Polysaccharide Lyase Family 6

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• Author: ^^^Emil Stender^^^
• Responsible Curator: ^^^Birte Svensson^^^

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Polysaccharide Lyase Family 6
3D structure	parralel β-helix
Mechanism	β-elimination
Charge neutralizer	calcium or water
Active site residues	known
CAZy DB link
http://www.cazy.org/PL6.html

Substrate specificities

PL6 currently contains 3 subfamilies [1] all of which contain members catalyzing the depolymerisation of alginate [2]. Alginate consisting of 1,4 linked β-D-mannuronic acid and α-L-guluronic acid arranged in poly-mannuronic acid blocks, poly-guluronic acid blocks or poly-mannuronic/guluronic acid blocks [3, 4]. Subfamily 2 and 3 have so far only shown specificity for poly-mannuronic/guluronic acid blocks [2], while subfamily 1 has been demonstrated to depolymerize poly-guluronic acid [5, 6], poly-mannuronic acid [7], poly-mannuronic/guluronic acid [2] as well as dermatan sulfate (formerly chrondroitin B) [2, 8, 9].

Kinetics and Mechanism

Figure 1. Syn – or anti – β-elimination catalyzed by PL6 enzymes acting on alginate. M represents mannuronic acid and G guluronic acid. n represents the continued sugar chain.

The β-elimination catalyzed by the PL6 enzymes results in the formation of a C4-C5 unsaturated sugar at the new non-reducing end. The first step is the neutralization of the acid group in the +1 subsite by a calcium [6, 9] or by water [5]. This lowers the pKa value of the C5-proton allowing for abstraction by the catalytic base (Figure 1). A catalytic acid then donates a proton to the glycosidic linkage resulting in the β-elimination. This can be done in syn with the acid and base on the same side of the sugar ring in the transition state (the case for D-mannuronic acid) or anti where they are on opposite sides of the sugar ring (the case for L-guluronic acid) [10, 11].

Catalytic Residues

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Three-dimensional structures

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Family Firsts

First stereochemistry determination

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First catalytic nucleophile identification

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First general acid/base residue identification

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First 3-D structure

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