12 Chitin-Protein Complexes Ordered Biopolymer Composites 1
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: May 12, 1982 | doi: 10.1021/bk-1982-0186.ch012
JOHN BLACKWELL, LOUIS T. GERMINARIO , and MARK A. WEIH Case Western Reserve University, Department of Macromolecular Science, Cleveland, OH 44106
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The skeletal materials of insects and many other "lower" animals are biopolymer composites, in which c h i t i n (polysaccharide) f i b r i l s are separated by a matrix of protein. X-ray fiber diagrams show that in many of these systems both the fibrous and matrix components are ordered. We have developed a model for the three-dimensional structure of a typical insect chitin-protein complex, that from the ovipositor of the ichneumon fly Megarhyssa. The x-ray pattern shows that crystalline α- c h i t i n f i b r i l s are arranged hexagonally with a center to center distance of 7•25nm. This hexagonal packing is confirmed by electron microscopy of stained ovipositor sections. The protein forms a h e l i c a l sheath around each c h i t i n fibril, with six protein subunits repeating i n 3•06nm along the fiber axis. Fourier reconstruction of the electron micrographs also reveals a sixfold arrangement of subunits i n the protein matrix. The structure provides a basis for the discussion of fiber-matrix interactions in these and other structural tissues, such as collagen and cellulose containing systems.
The polysaccharide c h i t i n forms the component of the skeletal materials of many lower animals, (1, 2^, 3) and serves in an analogous way to collagen i n mammalian connective tissues and cellulose i n plant c e l l walls. A l l of these tissues can be classified as fiber-matrix composites: i n the c e l l walls of higher plants the cellulose f i b r i l s are surrounded by other polysaccharides, proteins, and also l i g n i n i n some cases; i n 1 2
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0097-6156/82/0186-0149$5.00/0 © 1982 American Chemical Society Carraher and Gebelein; Biological Activities of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by CORNELL UNIV on May 31, 2017 | http://pubs.acs.org Publication Date: May 12, 1982 | doi: 10.1021/bk-1982-0186.ch012
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BIOLOGICAL ACTIVITIES OF POLYMERS
mammalian connective tissues collagen f i b r i l s are surrounded by a gel of proteoglycans and other proteins; and i n chitinous tissues the f i b r i l s are i n a matrix of protein. In addition, the animal tissues are frequently c a l c i f i e d . The mechanical properties of these skeletal materials necessarily depend on the structures of the fiber and matrix phases and their mutual interaction. At present we have extensive knowledge of the physi c a l structures of a l l three types of fibers, but information with regard to the structure of the matrix has been more d i f f i cult to obtain. For the cellulose and collagen containing tissues, there has been extensive study of the matrix chemistry, but x-ray studies indicate that the physical structures are amorphous. The chitinous tissues have received less attention, but i t has been shown that i n certain of these systems the matrix proteins are ordered, so that i t i s possible to develop a three-dimensional model for the fiber-matrix structure and interactions. The present paper describes our studies using x-ray diffraction and electron microscopy to derive the structure of one of the more ordered chitin-protein complexes, that from ovipositor of the ichneumon f l y M&g
