Pectinesterase

Related Reading

Pectinesterase is also known as pectinase, pectin methylesterase, and pectin oxidase. The enzymes that catalyze the hydrolysis of methoxy esters of pectin to produce pectic acid and methanol are not only widely distributed in higher plants, but also known to exist in molds and bacteria. The substrate specificity of the deesterification reaction is not yet fully understood. Therefore, enzymes almost only act on methyl esters in higher plants, so there are opinions that the name pectin methylesterase should be used for this enzyme. The activity determination is based on pectin as a substrate, and the carboxyl groups produced under certain conditions are quantified with alkali; there is also a method to quantify the methanol produced. The optimum pH is 8-4, depending on the source of the enzyme. Some cations, especially Na⁺, Ca²⁺, etc. can increase the activity of this enzyme.

Structure

The N-terminal propeptide of pectin esterase is variable in size and sequence, and shows a low level of amino acid identity. Alternatively, the C-terminal catalytic region is highly conserved and constitutes a mature enzyme. The first three-dimensional structure to solve plant pectin esterase is the isoform of carrot root, which is composed of right-handed parallel β-helices, and the transmembrane domain is combined with pectin. Similarly, several pectin esterase structures have been elucidated in fungi and E. coli, and they have most of the structural motifs commonly found in plants.

Figure 1. Protein structure of Pectinesterase.

Functions

The manipulation of pectin esterase expression can affect many physiological processes. In plants, the effect of pectin esterase on plant cell wall components can produce two diametrically opposed effects. The first is to promote the hardening of the cell wall by generating blocks of unesterified carboxyl groups that can interact with calcium ions to form a pectin-like gel. The other is that the release of protons may stimulate the activity of cell wall hydrolytic enzymes, resulting in cell wall relaxation. Pectin esterase plays a role in regulating the mechanical stability of the cell wall during fruit ripening, and in cell wall extension, shedding, stem elongation, tuber yield and root development during pollen germination and pollen tube growth. Pectin esterase has also been shown to play a role in the response of plants to pathogen attack. Tobacco cell wall-associated pectin esterase is involved in host cell receptor recognition of the tobacco mosaic virus movement protein, and it has been shown that this interaction is necessary for virus cell transfer.

Molecular biology and biochemistry

Pectin esterase is thought to be secreted into the cytoplasm using highly methylated pectin, although at some point in the secretory pathway, the N-terminal propeptide is cleaved. At present, although it has been hypothesized that this proregion can act as an intramolecular chaperone to ensure correct folding or inactivation activity until the insertion of PE into the cell wall is completed, the role of this proregion is not yet clear. Recently, special attention has been paid to the molecular research of pectin esterase, which has led to the characterization of several related isoforms in various higher plant species. Some of these pectinesterases are ubiquitously expressed, while others are specifically expressed during fruit ripening, pollen grain germination or stem elongation. These data indicate that pectin esters are encoded by a family of genes that are differentially regulated in cell types in response to specific developmental or environmental cues.