pH-induced conformational change of natural cyclic lipopeptide surfactin and the effect on protease activity

The cyclic lipopeptide surfactin (SF) is one of the promising environmental friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF shows excellent surface properties at various pH, together with lower toxicity and higher biodegradability than commonly used petroleum-based surfactants. However, the effect of the dissociation degree of SF on self-assembly is still incompletely understood, even though two acidic amino acid residues (Asp and Glu) are known to influence eventual surface and biological functions. Here, we report changes in the secondary structure of SF induced by increased pH, and the effect on protease activity. We found that the β-sheet and β-turn formation of SF are significantly enhanced through increased dissociation of Asp and Glu as revealed by a titration experiment of SF solution to estimate apparent pK₁ and pK₂ values together with circular dichroism spectroscopy. We also studied the activity of the common detergent enzyme subtilisin in SF solution at above its pK₂ (pH 7.6) to understand the role of the dissociation degree in the interaction with the protein. The mixing of SF having a unique cyclic topological feature with subtilisin suppressed the decrease in protease activity observed in the presence of synthetic surfactants such as sodium dodecyl sulfate and polyoxyethylene alkyl ether. Thus, SF has great potential for use in laundry detergent formulations, to improve the stability and reliability of detergent enzymes.