Pectinesterase

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 de-esterification 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.

Recent studies^{[citation needed]} 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, cell wall extension during pollen germination and pollen tube growth, abscission, stem elongation, tuber yield and root development. Pectinesterase has also been shown to play a role in a plants response to pathogen attack. A cell wall-associated pectinesterase of Nicotiana tabacum is involved in host cell receptor recognition for the tobacco mosaic virus 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 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.

Pectins form approximately 35% of the dry weight of dicot 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 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.