Contractile protein complex

Skeletal muscle moves the bones attached to joints. These muscles are composed of bundles of long, multinucleated cells. The cytoplasm contains a high concentration of a special macromolecular contractile-protein complex, actomyosin (Chap. 5). There is also an elaborate membranous network called the sarcoplasmic reticulum that has a high Ca2+ content. The contractile-protein complex has a banded appearance under microscopy. 

In this review, the contractile protein complex is called actomyosin, and its components actin and L-myosin. To avoid ambiguity, the word myosin is used only for preparations which may contain actomyosin and L-myosin, i.e., for all preparations made before the discovery of actin, and for the A and B myosins of Szent-Gyorgyi. 

Other examples of M.c. are anthranilate synthase and tryptophan synthase, which are involved in microbial Aromatic biosynthesis (see), and citrate lyase. The definition of M. c. may be extended to include the membrane-bound respiratory chain, the contractile protein complexes of muscle, and the ribosome (which also contains RNA), etc. 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Any dialogue on meat flavor development and deterioration requires a brief discussion of muscle structure. Muscle has a highly compact and complex multicellular structural organization (Figure 2). Individual muscle cells contain numerous mitochondria and nuclei. They also contain contractile elements as the bulk of their structure. While the sarcoplasm of muscle (the aqueous non-organellar component) is small compared to the cytoplasm of non-muscle cells, it does have a highly evolved system of membranes called the SR/L representing an acronym for sarcoplasmic reticulum/lysosomal membrane system (11). The SR/L surrounds each contractile element (Fig. 9-13 in 12 Fig. 7-10 in 13). The close proximity of the SR/L to the contractile proteins situates the proteins in a location that is optimal for their hydrolysis by lysosomal hydrolases (12, 13). 

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. 

CONTRACTILITY AND CONTRACTILE PROTEINS. The fundamental property of living matter on which its power uf movement depends is termed contractility. In the simplest forms of living things, it is evident in the flowing movement of Ihe material of the cel . In more complex forms, ihe properly is centralized in muscle tissues. Muscle cells are elongate and are so arranged that necessary movements result from their shortening when stimulated. 

The contractile proteins of the myofibril include three troponin regulatory proteins. The troponin complex includes three protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component). The subunits exist in a number of isoforms. The distribution of these isoforms varies between cardiac muscle and slow- and fast-twitch skeletal muscle. Only two major isoforms of troponin C are found in human heart and skeletal muscle. These are characteristic of slow- and fast-twitch skeletal muscle. The heart isoform is identical with the slow-twitch skeletal muscle isoform. Isoforms of cardiac-specific troponin T (cTnT) and cTnl also have been identified and are the products of unique genes. All cardiac troponins are localized primarily in the myofibrils (94%-97%), with a smaller cytoplasm fraction (3%-6%). 

Ultracentrifugation of thrombosthenin as well as of thrombosthenin M led to conflicting results. As is known from the contractile proteins of muscular origin, poorly defined complexes of variable size may form very easily, making molecular weight determinations difficult. The same conditions were encountered with the contractile protein from human platelets (Bettex-Galland and von Tavel, unpublished observations, 1963). 

The creatine synthesized in the liver is transported through the bloodstream to skeletal and heart muscle. It enters the mitochondria, where it is phosphorylated to crealine-P Creatine kinase catalyzes this reversible addition of a phosphate group, as shown in Figure 4.34. Creatine-P is unique in that its only known function is as an energy buffer. The creatine P formed in the mitochondria travels to the contractile proteins in the cytoplasm of the muscle fiber. The polymer, or complex, of contractile proteins is called a myofibril. Contraction of a myofibril is coupled to the hydrolysis of ATP to ADP. The immediate replenishment of ATP is catalyzed by a second creatine kinase, residing on the myofibril, that catalyzes the conversion of creatine-P to creatine. This reversal of the reaction takes place in the... 

FIGURE 4 Complexity of actions of PKG in various smooth muscle cells. PKG may inhibit phospholipase C (PLC) activation or IP3 receptor gating to inhibit the mobilization of [Ca +l, within the cell. PKG may also stimulate Ca + removal from the cytoplasm by activating BK channels or stimulating Ca + ATPase activity through the phosphorylation of phospholamban (PLB). Other modes of action not depicted in the model include inhibition of contractile protein function, inhibition of L-type channels, and regulation of cytoskeletal events. 

Actin was first identified as part of the protein complex acto-myosin responsible for producing the contractile force in skeletal muscle. Actin is one of the most strictly conserved... 

Actins contractile proteins found in many cell types. Actin is an essential component of the contractile complex of Muscle proteins (see). Microvilli, microspikes (filopodia) and stereocilia (hair cells in the cochlea of the ear and related organs) consist of actin associated with other proteins. Monomeric actin (G-actin) has 41,720. Microfilaments in the cell cytoplasm consist of filaments of polymerized actin (F-ac-tin) (see Cytoskeleton). A consensus model of F-actin shows a helical filament with a diameter of 90-100 A, in which the long axes of the monomers are nearly perpendicular to the filament axis. The positions of... 

Thrombin attacks synthetic esters, namely tosyl l-arginine methyl ester (TAMe), and when thrombin is treated with DIPT, the loss of esterase activity is proportional to the loss of clotting activity. Studies with synthetic esters have suggested that, as with trypsin, the reaction between thrombin and its substrates occurs in three steps an enzyme-substrate complex is formed the acyl portion of the ester is transferred to the enzyme, which becomes acylated while the alcoholic portion of the ester is released and the enzyme is deacylated and the peptide is transferred to water. The natural substrate of thrombin is fibrinogen. In addition to converting fibrinogen to fibrin, thrombin releases the contractile protein from platelets, activates fibrinase, and may participate in the conversion of prothrombin to thrombin. 

It is now generally accepted, primarily as a consequence of the work of Szent-Gy5i 3d and his school, that actomyosin is the contractile element of muscle and that contraction of muscle is essentially a reaction of actomyosin, ATP, and ions. If an agent such as epinephrine, present in trace amount, produces major changes in the tension of cardiac muscle, this must mean in the last analysis that actomyosin must be affected, since it is this protein complex which is responsible for tension. After the nature of that part of the machine directly involved in the contraction of actomyosin had been worked out in a general manner, it became possible for Szent-Gyorgyi to ask whether epinephrine (or other hormones or drugs known to influence contraction in intact muscle cells) acts directly upon... 

Troponin (ca. 5% of the contractile proteins) sits on the actin filaments (cf. Fig. 12.10) and controls the contact between the filaments of myosin and actin during muscle contraction by means of a Ca concentration-dependent change in conformation (cf. 12.3.2.1.5). It is a complex of three components, T, I, and C. Troponin T consists of a peptide chain with 259 amino acid residues and binds to tropomyosin. Troponin I (179 amino acid residues) binds to actin and inhibits various enzyme activities (ATPase). Troponin C (158 amino acid residues) binds Ca ions reversibly through a change in conformation. 

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