Structural insights into the substrate-binding cleft of AlyF reveal the first long-chain alginate-binding mode

Advances in alginate lyases and the potential application of enzymatic prepared alginate oligosaccharides: A mini review

Alginate is mainly a linear polysaccharide composed of randomly arranged β-D-mannuronic acid and α-L-guluronic acid linked by α, β-(1,4)-glycosidic bonds. Alginate lyases degrade alginate mainly adopting a β-elimination mechanism, breaking the glycosidic bonds between the monomers and forming a double bond between the C4 and C5 sugar rings to produce alginate oligosaccharides consisting of 2-25 monomers, which have various physiological functions. Thus, it can be used for the continuous industrial production of alginate oligosaccharides with a specific degree of polymerization, in accordance with the requirements of green exploitation of marine resources. With the development of structural analysis, the quantity of characterized alginate lyase structures is progressively growing, leading to a concomitant improvement in understanding the catalytic mechanism. Additionally, the use of molecular modification methods including rational design, truncated expression of non-catalytic domains, and recombination of conserved domains can improve the catalytic properties of the original enzyme, enabling researchers to screen out the enzyme with the expected excellent performance with high success rate and less workload. This review presents the latest findings on the catalytic mechanism of alginate lyases and outlines the methods for molecular modifications. Moreover, it explores the connection between the degree of polymerization and the physiological functions of alginate oligosaccharides, providing a reference for enzymatic preparation development and utilization.

Structure Characteristics, Biochemical Properties, and Pharmaceutical Applications of Alginate Lyases

Alginate, the most abundant polysaccharides of brown algae, consists of various proportions of uronic acid epimers α-L-guluronic acid (G) and β-D-mannuronic acid (M). Alginate oligosaccharides (AOs), the degradation products of alginates, exhibit excellent bioactivities and a great potential for broad applications in pharmaceutical fields. Alginate lyases can degrade alginate to functional AOs with unsaturated bonds or monosaccharides, which can facilitate the biorefinery of brown algae. On account of the increasing applications of AOs and biorefinery of brown algae, there is a scientific need to explore the important aspects of alginate lyase, such as catalytic mechanism, structure, and property. This review covers fundamental aspects and recent developments in basic information, structural characteristics, the structure-substrate specificity or catalytic efficiency relationship, property, molecular modification, and applications. To meet the needs of biorefinery systems of a broad array of biochemical products, alginate lyases with special properties, such as salt-activated, wide pH adaptation range, and cold adaptation are outlined. Withal, various challenges in alginate lyase research are traced out, and future directions, specifically on the molecular biology part of alginate lyases, are delineated to further widen the horizon of these exceptional alginate lyases.

Exploring molecular determinants of polysaccharide Lyase family 6-1 enzyme activity

The Polysaccharide Lyase Family 6 (PL6) represents one of the 41 polysaccharide lyase families classified in the CAZy database with the vast majority of its members being alginate lyases grouped into three subfamilies, PL6_1-3. To decipher the mode of recognition and action of the enzymes belonging to subfamily PL6_1, we solved the crystal structures of Pedsa0632, Patl3640, Pedsa3628 and Pedsa3807, which all show different substrate specificities and mode of action (endo-/exo-lyase). Thorough exploration of the structures of Pedsa0632 and Patl3640 in complex with their substrates as well as docking experiments confirm that the conserved residues in subsites -1 to +3 of the catalytic site form a common platform which can accommodate various types of alginate in a very similar manner but with a series of original adaptations bringing them their specificities of action. From comparative studies with existing structures of PL6_1 alginate lyases, we observe that in the right-handed parallel β-helix fold shared by all these enzymes, the substrate binding site harbors the same overall conserved structures and organization. Despite this apparent similarity, it appears that members of the PL6_1 subfamily specifically accommodate and catalyze the degradation of different alginates suggesting that this common platform is actually a highly adaptable and specific tool.

Substrate-Binding Mode and Intermediate-Product Distribution Coguided Protein Design of Alginate Lyase AlyF for Altered End-Product Distribution

Recently, we reported alginate lyase AlyF that predominantly produced trisaccharides (the trisaccharide content is 87.0%), and the determination of its substrate-binding mode facilitated its protein engineering for new product distribution. To clarify the relationship between the substrate-binding pocket and end-product distribution, the open binding pocket change was initially designed. The resulting F128T_W172R mutant of AlyF exhibited different intermediate-product distributions but still similar end-product distributions. However, these observations suggested that cleavage pattern changes for intermediate products might contribute to an altered end-product distribution. Structural analysis indicated that the sugar-binding affinity at subsite -2 should be redesigned to achieve this goal. Thus, residue Arg266, which is involved in sugar binding at subsite -2, was selected for site-saturation mutagenesis in the F128T_W172R mutant. The dominant end products of the F128T_W172R_R226H mutant were altered to disaccharides and trisaccharides (the disaccharide content increased to 40.5%).