Oxidative conjugation of catechols with proteins in insect skeletal systems

Abstract

Cuticle sclerotization or tanning is a vital process that occurs during each stage of insect development to harden and stabilize the newly secreted exoskeleton. The structural polymers protein and chitin make up the bulk of the cuticle, and chemical interactions between these biopolymers with quinonoid tanning agents are largely responsible for the physical properties of the mature exoskeleton. The oxidative conjugation of catechols with cuticular proteins plays an important role in this metabolism. The main hypothesis for cuticle sclerotization involves the formation of adducts and cross-links between nucleophilic imidazole nitrogens of histidyl residues in the proteins and electrophilic ring or side-chain carbons of ortho-quinones and para-quinone methides derived from the catechols, N-acetyldopamine, N-beta-alanyldopamine, and 3,4-dihydroxyphenylethanol. C–N and C–O linkages between these quinone tanning agents and proteins in cuticles from a variety of insects from several orders have been elucidated. cDNAs for both the tyrosinase and laccase types of phenoloxidases that catalyze the cross-linking reactions have been isolated and sequenced. The sequences of laccase cDNAs from two insect species were more similar to fungal laccases than to those from plants. These results provide insights into how insects use structural proteins, catechols, and oxidative enzymes to form catechol–amino acid adducts during sclerotization.