Oxidative conjugation of catechols with proteins in insect skeletal systems

Cuticle sclerotization or tanning is a vital process that occurs during eac h stage of insect development to harden and stabilize the newly secreted ex oskeleton. The structural polymers protein and chitin make up the bull; of the cuticle, and chemical interactions between these biopolymers with quino noid tanning agents are largely responsible for the physical properties of the mature exoskeleton. The oxidative conjugation of catechols with cuticul ar proteins plays an important role in this metabolism. The main hypothesis for cuticle sclerotization involves the formation of adducts and cross-lin ks between nucleophilic imidazole nitrogens of histidyl residues in the pro teins and electrophilic ring or side-chain carbons of ortho-quinones and pa ra-quinone methides derived from the catechols, N-acetyldopamine, N-beta-al anyldopamine, and 3,4-dihydroxyphenylethanol. C-N and C-O linkages between these quinone tanning agents and proteins in cuticles from a variety of ins ects from several orders have been elucidated, cDNAs for both the tyrosinas e and laccase types of phenoloxidases that catalyze the cross-linking react ions 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 prot eins, catechols, and oxidative enzymes to form catechol-amino acid adducts during sclerotization. Published by Elsevier Science Ltd.