Pectinesterase (EC 3.1.1.11; systematic name pectin pectylhydrolase) is a ubiquitous cell-wall-associated

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

that presents several

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...

that facilitate plant cell wall modification and subsequent breakdown. It catalyzes the following reaction: :pectin + ''n'' H₂O = ''n'' methanol + pectate It is found in all higher plants as well as in some

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. Pectinesterase functions primarily by altering the localised pH of the cell wall resulting in alterations in cell wall integrity. Pectinesterase catalyses the de-esterification of

pectin Pectin ( ': "congealed" and "curdled") is a heteropolysaccharide, a structural polymer contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal chemical component of pectin is galact ...

into pectate and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

. 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 ''Pectobacterium carotovorum'' is a bacterium of the family Pectobacteriaceae; it used to be a member of the genus ''Erwinia''. The species is a plant pathogen with a diverse host range, including many agriculturally and scientifically impo ...

'' and in fungal pathogens such as ''

Aspergillus niger ''Aspergillus niger'' is a mold classified within the ''Nigri'' section of the ''Aspergillus'' genus. The ''Aspergillus'' genus consists of common molds found throughout the environment within soil and water, on vegetation, in fecal matter, on de ...

'', pectinesterase is involved in maceration and soft-rotting of plant tissue. Plant pectinesterases are regulated by pectinesterase inhibitors, which are ineffective against microbial enzymes.

Function

Recent studies have shown that the manipulation of pectinesterase expression can influence numerous physiological processes. In plants, pectinesterase plays a role in the modulation of cell wall mechanical stability during

fruit ripening Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes Sweetness, sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does no ...

, cell wall extension during pollen germination and

pollen tube A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells fr ...

growth,

abscission Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a ...

, stem elongation,

tuber Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...

yield and root development. Pectinesterase has also been shown to play a role in a plants response to

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

attack. A cell wall-associated pectinesterase of ''

Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an annually grown herbaceous plant of the genus ''Nicotiana''. ''N. tabacum'' is the most commonly grown species in the genus ''Nicotiana,'' as the plant's leaves are commercially harvested to be ...

'' is involved in host cell receptor recognition for the

tobacco mosaic virus Tobacco mosaic virus (TMV) is a positive-sense single-stranded RNA virus species in the genus '' Tobamovirus'' that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteris ...

movement protein and it has been shown that this interaction is required for cell-to-cell translocation of 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

carboxyl In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl group (e.g. ...

groups that can interact with

calcium ions Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction ...

forming a pectate gel. The other being that proton release may stimulate the activity of cell wall

hydrolases In biochemistry, hydrolases constitute a class of enzymes that commonly function as biochemical catalysts that use water to break a chemical bond: :\ce \quad \xrightarrowtext\quad \ce This typically results in dividing a larger molecule into s ...

contributing to cell wall loosening.

Esterification of pectin

Pectins form approximately 35% of the dry weight of

dicot The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, ...

cell 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: homogalacturonan (HGA); rhamnogalacturonan I (RGI); rhamnogalacturonan 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

electrostatic Electrostatics is a branch of physics that studies slow-moving or stationary electric charges. Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word (), mean ...

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

. 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

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

and

is usually much lower than this.

Molecular biology and biochemistry

PE proteins are synthesised as pre-proteins of 540–580

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

possessing 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 sequences for

N-glycosylation ''N''-linked glycosylation is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), i ...

in the mature protein, although at least one site is present in the amino-terminal extension region. 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 ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...

'' 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 Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

pre-region is required for targeting the enzyme to the

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

and consists of about 25 amino acid residues. These

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

regions contain several

glycosylation Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...

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.

Plant isoforms

Several pectinesterase isoforms differing in

molecular weight A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

isoelectric point The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electric charge, electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). Howe ...

and biochemical activity have been identified in

dicotyledonous The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, ...

plants. 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.

Structure

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-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When t ...

catalytic region is highly conserved and constitutes the mature enzyme. The first three-dimensional structure solved for a plant pectinesterase was for an isoform from

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in colour, though heirloom variants including purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild ...

(''Daucus carota'') root and consists of a right-handed parallel β-helix as seen in all the

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

esterase In biochemistry, an esterase is a class of enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis (and as such, it is a type of hydrolase). A wide range of different esterases exist that differ ...

family CE-8, a

transmembrane A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently u ...

domain and a pectin binding cleft.; Similarly several pectinesterase structures have been elucidated in fungi and ''E.coli'' and share most of the structural motifs seen in plants. 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.

References

External links

* {{Portal bar, Biology, border=no EC 3.1.1