**2. Pectinase substrates**

The name "pectinase" indicates that it is an enzyme whose substrate is "pectin." This name is derived from the Greek word "πηκτικός" pēktikós, which means congeal or solidify. Pectin is the descriptive name given to a diverse group of the compounds that are responsible for gel formation. They are normally extracted from fruits and processed to be used, particularly in jams and jellies. Chemically, pectin is an extensive and heterogeneous group of polysaccharides of high molecular weights, whose backbone structure contains galacturonic acid as the main unit. The free carboxylic group of galacturonic acid may be stabilized by divalent ions. Consequently, pectin is generally found in the form of calcium and magnesium pectates. Pectin in native form is present in the primary plant cell wall of dicotyledonous and some monocotyledonous plants [6] as well as in the middle lamella [7]. It may be interlinked with other macromolecules to form insoluble protopectin [8]. The structure and composition of pectic macromolecules depend on the plant source, and some of them may be complex branched heteropolysaccharides containing more than 17 different glycosyl residues, as described in detail in other chapters of this book.

Pectic substances represent about 0.5–4.0% of the weight of fresh plants [9] and maybe expanded to reach 10–30% of the total weight in some plants like turnips, pineapple, tomato pulp, and citrus peels [10]. They contribute to the firmness and structure of plant tissues; they confer rigidity on cell walls [11] and protect the plant from droughts, withering, hazardous microorganisms, and an array of other plant pathogens.

Pectic substances could be taken as indicators of maturity and ripening of vegetable and fruits during growing as well as evolution of texture through storage. Nutritionally, insoluble pectic substances of fruits and vegetables are a significant part of the dietary fibre, which provides the beneficial effect of protecting consumers from chronic diseases, especially diabetes and colorectal cancer [12, 13]. As for the soluble pectic substances, which are normally extracted from agroindustrial by-products, such as citrus and apple peels are traditionally used as gelling and/or thickening substances in the field of food processing and of other industries.

The main unit in the chemical structure of the pectin and pectic substances is α-D-galacturonic acid which is linked by (1 ! 4) bonds. The side chains of the pectic macromolecule may include even 17 different types of monosaccharides, of which α-L-arabinofuranose, α-D-galactopyranose, α-L-rhamnopyranose, and β-Lxylofuranose are the most abundant units. As a matter of fact, pectic substances include two different high molecular weight fractions of polysaccharides: homoand hetero-polysaccharides, and thus, according to their monosaccharide composition and structure, pectic substances can be classified following a simplified system into the following main groups: homogalacturonans, rhamnogalacturonans type I, rhamnogalacturonans type II, xylogalacturonans, and other heterogalacturonans.

**Homogalacturonan.** The homopolysaccharide homogalacturonan (**HG**) is a linear chain of (1 ! 4) linked α-D-galactopyranosyluronic acid (*Galp*A) residues in which some of the carboxyl groups are either methyl esterified or O-acetylated at C3 or C2 depending on the plant of pectin source. As homogalacturonans are linear polysaccharide chains, they could be described as "smooth" regions of pectic macromolecules.

**Rhamnogalacturonan-I.** The heteropolysaccharide group named rhamnogalacturonan-I **(RG-I)** includes pectic substances with a backbone structure consisting of the repeating disaccharide [(1 ! 4)-α-D-*Galp*A-(1 ! 2)-α-L- *Rhap*- (1!)] where α-L-*Rhap* stands for α-L-rhamnopyranose. The backbone of *Galp*A residues may be O-acetylated on C-2 and/or C-3 [14]. There is no conclusive chemical evidence that the *Galp*A residues are methyl esterified, however, an enriched RG-I-like wall fraction from flax has been reported to contain methyl esters [15].

**Rhamnogalacturonan-II.** Rhamnogalacturonan-II (**RG-II**) is a group of nonsoluble pectic polysaccharides found normally in plant cell walls that are solubilised by treating the cell wall with endopolygalacturonases. They have molecular weights ranged between 5 and 10 kDa. The backbone of RG-II contains at least eight repeating units of 1 ! 4-linked α-D-*Galp*A residues and it is substituted with highly complex side chains that contain at least 12 different glycosyl residues [16]. Two structurally distinct disaccharides (chains C and D) are attached to C-3 of the backbone and two structurally distinct oligosaccharides (chains A and B) are attached to C-2 of the backbone (**Figure 1**).

Depending on the plant source and the method of isolation, between 20 and 80% of the *Rhap* residues are substituted at C-4 with neutral and acidic oligosaccharides. The oligosaccharides contain linear and branched α-L-arabinofuranose and β-Dgalactopyranose residues. Some of these side chains may be terminated with α-Lfructopyranose, β-D-glucuronic acid, and 4-O-methyl β-D-glucuronic acid residues [18, 19].

**Other heterogalacturonans.** Among heterogalacturonans xylogalacturonans (**XG**) have been gaining more attention and found mainly in fruit pectins [20]. They have a homogalacturonan backbone with frequent single xylose residues linked β-(1 ! 3) to about half of the galacturonic acid residues.

Rhamnogalacturonans I and II and xylogalacturonans are branched-chain polysaccharides, and some authors describe them as "hairy" regions of pectic macromolecules [2].

As mentioned above, this classification is made according to the monosaccharide composition and structure of the pectic substances. Nevertheless, pectic substances can be also grouped into four categories according to their molecular weight, watersolubility, and degree of methoxylation of their carboxylic acid groups, namely:

*Fungal Pectinases in Food Technology DOI: http://dx.doi.org/10.5772/intechopen.100910*

#### **Figure 1.**

*Model structure of pectin as methyl ester polygalacturonic acid branched at RGI, RG-II, and XG (modified from [17]).*

pectic acid, pectinic acid, pectin, and protopectin [8]. Whereas protopectin is water-insoluble, the other three are either totally or partially soluble in water.

**Protopectin** is the parent form of pectic substances and upon restricted hydrolysis yields pectin, pectinic, and pectic acids, as well as other derivatives. It is the term used to describe the native water-insoluble pectic substances found in the plant cell wall and middle lamella from which soluble pectic substances are produced.

**Pectic acid** is a product of the hydrolysis of pectin it is mainly polygalacturonic acid in the form of a linear polymer of the repeating unit of galacturonic acid, which contains negligible amounts of methoxyl groups. Free carboxylic groups could be partially or completely neutralised by sodium, potassium, or ammonium ions. Salts of pectic acid are called pectates.

**Pectin** represents the main carbohydrate component of the primary cell wall and middle lamella which accounts for about one-third of the total cell wall material [21]. As mentioned above, the main unit of pectin is galacturonic acid esterified at its carboxylic group with methanol, and the methoxylation ratio of galacturonic acid residues fluctuates between 60 and 90%.

**Pectinic acid** is the intermediate in methyl ester content between pectic acid and pectin (0 ≤ methoxylation ratio < 60%). Pectinates is a common name of pectinic acid salts.

The molecular weights of pectic substances range from 25 to 360 kDa and their degree of methoxylation fluctuates depending on the plant source and method of extraction. Free hydroxyl groups of galacturonic acid are partly or completely

neutralised by sodium, potassium, or ammonium ions, and some of the hydroxyl groups on C2 and C3 may be acetylated [22].
