Polysaccharide epimerases

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• Author: ^^^Margrethe Gaardlos^^^ and ^^^Anne Tondervik^^^
• Responsible Curator: ^^^Finn Aachmann^^^

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Mannuronan C5-epimerases

Substrate specificity

Mannuronan C5-epimerases are a group of enzymes that catalyze epimerization at the polymer-level of β-d-mannuronic acid residues (hereafter denoted M) into α-l-guluronic acid residues (hereafter denoted G) in alginate [1, 2, 3]. Alginate is an anionic polysaccharide made by brown seaweeds, some species of red algae, and the gram-negative bacterial genera Pseudomonas and Azotobacter [4, 5, 6, 7, 8]. The function of alginate in the different organisms are various, and related to structure, protection and surface adhesion [9, 10, 11, 12]. Alginate is a copolymer of the two 1-4 linked epimers [13, 14, 15], and by changing the composition of the two monomers the epimerases fine-tune the properties of the polymer [16].

At first, alginate is made as a homopolymer of M in the cell. Epimerases then convert some of the M residues in the polymer into G-residues [3, 17, 18]. This epimerization is not random and creates block structures of M, G or alternating MG [19, 20]. Alginate residues that are oxidized or acetylated are not substrates for the epimerases, and acetylation of alginate could be a way to control epimerization in nature [21, 22].

A. Haworth structures of d-mannuronic acid (M) and l-guluronic acid (G). B. The three different block structures found in alginate: non-epimerized M-blocks, MG-blocks and G-blocks.

Mannuronan C5-epimerases exist both in algae and in bacteria [1, 23]. Gene analyses propose as many as 31 different genes encoding putative mannuronan C-5 epimerases in the brown algae Ectocarpus [24]. However, the algal epimerases are difficult to express and it is the bacterial enzymes that have been studied most extensively [24, 25]. Two categories of bacterial mannuronan C-5-epimerases have been described: the periplasmic AlgG and the extracellular and calcium dependent AlgE. AlgG creates single G residues in stretches of mannuronan, while the AlgE enzymes are processive and create MG-blocks and G-blocks. Pseudomonas is only known to produce AlgG [18, 26, 27], while A. vinelandii contains seven active AlgE enzymes in addition to AlgG [28, 29, 30, 31]. A mutant strain of P. fluorescens without the algG gene creates pure mannuronan \autocite{Gimmestad2003}. This strain can be used to produce unepimerized substrate, which is useful for the study of the epimerization reaction. Methods for studying this are discussed in Section \ref{sec:catmec}.

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References

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