**3.3 Depolymerase**

Depolymerase, a range of depolymerizing enzymes degrade the pectic substance through cleaving of α-(1 → 4)-glycosidic bonds in DGalA units either by trans elimination or hydrolysis [43]. Split the -(1,4)-glycosidic bonds in pectins either by hydrolysis (polygalacturonase) or by transelimination (lyases).

	- i.Polygalacturonases (PG), that split the glycosidic linkage in the presence of water molecules across the oxygen bridge. Forming a D-galacturonic acid monomer. The structure confirmation of these enzymes loses when it reacts with pectin, which may occur due to the presence of free carboxylic groups in the target molecules. The viscosity of the interaction solution reduces with an increase of reducing end-groups. PG is the most enzyme studied and industrially applied because of its depolymerization specificity via the hydrolysis process [44, 45]. Depending on the pattern of action, PG is categorized into:
	- Exo-PG, which targets the terminal groups of the pectic molecule, lowering of chain length gradually.
	- Endo-PG, that attacks all chain links arbitrarily, resulting in more incisive and faster consequences.

However, rhamnopolygalacturonase catalyzes cleavage within or at the nonreducing terminals of the rhamnogalacturonan core chains [46, 47]. Various microorganisms can produce PG with several biochemical properties and modes of action. Most PGs stimulate the hydrolysis rate at an optimum temperature ranging from 30 to 50°C with ideal pH that ranges from 3.5 to 5.5. It is reported that almost both exo-PG and endo-PG are synthesized in acidic conditions. While some exo-PG are produced at high basic pH (11.0) by certain bacterial and fungal species as *Bacillus* sp. KSM-P410, *Bacillus licheniformis*, and *Fusarium oxysporum* [48]. Whereas rhamno-PG is more efficient and stable at pH 4.0 and temperature 50°C. The molecular weight average for exo-PG and endo-PG is 38 – 65 kDa, while rhamno-PG is 66 kDa [49].

	- Exo-PMG, that targets the terminal groups of the non-reducing end of pectin, releasing methyl mono-galacturonate.

• Endo-PMG, that attacks all chain links randomly. Resulting in more incisive and faster consequences of oligomethyl-galacturonates.

*Lyases* (trans eliminases), in which trans-eliminative breakdown for pectinate polymers or pectate through catalyze the Polygalacturonate depolymerization and pectin esterification, by splitting the C-4 of the glycosidic linkage followed by hydrogen removal from the C-5 releasing an unsaturated product with the unsaturated bond between C-4 and C-5. For activation, some cytoplasmic or intracellular lyases, need ions as Ni2+, Co2+, and Mn2+ [50].

According to the acted substrate, lyase is divided into two types [51]:

	- Exo-PGL, target the non-reducing terminal of pectic acid, releasing unsaturated di-galacturonates.
	- Endo-PGL, which works in an unsystematic cleavage fashion on the substrate, producing unsaturated oligogalacturonates.
	- Oligo-D-galactosiduronate lyase, which acts on the terminal position of unsaturated di-galacturonate, released initially by the pectate lyase action, forming mono-galacturonates [53].
	- Exo-PMGL, that degrades pectin through stepwise transeliminative cleavage, releasing unsaturated methylmonogalacturonates [55].
	- Endo-PMGL, which acts randomly on the pectin by cleaving α-1,4 glycosidic linkages, producing unsaturated methyloligogalacturonates.

Overall, pectin lyases originated mainly from microorganisms, which lead to change in the biochemical properties according to each microbe. These enzymes working efficiently in the temperature range 40-50°C, and alkaline pH 7.5-10.0. The molecular weight of lyases is ranging from 22-90 kDa, while PMGL reached 89 kDa from *Aureobasidium pullulans* LV-10 and 90 kDa from *Pichia pinus*. Whereas the molecular weight for PGL of 55 and 74 kDa was reported in *Yersinia enterocolitica,* and *Bacteroides thetaiotaomicron*, respectively. The point of isoelectric for some lyases are ranged from 5.2 to 10.7. While others are still unexplored [56]. Many enzymes act in the adjacent chains of RGI and RGII as exogalactanase, endogalactanase, α- and β-galactosidase, α-L-arabinofuranosidase, exoarabinase, and endoarabinase [57].
