Skeletal muscle changes structure

Magnesium is also of interest as a replacement for Ca(ll) in calcium-requiring enzymes. In some of these, the replacement is simple (Lewinski and Lebioda, 1986), and in others it cannot occur. NMR studies show that magnesium can bind in the calcium sites of troponin C (Tsuda et al., 1990). The structure of turkey skeletal muscle troponin C has recently been reported (Herzberg and James, 1985). In one domain the replacement of Ca(II) by Mg(II) causes a conformational change, but in the other domain it does not. 

The breakdown of glycogen in skeletal muscles and the liver is regulated by variations in the ratio of the two forms of glycogen phosphorylase. The a and b forms differ in their secondary, tertiary, and quaternary structures the active site undergoes changes in structure and, consequently, changes in catalytic activity as the two forms are interconverted. 

Calsequestrin is a calcium-storage protein found in the sacroplasmic reticulum, which binds about 50 calcium ions per monomer (molecular weight 40 000) with binding constants in the range 103-105 dm3 mol. Release and uptake of Ca2+ during muscle contradion and relaxation involve this store. Calsequestrin from rabbit skeletal muscle has a random coil conformation in the absence of calcium. Binding of Ca2+ is associated with a change to a more compact structure.267... 

Bushell AJ, Klenerman L, Taylor S, Davies H, Grierson I, Helliwell TR, Jackson MJ (2002b) Ischemic preconditioning of skeletal muscle. 1. Protection against the structural changes induced by ischemic/reperfusion injury. J Bone Joint Surg 84-B l 184-1188... 

Figure 10-4. The structure of the calciumbinding protein troponin from chicken skeletal muscle. Although this is an exceptionally complicated ligand to a coordination chemist, the binding of calcium ions is to the hard donor sites that might be predicted. Binding of the calcium triggers a conformational change.

Yagi, N. (2003). An X-ray diffraction study on early structural changes in skeletal muscle contraction. Biophys. J. 84, 1093-1102. 

A protein with a similar dumbell shape and structure is troponin C of skeletal muscles. Troponin C binds to a complex of proteins that assemble on the thin actin filaments of muscle fibers and control con-trachon in response to changes in the calcium ion con-centrahon (Chapter 19). Other proteins that contain EF-hand mohfs and are therefore responsive to Ca + include spectrin of cell membranes, clathrin light chains from coated vesicles, the extracellular osteonectin of bones and teeth, ° and a birch pollen anhgen. 2 Another group of 17 or more small SlOO EF-hand proteins play a variety of other roles. One of these, which has a high affinity for Zn +, has been named psoriasin because of its 5-fold or greater... 

It is now widely believed that in skeletal muscle a depolarization-induced change in the structure of the DHPR a 1 subunit directly influences the RyR in such a way as to markedly increase its conductance for Ca +. Since the resulting increase in [Ca +]i can open other RyR channels, this produces a surge of Ca + release. In contrast, cardiac muscle expresses a different a 1 DHPR subunit than skeletal muscle. The cardiac subunit has much higher channel conductance and faster kinetics than the skeletal muscle type, and so admits much more extracellular calcium during the action potential. Thus, CICR does play an important role in cardiac muscle. In both cases, high [Ca +]i reduces RyR conductance, by direct binding of Ca + (or Ca +-calmodulin) to RyR, by activation of a kinase that phosphorylates RyR, and probably both. At the same time, Ca-calmodulin (Ca-CaM) activates a protein kinase that phosphorylates the SR Ca-ATPase, which increases its activity 10- to 100-fold. These two mechanisms combine to terminate the Ca " " spike. 

Fig. 6.8 Tetrameric Ca2+ channels and control of Ca2+ release, a) A change in the membrane potential (V) induces a conformational change in the dihydropyridine receptor of skeletal muscle this is transmitted as a signal to the structurally coupled ryanodin receptor. Opening of the Ca2+ channel takes place and efflux of Ca2+ from the sarcoplasmic reticulum into the cytosol occurs, b) In cardiac muscle, the release of Ca2+takes place by a Ca2+-induced mechanism. A potential change V induces...

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