Engineering the structure of emulsans via whole cell biotransformations

Our work addressed the potential to regulate the lipid composition of emulsans in order to ‘tailor’ emulsan physical and biological properties. Emulsan, an extracellular lipoheteropolysaccharide generated by the bacterium Acinetobacter calcoaceticus, was known to form oil-in water emulsions. The key concept demonstrated in this research was the extraordinary promiscuity of A. calcoaceticus to incorporate non-native structures into polymers (Gorkovenko, 1997). Side chain lipids that were highly fluorinated, hydroxylated in unusual positions (Zhang et al, 1997), and had high levels of unsaturated (Gorkovenko et al, 1999) were incorporated as side-chains along polysaccharide chains (see Table 1). Importantly, these alterations in structural features resulted in large changes in emulsification behavior. For example, it was shown that, as the degree of fatty acid substitution of the polysaccharide backbone increased to about one fatty acid per seven sugar residues, the emulsification activity was optimized. Since these polymers have analogous structures to endotoxins, work was begun in collaboration with Professor Juliet Fuhrman (Tufts University) to explore whether these lipoheteropolysaccharides could function for immunoregulation. Studies thus far, by macrophage screening and a mouse studies showed that when properly designed, these ‘tailorable’ lipoheteropolysaccharides have comparable adjuvant activity to an industry standard.

Scheme 20. Emulsan structure

Table 1. Incorporation of unusual and non-natural fatty acids with high degrees of fluorination and -hydroxylation as side chains of novel emulsans.

References

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