Pectin methyl esterase (PME)

Pectin methyl esterase (PME) [3.1.1.11) is an enzyme that plays a critical role in the metabolism of pectin, a complex carbohydrate that is an essential component of the plant cell wall. This enzyme is widely distributed in all higher plants and that was established in mid-20th Century (Lineweaver et al., 1951).

The enzyme is responsible for the de-esterification of pectin, which involves the removal of methyl ester groups from pectin molecules. This process alters the physical properties of the pectin, making it more susceptible to degradation by other enzymes, such as polygalacturonase, which can break down the pectin into simpler sugars that can be used by the plant for energy.

PME is found in many plant species, including fruits and vegetables, and is involved in a wide range of biological processes, such as fruit ripening, cell wall remodeling, and response to environmental stresses. The enzyme is also of key importance in the food industry, where it is used as a processing aid to modify the texture and consistency of food products.

Structure and Function of Pectin Methyl Esterase

PME belongs to a family of enzymes known as esterases, which catalyze the hydrolysis of ester bonds in a wide range of organic compounds. The enzyme is a glycoprotein with a molecular weight of approximately 30-35 kDa, and is composed of a single polypeptide chain folded into a compact structure with a core of eight parallel beta-strands and several alpha-helices.

PME acts on the pectin polymer by binding to the carboxyl group of the methyl ester and catalyzing the hydrolysis of the ester bond, resulting in the release of methanol and the formation of a carboxylate anion. This reaction alters the physical properties of the pectin, making it more susceptible to cleavage by other enzymes, such as polygalacturonase, which can then break down the pectin into simpler sugars that can be used by the plant for energy.

Regulation of Pectin Methyl Esterase Activity

The activity of PME is regulated by several factors, including pH, temperature, and the presence of inhibitors or activators. The enzyme has an optimum pH range of 7.5 to 8.5, which corresponds to the pH of the plant cell wall. At pH values below 6.0, the enzyme activity decreases rapidly due to the protonation of the carboxylate group, which inhibits the binding of the substrate.

Temperature also plays a critical role in the activity of PME, with the enzyme having an optimum temperature range of 25 to 40°C. At temperatures above 50°C, the enzyme denatures and loses its activity.

The activity of PME can also be modulated by the presence of inhibitors or activators. For example, calcium ions have been shown to activate PME activity in vitro. 

In addition, several proteins have been identified that can interact with PME and modulate its activity, such as pectin methylesterase inhibitors (PMEIs). These are small proteins that bind to PME and prevent it from binding to the substrate. A PMEI has been isolated from kiwi fruit. It forms a 1:1 non-covalent complex. The polypeptide chain is 152 amino acid residues long and contains 5 cysteine amino acids. Four are connected to each other through disulphide bridges (Giovane et al., 2004). Similar inhibitors have been isolated in Arabidopsis.

Salt concentration by virtue of ionic strength in solution promotes PME activity between the ranges of 0.15 and 0.2 M and when the pH range is between 7 and 8 (Aung et al., 1965). 

Role of Pectin Methyl Esterase in Plant Development and Response to Environmental Stresses

PME is involved in several important biological processes in plants, including fruit ripening, cell wall remodeling, and response to environmental stresses. During fruit ripening, PME activity increases, leading to de-esterification of pectin and the softening of the fruit. This process is essential for the development of the characteristic texture and flavour of many fruits, such as tomatoes, apples, and pears.

Pectin methyl esterase (PME) also plays an important role in plant cell wall remodeling through its pectin de-esterification catalysis. The cell wall is a complex structure that provides support and protection to the plant cell, and it is composed of several different types of polysaccharides, including cellulose, hemicellulose, and of course pectin.

Pectin is a complex polysaccharide that is composed of several different types of sugars, including galacturonic acid, rhamnose, and arabinose. The degree of methylation of the pectin molecule affects its physical properties, including its solubility, viscosity, and gelation properties. PME catalyzes the hydrolysis of the methyl ester groups in the pectin molecule, resulting in the release of methanol and the formation of carboxylate groups.

The de-esterification of pectin by PME alters the physical properties of the pectin, making it more susceptible to cleavage by other enzymes, such as polygalacturonase (PG), which breaks down the pectin into simpler sugars. This process leads to the remodeling of the plant cell wall by loosening the cell wall matrix, allowing for cell expansion and growth.

PME activity has been shown to be important in several different developmental processes in plants, including fruit ripening and seed germination. For example, during fruit ripening, PME activity increases, leading to the softening of the fruit. In addition, PME activity has been shown to be important in the germination of seeds, where it plays a role in the breakdown of the seed coat.

PME activity is also important in response to environmental stresses, such as drought and salinity. In response to these stresses, plants produce abscisic acid (ABA), a plant hormone that regulates several physiological processes, including stomatal closure and seed dormancy. ABA has been shown to regulate the expression of PME genes, leading to changes in pectin structure and cell wall remodeling.

So we find PME plays a critical role in plant cell wall remodeling by catalyzing the de-esterification of pectin. This process leads to the loosening of the cell wall matrix, allowing for cell expansion and growth. PME activity is important in several different developmental processes in plants, as well as in response to environmental stresses. Understanding the regulation of PME activity is important for the development of new strategies to improve plant growth and yield under different environmental conditions.

References

Aung, T. and Ross, E. (1965). Heat sensitivity of pectinesterase activity in papaya puree and of catalase-like activity in passion fruit juice. J. Food Sci. 30, pp. 144

Giovane, A., Servillo, L., Balestrieri, C., Raiola, A., D’avino, R., Tamburrini, M., … & Camardella, L. (2004). Pectin methylesterase inhibitor. Biochimica et Biophysica Acta (BBA)-Proteins and Proteomics, 1696(2), pp. 245-252 (Article).

Lineweaver, H. and Jansen, E.F. 1951. Pectic enzymes. In “Advances in Enzymology and Related Subjects of Biochemistry,” ed. Nerd, F.F., vol. 11, . Interscience Publ., New York. pp. 267-295