 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 fungiand bacteriais usually much lower than this.
Pectinesterase molecular biology and biochemistry
PE proteins are synthesised as pre-proteins of 540-580
amino acidspossessing a signal sequence and a large amino-terminal extension of around 22 kDa. This terminal extension is eventually removed to yield a mature protein of 34-37 kDa. Most PEs lack consensus sequencesfor N-glycosylationin the mature protein, although at least one site is present in the amino-terminal extension region.
Spatial and temporal regulation of pectinestersae 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 framesthat are annotated as pectinesterases, most of which are encoded as large pre-proproteins. The signal peptidepre-region is required for targeting the enzyme to the endoplasmic reticulumand consists of about 25 amino acid residues. These N-terminalregions contain several glycosylationsites 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 pectinesterses are encoded by a family of genes that are differentially regulated in cell type in response to specific developmental or environmental cues.
tructure of pectinesterase
The N-terminal pro-peptides of pectinesterase are variable in size and sequence and show a low level of amino acid identity. Alternatively the
C-terminalcatalytic region is highly conserved and constitutes the mature enzyme. To date the only known three-dimensional structure for a plant pectinesteraseis for an isoform from carrot("Daucus carota") root and consists of a right-handed parallel β-helix as seen in all the carbohydrate esterasefamily CE-8, a transmembranedomain and a pectin binding cleft. Similarly several pectinesterase structures have been elucidated in fungi and share most of the structural motifs seen in plants.
Pectinesterase isoforms in plants
Several pectinesterase isoforms differing in
molecular weight, isoelectric pointand biochemical activity have been identified in dicotyledonousplants. Pectinesterase isoforms are encoded by a family of genes, some of which are constitutively expressed throughout the plant, whereas others are differentially expressed in specific tissues and at different developmental stages. Isoforms of pectinesterase differ in various biochemical parameters such as relative molecular mass, isoelectric point, optimum pH, substrate affinity, ion-requirement and location.
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Look at other dictionaries:
pectinesterase — noun Date: 1945 an enzyme that catalyzes the hydrolysis of pectins into pectic acids and methanol … New Collegiate Dictionary
pectinesterase — pec·ti·nes·ter·ase (pĕk tə nĕsʹtə rās , rāz ) n. An enzyme found in certain plants, bacteria, and fungi that catalyzes the hydrolysis of pectin to pectic acid and methanol. Also called pectase. * * * … Universalium
pectinesterase — noun A ubiquitous cell wall associated enzyme that presents several isoforms that facilitate plant cell wall modification and subsequent breakdown, found in all higher plants as well as in some bacteria and fungi … Wiktionary
pectinesterase — SYN: pectase. * * * pec·tin·es·ter·ase .pek tə nes tə .rās, .rāz n an enzyme that catalyzes the hydrolysis of pectins into pectic acids and methanol called also pectase … Medical dictionary
pectinesterase — pec·tin·esterase … English syllables
pectinesterase — |pektə̇n+ noun Etymology: pectin + esterase : an enzyme that catalyzes the hydrolysis of pectins into pectic acids and methanol and that occurs especially in higher plants called also pectase … Useful english dictionary
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