Plant defence-inducing N-acylglutamines from insect guts: structural diversity and microbe-assisted biosynthesis

Abstract

N-Acylglutamines are ubiquitously present in the regurgitants of herbivorous insects. The compounds comprise a larger family containing fatty acids of different degrees of unsaturation and chain length. Further modifications occur in the fatty acid moiety by oxidative transformations to secondary alcohols and epoxides. By hydrolysis (epoxide), acylation or phosphorylation of the alcohols the diversity of the compounds is further increased. Screening of cultivable gut bacteria from several Lepidopterans revealed that many of the commensal microorganisms are principally able to synthesize the N-acyglutamines. From Microbacterium arborescens the first pure N-acyl synthase was isolated and characterized as an homooligomeric enzyme with a protomer size of 17.2 kDa. The enzyme displays a pH optimum at pH8 and second plateau of even higher activity between pH 9-12 corresponding to the strongly alkaline conditions in the insect gut. The enzyme catalyses amide formation between a wide range of free fatty acid and most proteinogenic amino acids. Preceeding activation of the acyl moiety is not required.