11 - Quinone-Tanned Scleroproteins

Abstract

This chapter focuses on quinone-tanned scleroproteins. It defines a scleroprotein functionally as a protein contributing mechanical strength to supporting structures in animals. The most persistent properties of the scleroproteins are their insolubility in vitro, and their resistance to proteinases and various hydrolytic solvents. This stability can be attributed to two factors: (1) special arrangement of amino acids and (2) covalent cross-linking of the primary valence chains. The insect cuticle constitutes one of the few invertebrate structures in which sclerotization has been scrutinized. Instead of using 3,4-dihydroxyphenylalanine (DOPA) proteins, insects mix the o-diphenol N-acetyldopamine with oxidases, cuticular proteins, and chitin. By blending these four basic ingredients in various proportions, insects have achieved an intricate capacity to modulate the mechanical properties of their cuticles—for example, the hard mandibles, resilient femurs, and extensible abdomens of locusts. Such versatility is not inherent in the DOPA protein of periostracum. Periostracin, with a fixed amount of DOPA residues, is secreted by the mantle; the DOPA is oxidized, sclerotization ensues, and a highly regimented type of scleroprotein is formed.