Chitin-protein complex from

Chitin is classically a linear polymer of /3-d-(1 - 4)-linked 2-acetamido-2-deoxy-D-glucosyl residues. However, after comparing chitin isolated from different sources, it became evident that substantial variations in solubility, molecular weight, acetyl values, and specific rotation occur among samples. It would appear that chitin is not a single, polymeric entity, but rather, a family of closely related products derived from natural chitin-protein complexes.209... 

Figure 5. Proposed arrangement of the protein subunits around the chitin core in Megarhyssa chitin-protein complex. Key a, axial projection and b, projection perpendicular to fiber axis. (Reproduced, with permission, from Ref. 26. Copyright 1980, Academic Press.)...

A historical review of the development of our knowledge of the yeast cell wall was given by Phaff (I). Most information based on chemical studies has been derived, by far, from studies with cell walls from baker s yeast, Saccharomyces cerevisiae, and closely related species. The principal components of Saccharomyces walls are several types of glucan and a mannan-protein complex which may contain variable proportions of phosphate. A low content of chitin (ca. 1% ) may be present depending on the number of times a cell has produced buds. The reason for this is that chitin is present only in the bud scars (ca. 3 pm2 in area) produced on the surface of a mother cell (2), each at a different place on the cell surface. 

Chltln.—In a re-examination of X-ray diffraction data for chitin-protein systems, the complex from the ovipositor of the ichneumon fly Megarhssa has been used. It is proposed that the crystalline cr-chitin fibril is surrounded by a protein sheath of higher electron density to give a total fibril radius of 725 nm. Fibrils of this type are then packed on a hexagonal network vnth one fibril per unit cell. 

Carbohydrates form the major structural components of the cell walls. The most common form is cellulose which makes up over 30 per cent of the dry weight of wood. Other structural forms are hemicellulose (a mixed polymer of hexose and pentose sugars), pectins and chitin. Apart from contributing to the structure, some polymers also act as energy storage materials in living systems. Glycogen and starch form the major carbohydrate stores of animals and plants, respectively. Carbohydrate structure, like that of nucleic acids and proteins, is complex, and various levels of structure can be identified. 

In the chiton tooth, the organic framework components are synthesized and secreted by the cells into the extracellular space, and there they self assemble. By the time mineralization is about to occur the cells are tens of micrometers away from many of the mineralization sites. They must therefore operate by remote control. The mineralization sites themselves are within a complex chitin framework, the dimensions of which are in the nanometer range. The sea urchin larval spicule represents the exact opposite situation. Mineralization occurs in a vacuole defined by a membrane, and the entire apparatus is within a consortium of fused cells (the syncytium). The membrane of the syncytium tightly surrounds the growing spicule [74], Therefore, it has been proposed that the cells directly control spicule formation. The mineralization vacuole is subdivided by framework proteins. Nothing is known about the structure of the one nucleation site per spicule in the larvae, but in the adult a well-defined location, enclosed within a framework, has been identified as the nucleation site [83]. Dentin formation is intermediate between the two. It is an extracellular process, and the distances between cells or cell processes and mineralization sites are in the range of tens of micrometers or several micrometers respectively. Nucleation occurs within the fibril or at its surface and is associated with a site on the fibril surface some 7 or 8 nm wide [54]. The space available for crystal growth within the fibril is even smaller in one of the dimensions, namely 2 or 3 nm wide. 

A prominent FDA-approved chitin dressing is rapid deployment hemo-stat (RDH) (Marine Polymer Technologies) which costs 300 per dressing. One study shows that polymeric hber material based on P-NAG is more effective than alpha-chitin or chitosan, since these have a heterogeneous structure and are complexed with minerals and proteins. Moreover, the j3 structure (parallel orientation) of the hbers was found to be more effective than the a structure (antiparallel orientation). In another study, the hemostatic and antibacterial properties of chitosan dressings have been shown to be improved by the addition of polyphosphate polymers and silver nanoparticles respectively. One limiting factor is that all forms of chitin or chitosan bandages are not equally effective and the effectiveness varies from batch to batch. ... 

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