Polysaccharide Lyase Family 40

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• Author: Navindu Dinara Gajanayaka

• Responsible Curator: Jan-Hendrik Hehemann

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Polysaccharide Lyase Family PL40
3D Structure
Mechanism
Charge neutraliser
Active site residues
CAZy DB link
https://www.cazy.org/PL40.html

Substrate specificities

PL40 enzymes are categorized as ulvan lyases that primarily degrade ulvan, a sulfated polysaccharide found in green macroalgae (Ulva spp.). These enzymes specifically target the uronic acid-rich backbone regions of ulvan, particularly the GlcA/IdoA-Rha3S motif [1]. PL40 family members function as endolytic enzymes, cleaving internal glycosidic linkages to release oligosaccharides terminated with Δ4,5-unsaturated uronic acids [1, 2, 3, 4].

Kinetics and Mechanism

PL40 ulvan lyases employ a β-elimination mechanism in which abstraction of the C5 proton from the uronic acid residue triggers cleavage of the C-O4 glycosidic bond, generating a Δ4,5-unsaturated uronic acid at the non-reducing end of the product [4]. The catalytic machinery of PL40 Uly1040 involves key residues including histidine and tyrosine, along with Mn²⁺ and additional residues within the substrate binding pocket, which collectively organize and activate the catalytic center [4]. Enzymes within the PL40 family typically exhibit optimal activity at pH 7-8 and temperatures of 35-40°C [2, 4]. Their catalytic efficiency is often enhanced by divalent metal ions, including Mn²⁺, Fe²⁺, Mg²⁺, and Ca²⁺ [2, 4].

Catalytic Residues

To date, Uly1040 remains the only comprehensively characterized PL40 enzyme with detailed structural and mechanistic information, serving as the prototypical model for the entire family [4]. The catalytic mechanism centers on a conserved His/Tyr dyad, a histidine residue (His485) functions as the general base, abstracting the C5 proton from the uronic acid during β-elimination, while a tyrosine (Tyr305) acts as the general acid, donating a proton to the leaving group. The active site architecture includes several supporting residues that facilitate catalysis. Trp246 and Asn245 stabilize the transition state by neutralizing the negative charge on the uronic acid carboxyl group at the +1 subsite. Additionally, His487 and Asp358 coordinate with a bound Mn²⁺ ion to properly orient and activate the catalytic histidine (His485). Bioinformatic and phylogenetic analyses reveal that this His/Tyr catalytic dyad and its network of supporting residues are highly conserved across PL40 family members, suggesting a shared catalytic mechanism throughout the family [4].

Three-dimensional structures

The crystal structure of Uly1040 from Alteromonas macleodii, solved at 1.74 Å resolution (PDB ID 9VTK) , represents the only structurally characterized member of the PL40 family to date. Uly1040 exhibits a two-domain architecture characteristic of PL40 ulvan lyases. The N-terminal domain (residues 27–430) consists a distinctive (α/α)₆ toroid fold comprising 16 α-helices and 2 β-strands. This domain region critical catalytic machinery, including the catalytic base His485 and the Mn²⁺ coordination site. The C-terminal domain (residues 444–856) contains a more complex architecture with 7 α-helices and 29 β-strands organized into six antiparallel β-sheets. Together, these two domains create a deep substrate binding groove specifically creatred to accommodate ulvan's sulfated uronic acid–rhamnose backbone. The domains are connected by a short linker region (residues 431–443). This structural architecture strategically positions the conserved catalytic residues, including the general acid Tyr305 and supporting residues Trp246 and Asn245, to facilitate the β-elimination mechanism [4].

Family Firsts

First stereochemistry determination

P10_PLnc BN863_21990 from Formosa agariphila provided the first evidence of β-elimination activity in PL40 by generating Δ4,5-unsaturated uronic acid products [1].

First general acid/base residue identification

Uly1040 structure and mutagenesis first identified His485 (general base) and Tyr305 (general acid) as the conserved catalytic dyad for PL40 β-elimination [4].

First charge neutralizer

Trp246 and Asn245 were identified as the key charge neutralizer residues that stabilize the negative charge on the uronic acid carboxylate group at the +1 subsite during β-elimination [4].

First 3-D structure

Uly1040 at 1.74 Å showing the (α/α)₆ toroid and anti-parallel β-sheet domains [4].

References

1. Reisky, L., et al. (2019) A marine bacterial enzymatic cascade degrades the algal polysaccharide ulvan. Nat Chem Biol 15(8):803–812.

2. Gajanayaka, N.D., et al. (2024) Pseudoalteromonas agarivorans-derived novel ulvan lyase of polysaccharide lyase family 40: Potential application of ulvan and partially hydrolyzed products in cosmetic industry. J Ind Microbiol Biotechnol 52.

3. Fu, Z., et al. (2026) Mechanistic insights into catalysis of a novel polysaccharide lyase family 40 ulvan lyase from Thalassomonas sp. LD5. Int J Biol Macromol 336:149298.

4. Wang, H.-Q., et al. (2026) Structural and functional insights into Uly1040, an ulvan lyase from polysaccharide lyase family 40. Appl Environ Microbiol e02101-25.