Cross-linked polymers fibrous proteins

Proteins are commonly classified into two main classes, viz fibrous and globular proteins. The former are the principal structural components of such materials as feathers, hair, nails and silk they consist of essentially linear polymers which are not dispersible in water. Globular proteins occur in materials such as egg-white, enzymes, metabolic intermediates and milk they are relatively compact cross-linked polymers and are dispersible in water. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

The most important reaction in blood clotting is the conversion, catalyzed by thrombin, of the soluble plasma protein fibrinogen (factor 1) into polymeric fibrin, which is deposited as a fibrous network in the primary thrombus. Thrombin (factor 11a) is a serine proteinase (see p. 176) that cleaves small peptides from fibrinogen. This exposes binding sites that spontaneously allow the fibrin molecules to aggregate into polymers. Subsequent covalent cross-linking of fibrin by a transglutaminase (factor Xlll) further stabilizes the thrombus. 

Fibrin. Fibrin is a fibrous, nonglobular protein involved in the clotting of blood that forms three-dimensional matrices [15]. Upon cross-linking, fibrin is converted to a gel. Therefore, it is one of the most widely used natural polymers for cardiovascular TE [16]. Fibrin products are FDA approved for clinical use as surgical sealants [11]. 

---