PECTIN: STRUCTURE, FUNCTIONS, TYPES, FOODS, APPLICATIONS - BIOLOGY - 2025

The pectins are the group of polysaccharides of vegetable origin structurally more complex nature, whose main structure is composed of residues of D-galacturonic acid bound by glucosidic bonds of α-1,4-D type.

In dicotyledonous plants and some nongramineous monocots, pectins make up approximately 35% of the molecules present in primary cell walls. They are especially abundant molecules in the walls of growing and dividing cells, as well as in the "soft" parts of plant tissues.

Basic unit of pectin, galacturonic acid esterified to a methyl group (-CH3) (Source: Simann13 via Wikimedia Commons)

In cells of higher plants, pectins are also part of the cell wall and multiple lines of evidence suggest that they are important for growth, development, morphogenesis, cell-cell adhesion processes, defense, signaling, cell expansion, hydration of seeds, development of fruits, etc.

These polysaccharides are synthesized in the Golgi complex and are then transported to the cell wall by means of membrane vesicles. Pectins, being part of the plant cell wall matrix, are thought to function as a site for deposition and extension of the glucan network that has important roles in wall porosity and adherence to other cells.

Furthermore, pectins have industrial uses as gelling and stabilizing agents in food and cosmetics; they have been used in the synthesis of biofilms, adhesives, paper substitutes and medical products for implants or drug carriers.

Many studies point to its benefits for human health, since it has been shown that they contribute to the decrease of cholesterol and blood glucose levels, in addition to the stimulation of the immune system.

Structure

Pectins are a family of proteins essentially made up of galacturonic acid units covalently linked together. Galacturonic acid represents more or less 70% of the entire molecular structure of pectins and can be attached at the O-1 or O-4 positions.

Galacturonic acid is a hexose, that is, it is a sugar with 6 carbon atoms whose molecular formula is C6H10O.

It has a molecular weight of more or less 194.14 g / mol and differs structurally from galactose, for example, in that the carbon at position 6 is attached to a carboxyl group (-COOH) and not to a hydroxyl group (-OH).

Different types of substituents can be found on galacturonic acid residues, which more or less define the structural properties of each type of pectin; some of the most common are methyl groups (CH3) esterified to carbon 6, although neutral sugars can also be found in the side chains.

Domain combination

Some researchers have determined that the different pectins present in nature are nothing more than a combination of homogeneous or smooth domains (without branches) and others highly branched or "hairy", which are combined with each other in different proportions.

These domains have been identified as the homogalacturonan domain, which is the simplest of all and the one with the fewest "showy" side chains; the rhamnogalacturonan-I domain and the rhamnogalacturonan-II domain, one more complex than the other.

Due to the presence of different substituents and in different proportions, the length, the structural definition and the molecular weight of pectins are highly variable, and this also depends, to a great extent, on the type of cell and the species considered.

Types or domains

The galacturonic acid that makes up the main structure of pectins can be found in two different structural forms that constitute the backbone of three polysaccharide domains found in all types of pectins.

These domains are known as homogalacturonan (HGA), rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II). These three domains can be linked covalently, forming a thick network between the primary cell wall and the middle lamella.

Homogalacturonan (HGA)

It is a linear homopolymer composed of D-galacturonic acid residues linked together by glucosidic bonds of the α-1,4 type. It can contain up to 200 galacturonic acid residues and is repeated in the structure of many pectin molecules (it comprises more or less 65% of the pectins)

This polysaccharide is synthesized in the Golgi complex of plant cells, where more than 70% of its residues have been modified by esterification of a methyl group on the carbon belonging to the carboxyl group at position 6.

Chemical structure of homogalacturonan (Source: NEUROtiker via Wikimedia Commons)

Another modification that galacturonic acid residues in the homogalacturonan domain can undergo is acetylation (addition of an acetyl group) of carbon 3 or carbon 2.

In addition, some pectins have xylose substitutions at carbon 3 of some of their residues, which yields a different domain known as xylogalacturonan, abundant in fruits such as apples, watermelons, carrots, and in the seed coat of peas.

Ramnogalacturonan-I (RG-I)

This is a heteropolysaccharide made up of just under 100 repeats of the disaccharide made up of L-rhamnose and D-galacturonic acid. It represents between 20 and 35% of pectins and its expression depends on the type of cell and the moment of development.

Much of the rhamnosil residues in its backbone have side chains that possess individual, linear or branched L-arabinofuranose and D-galactopyranose residues. They may also contain fucose residues, glucose, and methylated glucose residues.

Ramnogalacturonan II (RG-II)

This is the most complex pectin and only represents 10% of cellular pectins in plants. Its structure is highly conserved in plant species and is formed by a homogalacturonan skeleton of at least 8 D-galacturonic acid residues linked by 1,4 bonds.

In their side chains, these residues have branches of more than 12 different types of sugars, linked through more than 20 different types of bonds. It is common to find rhamnogalacturonan-II in dimer form, with the two portions linked together by a borate-diol ester bond.

Features

Pectins are mainly structural proteins and, since they can associate with other polysaccharides such as hemicelluloses, also present in the cell walls of plants, they confer firmness and hardness to said structures.

In fresh tissue, the presence of free carboxyl groups in the pectin molecules increases the possibilities and the binding strength of calcium molecules between the pectin polymers, which gives them even more structural stability.

They also function as a moisturizing agent and as an adhesion material for the various cellulolytic components of the cell wall. In addition, they play an important role in controlling the movement of water and other plant fluids through the fastest growing portions of tissue in a plant.

The oligosaccharides derived from the molecules of some pectins participate in the induction of lignification of certain plant tissues, promoting, in turn, the accumulation of protease inhibitor molecules (enzymes that degrade proteins).

For these reasons, pectins are important for growth, development and morphogenesis, cell-cell signaling and adhesion processes, defense, cell expansion, seed hydration, fruit development, among others.

Pectin-rich foods

Pectins are an important source of fiber that is present in a large number of vegetables and fruits consumed daily by man, as it is a structural part of the cell walls of most green plants.

It is very abundant in the peels of citrus fruits such as lemons, limes, grapefruits, oranges, mandarins and passion fruits (passion fruit or passion fruit), however, the amount of pectin available depends on the state of maturity of the the fruits.

The greener or less ripe fruits are those with a higher content of pectins, otherwise those fruits that are too ripe or overdone.

Jam, sweet or jelly, one of the culinary applications of pectin (Image by RitaE on pixabay.com)

Other pectin-rich fruits include apples, peaches, bananas, mango, guava, papaya, pineapple, strawberries, apricots, and various types of berries. Among the vegetables that have abundant amounts of pectin are tomatoes, beans and peas.

Furthermore, pectins are commonly used in the food industry as gelling additives or stabilizers in sauces, galeas and many other types of industrial preparations.

Applications

In the food industry

Given their composition, pectins are highly soluble molecules in water, which is why they have multiple applications, especially in the food industry.

It is used as a gelling, stabilizing or thickening agent for multiple culinary preparations, especially jellies and jams, yogurt-based drinks, milkshakes with milk and fruits, and ice creams.

Pectin is popular for jam making (Image by Michal Jarmoluk on pixabay.com)

The industrial production of pectin for these purposes is based on its extraction from the peels of fruits such as apples and some citrus fruits, a process that is carried out at high temperature and in acidic pH conditions (low pH).

In human health

In addition to being naturally present as part of the fiber in many of the plant-based foods that humans consume daily, pectins have been shown to have “pharmacological” applications:

- In the treatment of diarrhea (mixed with chamomile extract)

- Block the adherence of pathogenic microorganisms to the stomach mucosa, avoiding gastrointestinal infections

- They have positive effects as immuno-regulators of the digestive system

- Lower blood cholesterol

- Decrease the glucose absorption rate in the serum of obese and diabetic patients

References

1. BeMiller, JN (1986). An introduction to pectins: structure and properties. Chemistry and function of pectins, 310, 2-12.
2. Dergal, SB, Rodríguez, HB, & Morales, AA (2006). Food chemistry. Pearson Education.
3. Mohnen, D. (2008). Pectin structure and biosynthesis. Current opinion in plant biology, 11 (3), 266-277.
4. Thakur, BR, Singh, RK, Handa, AK, & Rao, MA (1997). Chemistry and uses of pectin-a review. Critical Reviews in Food Science & Nutrition, 37 (1), 47-73. Thakur, BR, Singh, RK, Handa, AK, & Rao, MA (1997). Chemistry and uses of pectin-a review. Critical Reviews in Food Science & Nutrition, 37 (1), 47-73.
5. Voragen, AG, Coenen, GJ, Verhoef, RP, & Schols, HA (2009). Pectin, a versatile polysaccharide present in plant cell walls. Structural Chemistry, 20 (2), 263.
6. Willats, WG, McCartney, L., Mackie, W., & Knox, JP (2001). Pectin: cell biology and prospects for functional analysis. Plant molecular biology, 47 (1-2), 9-27.