Alginate lyase

Related Reading

Introduction

Alginate accounts for about 40% of the dry weight and is the most abundant polysaccharide in brown algae. It is formed by β-D-mannuronate (M) and α-L-guluronate (G) as monomer units. These units are linked in 3 different blocks, namely poly β-D-mannuronate (polyM), poly α-L-guluronate (polyG) and the heteropolymer (polyMG). Some bacteria can synthesize alginate, when they suffer from harmful factors such as antibiotics or drought, alginate can play a protective role. Alginate oligosaccharides are products of alginate depolymerization by alginate lyase or physicochemical methods. At present, they are receiving more and more attention because of their wide application in the food and pharmaceutical industries. The same, alginate lyase for mild degradation of alginate has also become the focus for various fields. Alginate lyase can degrade alginate by β-elimination of glycosidic bonds and produce unsaturated oligosaccharides with double bonds at the non-reducing end. Now alginate lyase, especially endolytic alginate lyase has been widely used in the production of alginate oligosaccharides, the preparation of protoplast of red and brown algae, and the elucidation of the fine structure of alginate. In addition, alginate lyase can also degrade bacterial polysaccharide biofilm, making it show great potential in the treatment of cystic fibrosis.

Source and Classification

Alginate lyase has been isolated from a variety of sources, including marine algae, marine mollusks, and various marine and terrestrial bacteria. In addition, some lyases have also been isolated from fungi and viruses (Table 1). According to the substrate specificity, alginate lyase can be grouped into two categories, one is G block-specific lyase (polyG lyase, EC4.2.2.11), the other is M block-specific lyase (PolyM lyase, EC4.2.2.3). According to different modes of action, it can be divided into endolytic and exolytic alginate lyase. endolytic alginate lyase can cleave the glycosidic bond inside the alginate polymer to release unsaturated oligosaccharides, while exolytic alginate lyase is responsible for further degradation of oligosaccharides into monomers. According to the analysis of the hydrophobic clusters of the primary structure, alginate lyase can be divided into 7 Polysaccharide Lyase (PL) families, PL-5, -6, -7, -14, -15, -17 and -18. Part of the endolytic bacterial alginate lyases are assigned to PL-5 and PL-7. The lyases isolated from marine mollusks and viruses are collected in the PL-14 family. Most alginate lyases from bacteria are classified into PL-5, -7, -15 and -17 families. The bifunctional alginate lyases belong to the PL-18 family, while other lyases are scattered in the other 6 families. In addition, alginate lyases can be divided into three categories based on their molecular masses, namely small (25-30 kDa), medium (about 40 kDa) and large lyases (> 60 kDa).

Table 1. Alginate lyase of different PL families from different sources and their substrate specificities (Zhu, B.W.; Yin, H. 2015)

Substrate Specificity

Alginate consists of 4 different types of linkages, such as M-M, M-G, G-G and G-M, and the extent of each linkage is different. Alginate lyases are classified according to their dominant cleaving action on different types of substrates, as shown in Figure 1. The alginate lyases preferring M-rich alginate are grouped into polyM lyase (EC 4.2.2.3), and the lyases preferring G-rich alginate are classified as polyG lyase (EC 4.2.2.11). Although an alginate lyase may be referred to as polyM lyase or polyG lyase, the enzyme generally shows moderate or low processivity for the other homopolymers. Currently, these problems are being solved by using technologies with high accuracy and high sensitivity. At present, there are some reports of alginate lyase with specific substrate specificities, such as the lyase from ATCC43367, which is reported to only cleave M-M linkages. The alginate lyase isolated from the marine mollusk Laambis sp only showed high activity for polyG. However, some alginate lyases show activity on both polyM and polyG, and these lyases are regarded as bifunctional lyases, such as the alginate lyase AlySJ-02 from Pseudoalteromonas sp. SM0524, AlyPEEC from Pseudoalteromonas sp IAM14594, AlyA from Pseudoalteromonas atlantica AR06, and Aly from Pseudoalteromonas sp. 272.

Figure 1. The substrate specificity of alginate lyase and structures of degradation products (Zhu, B.W.; Yin, H. 2015)