Properties and potential applications of mannuronan C5-epimerase: A biotechnological tool for modifying alginate

Bacterial Alginate Biosynthesis and Metabolism

“…Isomerases of the AlgE family have been shown to be differentially expressed during the life cycle of A. vinelandii and to generate different patterns of G-blocks on alginate [147]. Five of the algE genes of A. vinelandii are found clustered in the genome, while algE5 and algE7 are found isolated elsewhere (see Figure 1), and each correspondingly encoded enzyme appears to have different specificities in terms of substrate preferences and non-random formation of G-and GM-blocks on the alginate backbone [149]. These extracellular alginate epimerases from A. vinelandii are about 70% identical but share less than 10% sequence identity with AlgG [134].…”

“…Four to seven copies of a nine amino acid long motif are located in the N-terminal region of the R-modules and are known to be Ca 2+ -binding sites [153]. Calcium ions are important not only for the structural integrity of the protein but also for neutralizing the charges of ManA during the epimerization reaction, in a similar manner Arg 345 functions in AlgG [149,150]. In addition, the last R-module of each extracellular AlgE usually contains an unstructured peptide involved in the secretion of the enzyme [151].…”

“…Although the extracellular C5-epimerases AlgE1-7 play an important role in the addition of L-GulA units to the alginate produced by A. vinelandii, these enzymes have also been cloned and isolated to be used for in vitro modification of macroalgal-derived alginate [128,149]. AlgE1 and AlgE3 are the only epimerases that contain two catalytic A-modules, one of which introduces single G units that form alternating GM sequences, while the other can generate G-blocks with consecutive GulA units [65,154].…”

“…AlgE1 and AlgE3 are the only epimerases that contain two catalytic A-modules, one of which introduces single G units that form alternating GM sequences, while the other can generate G-blocks with consecutive GulA units [65,154]. AlgE2 and AlgE5 both consist of an A-module followed by four R-modules, and they primarily introduce short G-blocks on the alginate chain [149]. The same module organization is found in AlgE6, which, however, is able to introduce longer G-blocks on polymannuronic acid [147,153].…”

“…AlgE4 consists of only one A-module and one R-module and is the smallest enzyme among the extracellular C5-epimerases. Although it inserts a high proportion of GulA units into the alginate chain, this enzyme is unable to attach successive G-blocks to the polymer [65,149]. AlgE7 generally produces the same GM alternating products as AlgE1, AlgE3, and AlgE4.…”