Muscle, three-dimensional structure

FIGURE 17.13 The three-dimensional structure of an actin monomer from skeletal muscle. This view shows the two domains (left and right) of actin. 

Three-dimensional structures of Escherichia coli and cat muscle PK had been refined. These studies disclose the essential residues that determine the relative orientations of domains and the precise nature of intersubunit contacts (A3, M15). 

A3. Allen, S., and Muirhead, H Refined three-dimensional structure of cat-muscle (Ml) pyruvate kinase at a resolution of 2.6 A. Acta Crystallogr., Sect. D Biol. Crystallogr. D52, 499-504 (1996). 

Protein domains are the common currency of protein structure and function. Protein domains are discrete structural units that fold up to form a compact globular shape. Experiments on protein structure and function have been greatly aided by consideration of the modular nature of proteins. This has allowed very large proteins to be studied. The expression of individual domains has allowed the intractable giant muscle protein titin to be structurally studied (Pfuhl and Pastore, 1995). Protein domains can be found in a variety of contexts, (Fig. 1), in association with a range of unrelated domains and in a variety of orders. Ultimately protein domains are defined at the level of three-dimensional structure however, many protein domains have been described at the level of sequence. The success of sequence-based methods has been demonstrated by numerous confirmations, by elucidation of the three-dimensional structure of the domain. 

Figure 2. Representation of the three dimensional structure of skeletal muscle.

Luther, P. K. (2000). Three-dimensional structure of a vertebrate muscle Z-band Implications for titin and alpha-actinin binding./. Struct. Biol. 129, 1-16. 

Freundlich, A., and Squire, J. M. (1983). Three-dimensional structure of the insect (Lethocerus) flight muscle M-band./. Mol. Biol. 169, 439-453. 

Luther, P. K., and Squire, J. M. (1980). Three-dimensional structure of the vertebrate muscle A-band. II. The myosin filament superlattice./. Mol. Biol. 141, 409-439. Luther, P. K., and Squire, J. M. (2002). Muscle Z-band ultrastructure Titin Z-repeats and Z-band periodicities do not match./. Mol. Biol. 319, 1157-1164. 

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]. 

The amino acid compositions of two phosphorylases have been determined, and are shown in Table XVI. The complete, amino acid sequence or three-dimensional structure of a phosphorylase is not yet known, and the task of delineating it will be difficult because of the large size of the molecules. However, the sequence of amino acids about an important phosphoserine residue has been reported for rabbit and human muscle phosphorylase a. In the rabbit enzyme, the sequence is... 

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