Polypeptides muscle fibers

This complex consists of at least 25 separate polypeptides, seven of which are encoded by mtDNA. Its catalytic action is to transfer electrons from NADH to ubiquinone, thus replenishing NAD concentrations. Complex I deficiency has been described in myopathic syndromes, characterized by exercise intolerance and lactic acidemia. In at least some patients it has been demonstrated that the defect is tissue specific and a defect in nuclear DNA is assumed. Muscle biopsy findings in these patients are typical of those in many respiratory chain abnormalities. Instead of the even distribution of mitochondria seen in normal muscle fibers, mitochondria are seen in dense clusters, especially at the fiber periphery, giving rise to the ragged-red fiber (Figure 10). This appearance is a hallmark of many mitochondrial myopathies. 

In contrast with synthetic polymers, proteins are characterized by very high levels of structural order. Unlike synthetic polymers, proteins are characterized by absolutely uniform chain lengths and well-defined monomer sequences (primary structure) [3]. These features are two of the requirements that enable folding of linear polypeptide chains into structurally well-defined and functional proteins. Proteins play an important role in numerous processes in biology, e.g. as carriers for small molecules and ions (examples are presented in Chapter 2.2), as catalysts, or as muscle fibers, and their exquisite properties are closely related to their well-defined three-dimensional structure [3]. 

Protein fibers, such as silk, hair, finger n ils, and muscle fibers, are believed to consist of long polypeptide chains. These chains may be stretched out in an extended zig-zag form, or perhaps in a spiral. 

Figure 1.21 depicts an aitomativc pathway. Following the events of Figure 1,19, the polypeptide is inserted into the membrane, pacicaged into a secretory vesicle, and inserted into the plasma membrane via fusion of the vesicle with the PM, Membrane-bound proteins include nutrient transport proteins, hormone receptors, ion pumps, and proteins that transmit impulses along the length of a nerve or muscle fiber. 

There are many different applications of the DLS technique. The DLS method has been u.sed to determine the size of polymer lattices and resins and to monitor the grow th of particles during processes such as emulsification and polymerization. Micelles and microemulsions have been studied by DL.S methods. DLS is also widely applicable to the investigation of biopolymers and biocolloids. It has been used to study natural and synthetic polypeptides, nucleic acids, ribosomes, vesicles, viruses, and muscle fibers. 

This complex contains 11 polypeptide subunits of which only one is encoded by mtDNA. Defects of complex III are relatively uncommon and clinical presentations vary. Fatal infantile encephalomyopathies have been described in which severe neonatal lactic acidosis and hypotonia are present along with generalized amino aciduria, a Fanconi syndrome of renal insufficiency and eventual coma and death. Muscle biopsy findings may be uninformative since abnormal mitochondrial distribution is not seen, i.e., there are no ragged-red fibers. Other patients present with pure myopathy in later life and the existence of tissue-specific subunits in complex III has been suggested since one of these patients was shown to have normal complex 111 activity in lymphocytes and fibroblasts. 

Nicotinic receptors are part of a transmembrane polypeptide whose subunits form cation-selective ion channels (see Figure 2-9). These receptors are located on plasma membranes of postganglionic cells in all autonomic ganglia, of muscles innervated by somatic motor fibers, and of some central nervous system neurons (see Figure 6-1). 

Fibrous proteins provide the structural material of many tissues in the body. They are the chief constituents of hair, cartilage, and muscles. Fibrous proteins consist of lengthwise bundles of polypeptide chains (a fiber). Globular proteins consist of polypeptide chains folded into a spherical shape they are found in the bloodstream where they transport and store various substances, act as antibodies (fight infections), act as enzymes (catalysts), and participate in the body s various regulatory systems. 

The -Keratin Structure.—Hair and muscle can be reversibly stretched to about 100 per cent elongation. Some authors have expressed doubt as to whether this elongation is to be attributed to the polypeptide chains, but it seems to us that Astbury s contention that it should be is justified. With a fiber-axis length of 1.53 A per residue for the a helix, an extended chain in the 8-keratin structure would be predicted, on this assumption, to have a fiber-axis residue length of about 3.1 A. The principal meridional x-ray reflection of stretched hair, stretched muscle, and other proteins with the S-keratin structure has in fact a spacing reported by Astbury as about 3.32 A, which is presumably the fiber-axis residue length, and would thus correspond to 117 per cent extension of the a helix. That the jS-kera-... 

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