Contractility in the fibrous proteins

The a- and p-keratins exist in the oriented crystalline state and possess a high concentration of the cystine residues. They also undergo contraction when subjected to the action of a wide variety of reagents.(70) It is recognized that two distinctly different types of contractile processes can be observed in a-keratin fibers. One of these involves the interaction with reagents known to sever disulfide cross-links. As would be expected, in this case the observed dimensional changes are irreversible. 

In the other case, the integrity of the intermolecular cross-links is maintained and anisotropic dimensional changes occur as a result of interaction with reagents of the type illustrated in Fig. 8.17. 

The demonstration that the crystal-liquid phase transition can be conducted isothermally, by changing the concentration of the supernatant phase, portends the possibility of the utilization of fibrous macromolecules as the working substance of an engine that isothermally converts chemical energy to mechanical work. (3,54) 

It is reasonable to inquire at this point whether the principles that have been set forth above have any applicability to natural functioning contractile systems. Muscles are very intricately constructed fibrous structures developed by nature to convert chemical energy into mechanical work. Detailed and sophisticated electron microscopic and x-ray diffraction studies have established the fine structure of muscle. The chemical processes and enzymatic activity that are intimately involved 

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In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

Fig. 1. Schematic drawings of the viruses discussed in this chapter. (A) An icosahe-dral virus with fiber proteins inserted in its pentameric vertices. The gray box denotes domains with known structures for adenovirus, reovirus, and bacteriophage PRD1, in each case containing the head domain and proximal part of the triple /8-spiral shaft domain. (B) Contractile-tailed bacteriophage T4. T4 contains three different fibrous proteins, fibritin connected to the neck, the long (bent) fibers connected to the base plate, and the short fibers also connected to the base plate. Only two of each of the trimeric fibrous proteins are shown for clarity. The gray box denotes domains with known structure for the T4 short fiber.

Basically, there are three major groups of proteins in muscle tissue (a) the sarcoplasmic proteins of the muscle cell cytoplasm, (b) the myofibrillar proteins, soluble at high ionic strengths, that make up the myofibril or contractile part of the muscle, and (c) the stromal proteins comprised largely of the connective tissue proteins, collagen, and elastin. The myofibrillar proteins and the stromal proteins are fibrous and elongated they form viscous solutions with large shear resistance. These properties coupled with other lines of indirect evidence indicate that the physical properties of the myofibrillar and stromal proteins are directly related to the texture and tenderness of meat (34). 

Ans. This classification is based on solubility in the aqueous environment. Fibrous proteins are insoluble while globular proteins are soluble. Globular proteins are globe-shaped, i.e., more or less spherical. Fibrous proteins have shapes in which one dimension is much longer than the other dimensions, i.e., they are elongated. Fibrous proteins function as structural and contractile proteins (Table 20-1). The functions of globular proteins are those such as the transport, enzyme, defense, and regulatory functions which require solubility in the aqueous environment. 

Based on a series of designed elastic-contractile model proteins. Figure 1.2 exhibits a family of curves whereby stepwise linear increases in oil-like character give rise to supra-linear increases in curve steepness, that is, in positive cooperativity. More oil-like phenylalanine (Phe, F) residues with the side chain -CH2-C6H5 replace less oil-like valine (Val, V) residues with the side chain -CH-(CH3)2. Here the structural symmetry is translational with as many as 42 repeats (Model protein v) of the basic 30-residue sequence, and the structure is designed beginning with a repeating five-residue sequence of a fibrous protein, the mam-mahan elastic fiber. 

Two major kinds of proteins are tough fibrous proteins, which compose hair, tendons, muscles, feathers, and silk, and spherical or oblong-shaped globular proteins, such as hanoglobin in blood or the proteins, which comprise enzymes. Proteins serve many functions. These include nutrient proteins, such as casein in milk structural proteins, such as collagen in tendons contractile proteins, such as those in muscles and regulatory proteins, such as insulin, which regulate biochemical processes. 

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