Dextranase

Dextranase (EC 3.2.1.11; α-1,6-glucan 6-glucanohydrolase) is an enzyme that hydrolyzes the α-1,6-glycosidic linkage in dextran polymers. The enzyme is widely distributed in variety of fungi and bacteria. The main domain of the enzyme is a right-handed parallel β helix, which is connected to a β sandwich domain at the N terminus. Dextranase cleaves the linkages within the dextran molecule and releases shorter isomaltosaccharides. The use of dextranase in the sugar-processing can reduce the viscosity of the sugar juice, which caused by the dextran contamination.

Classification

Dextranase enzymes are found in two glycoside hydrolase families: GH family 49 and GH family 66, with no sequence similarity between the two families. The bacterial dextranases from the Streptococcus species have been classified into family 66. Both bacterial dextranases from the Arthrobacter species and fungal dextranases from Penicillium species have been classified into family 49. In family 49, dextran 1,6-α-isomaltotriosidase from Brevibacterium fuscum var. dextranlyticum and isopullulanse from Aspergillus niger are also found.

Structure

• Dextranase from Penicillium minioluteum (family 49)

Structure of dextranase from Penicillium minioluteum folds into two domains. The first domain consists of 200 residues forming 13 β strands. Nine of those strands are folded into a β sandwich. The strands in this β sandwich are all antiparallel, with the exception of the interaction between β strands 5 and 13. In the interior of the β sandwich, there are 14 hydrophobic residues that are completely conserved in GH 49. The other domain has a right-handed parallel β-helical fold that contains three parallel β sheets, which are named PB1, PB2, and PB3. The β helix contains ten complete coils. In the N-terminal end of the β helix, there are two incomplete coils that only contain the PB2 and PB3 β strands, and, in the C terminus, the last coil contains only PB1 and PB2. The two domains interact over a large area. The β sheet and the loops in the C-terminal end of the β sandwich fold interact with the surface of PB3 and the T3 loops from coils 1 to 9 in the parallel β helix. The interface has 29 amino acids that are completely conserved in GH 49.

Figure 1. The Structure of dextranase from Penicillium minioluteum successively colored from red in the N-terminus to blue in the C-terminus. (Larsson, et al. 2003)

• Dextranase from Streptococcus mutans (family 66)

Dextranase from Streptococcus mutans formed a multidomain structure composed of three domains, designated as domains N, A, and C. The topology of domain N (Asn100–Asp210) resembled a C2-type immunoglobulin fold, consisting of seven antiparallel β-strands forming three-stranded and four-stranded β-sheets. Domain A (Asp211–Lys593) was mainly composed of a (β/α)8-barrel, which is a catalytic domain in many glycoside hydrolases. There were five relatively large looped regions in this domain, ranging from 23 to 50 residues in length, and the 50-residue-long loop 3 associated with loop 2 to form a small subdomain structure, which was similarly positioned in comparison with the domain B of α-amylases, forming one side wall of the catalytic cleft. Domain C (Val594–Ile732) adopted an antiparallel β-sandwich structure, consisting of 10 β-strands, but basically belonged to two four-stranded Greek key motifs, which are found in many GHs.

Figure 2. Structure of dextranase from Streptococcus mutans. (Suzuki N, et al. 2012)

Application

In sugar production, dextrans are undesirable compounds synthesized by contaminant microorganisms from sucrose, increasing the viscosity of the flow and reducing industrial recovery, bringing about significant losses. The contents of these polysaccharides in sugar cane prior to harvest are very low or almost zero. They are formed by the action of the dextransucrase enzyme from contaminant microorganisms that home to the plant sap or that attack it when its rind is damaged.

The use of the dextranase enzyme is the most efficient method for hydrolyzing the dextrans at sugar mills. The fungal dextranases showed the highest reaction rate at low Brix, with pH and temperature close to 5.0 and 50ºC, respectively, that is, conditions existing in juice extraction. Some of these dextranases formulated in enzymatic preparations have been efficiently used for hydrolyzing dextrans in sugar mill juices. In more advanced stage of the process, where the dextrans have already caused losses, the conditions of temperature and Brix are high. However, although the volumes are smaller, the treatment with these enzymes in syrup showed the need to increase the dose, equaling dextranase consumption. Some thermo tolerant bacterial dextranases identified up to now showed a much reduced specific activity that makes their industrial use unfeasible. The fungal dextranases from Chaetomium sp. have shown the best results on dextrans treatment both in juices and syrups.

References