Polysaccharide Lyase Family 20

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• Author: Naotake Konno
• Responsible Curator: Shinya Fushinobu

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Polysaccharide Lyase Family PL20
3D Structure	β-jelly roll
Mechanism	β-elimination
Charge neutraliser	none
Active site residues	unknown
CAZy DB link
https://www.cazy.org/PL20.html

Substrate specificities

Polysaccharide lyases of family 20 cleave β-1,4 linkages in polyglucuronate (β-1,4-glucuronan lyase; EC 4.2.2.14). The first PL20 enzyme was cloned from filamentous fungus Trichoderma reesei (TrGL) [1]. TrGL was highly specific for β-1,4-glucuronan prepared from regenerated cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation (cellouronate). Polyglucuronate is a minor polysaccharide compared with other polyuronates. But β-1,4-glucuronan structures are found in water-soluble polysaccharides from bacteria, fungi, and algae.

Kinetics and Mechanism

TrGL has been characterized as a β-1,4-glucuronan lyase; TrGL catalyzed endolytic depolymerization of β-1,4-glucuronan by β-elimination [1]. The enzyme was most active at pH 6.5 and 50°C, and its activity and thermostability increased in the presence of calcium ions.

Catalytic Residues

There are approximately 40 completely conserved amino-acid residues in PL20 members [2]. Possible catalytic residues have been predicted based on structural comparison between TrGL and PL7 alginate lyase A1–II’ [2, 3]. The charge neutralizer, the catalytic base, and the catalytic acid in TrGL are predicted to be Gln91, His53 and Tyr200, respectively. However, in order to clarify the substrate recognition mechanism and the identity of the catalytic residues of PL20, further studies will be required.

Three-dimensional structures

Figure 1: PL20 glucuronan lyase from Trichoderma reesei (PDB ID 2zzj).

The ligand-free structure of TrGL was the first PL20 structure to be reported (Figure 1, PDB ID 2zzj, 1.8 Å resolution) [2]. TrGL has a typical β-jelly roll fold. A calcium binding site, which appears to contribute to the stability, was found at a position far from the cleft. However, no calcium binding site in the cleft has been identified.

Family Firsts

First catalytic activity

TrGl from Trichoderma reesei [1].

First 3-D structure

TrGl from Trichoderma reesei [2].

References

1. Konno N, Igarashi K, Habu N, Samejima M, and Isogai A. (2009). Cloning of the Trichoderma reesei cDNA encoding a glucuronan lyase belonging to a novel polysaccharide lyase family. Appl Environ Microbiol. 2009;75(1):101-7. DOI:10.1128/AEM.01749-08 |
2. Konno N, Ishida T, Igarashi K, Fushinobu S, Habu N, Samejima M, and Isogai A. (2009). Crystal structure of polysaccharide lyase family 20 endo-beta-1,4-glucuronan lyase from the filamentous fungus Trichoderma reesei. FEBS Lett. 2009;583(8):1323-6. DOI:10.1016/j.febslet.2009.03.034 |
3. Ogura K, Yamasaki M, Mikami B, Hashimoto W, and Murata K. (2008). Substrate recognition by family 7 alginate lyase from Sphingomonas sp. A1. J Mol Biol. 2008;380(2):373-85. DOI:10.1016/j.jmb.2008.05.008 |