Sclerotinis a component of thecuticleof variousArthropoda,most familiarlyinsects.It is formed bycross-linkingmembers of particular classes ofproteinmolecules, a biochemical process calledsclerotization,a form oftanningin which quinones are enzymatically introduced into the cuticle, and react with terminal and lysine-related amino groups in the proteins to form strong links between the molecules.[1]The resulting material greatly increases the rigidity of an insect'schitinousexoskeleton, which is otherwise fairly soft. It is particularly prominent in the thicker, armoured parts ofinsectandarachnidintegument, such as in the biting mouthparts andscleritesof scorpions and beetles.[2]
As it matures, freshly formed sclerotin becomes a hard, horn-like substance with a range of yellow-brown colors. As animals adapted to life on land, increasingly diverse needs for organic stiffening components arose (as opposed to mineral stiffening components such as calcium carbonates and phosphates). Among the invertebrates, this need was met largely by the development of sclerotins and other cross-linked proteins that allowed insects to adapt to existence on the land and later to develop wings.[3]
Sclerotin is biochemically variable; different species incorporate different proteins in different proportions, and the same insect will use different compositions in forming the different components of its body. For example, a caterpillar may have sclerotized jaws, head capsule, and legs, with the rest of the body soft and membranous. In general, sclerotized cuticle is formed by cross-linking the various protein molecules with phenolic compounds – a tanning process under enzymatic control.[1]In some of theApterygota,however, at least some of the cross-linking is by disulphide bonds reminiscent of protein cross-linking in the formation ofkeratin.[4]This has led many authors to refer to such cross-linked proteins in invertebrates as keratin, but modern analyses have shown that the term is inappropriate; keratins are a fairly well-defined set of proteins comprising particular chains in different proportions cross-linked in particular ways. Invertebrate connective tissue proteins based on disulphide links appear to be radically different.[5][6]
See also
editReferences
edit- ^abChapman, R.F. (1969)The Insects: Structure and Function.Elsevier, New York. Chapter 22.37 - "Expansion of the new cuticle is brought to an end by the onset of tanning"
- ^Klots, Alexander B.; Klots Elise B. (1977).1001 Questions Answered About Insects.Courier Dover Publications. p. 29.ISBN0486234703.
- ^Wigglesworth, Vincent Brian (1984).Insect Physiology.Springer.ISBN0412264609.
- ^Richards, O. W.; Davies, R.G. (1977).Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology.Berlin: Springer.ISBN0-412-61390-5.
- ^Weber, Klaus et al; "Amino acid sequences and homopolymer-forming ability of the intermediate filament proteins from an invertebrate epithelium" The EMBO Journal vol.7 no.10 pp.2995-3001, 1988
- ^BROWN, C. H.; Keratins in Invertebrates; Nature 166, 439 (9 September 1950);doi:10.1038/166439a0