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CHEM 361B CHEM MOLECULAR-ARCHITECTURE-IN-MUSCLE-CONTRACTILE-A_2005_ADVANCES-IN-PROTEIN-CHEM
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CHEM 361B CHEM MOLECULAR-ARCHITECTURE-IN-MUSCLE-CONTRACTILE-A_2005_ADVANCES-IN-PROTEIN-CHEM
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MOLECULAR ARCHITECTURE IN MUSCLECONTRACTILE ASSEMBLIES
By JOHN M. SQUIRE,* HIND A. AL‐KHAYAT,* CARLO KNUPP,* AND
PRADEEP K. LUTHER{
*Biological Structure and Function Section, Biomedical Sciences Division,
Imperial College London, London SW7 2AZ, United Kindom;
{Biophysics Group, Department of Optometry and Vision Sciences, Redwood
Building,
Cardiff University, Cardiff CF10 3NB, Wales
I. Introduction.................................................................................................................17
II. Hierarchy......................................................................................................................19
A. Components and Organization of the Sarcomere . . . . . . . . . . . . . . . . . . . . . 23
B. Actin Filaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
C. Vertebrate A‐Band Lattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
D. The Sliding Filament Model and the Crossbridge Cycle . . . . . . . . . . . . . . . 31
III. Actin Filament Structure and the Z‐Band..................................................................34
A. The Actin Monomer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B. F‐Actin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
C. The Thin Filament and Troponin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
D. Filament Organization in the Contractile Units of Different
Muscle Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
E. The Z‐Band. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
F. Filament Organization in the Vertebrate I‐Band . . . . . . . . . . . . . . . . . . . . . . 49
IV. Myosin Filament Structure and the M‐Band.............................................................51
A. The X‐Ray Diffraction Approach to Myosin Filament Structure . . . . . . . . 51
B. Myosin Head Organization in Relaxed Vertebrate Myosin Filaments . . . 56
C. Further A‐Band Analysis: C‐Protein, Titin, and the Vertebrate M‐Band.............61
D. Invertebrate Myosin Filaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
E. Crossbridge Arrangements on Different Myosin Filaments:
Variations on a Theme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
F. Conclusion: Implications about the Crossbridge Mechanism. . . . . . . . . . . 77
References..........................................................................................................................79
I. IntrodUCTION
The pioneering days of molecular biology in the 1950s and 1960s saw
the determination of a variety of unknown molecular structures using an
array of then novel techniques. The double helix of DNA was solved using
high‐angle X‐ray fiber diffraction data (Watson and Crick, 1953; Wilkins
et al., 1953). The technique of protein crystallography came of age when
structures of key globular proteins were determined for the first time
(Kendrew, 1963; Muirhead and Perutz, 1963). At the same time, electron
microscopy and low‐angle X‐ray fiber diffraction helped to define the
ADVANCES IN 17 Copyright 2005, Elsevier Inc.
PROTEIN CHEMISTRY, Vol. 71 All rights reserved.
DOI: 10.1016/S0065-3233(04)71002-5 0065-3233/05 $35.00
,18 SQUIRE ET AL.
FIg. 1. (A) Schematic illustration of the hierarchy of muscle. Skeletal muscle is
composed of fibers about 20 to 100 mm in diameter and very long. Fibers in the light
microscope appear cross‐striated and the muscles they come from are known as striated
, MOLECULAR ARCHITECTURE IN MUSCLE CONTRACTILE 1
structure of the muscle sarcomere in terms of separate actin and myosin
filaments that slide past each other when a muscle shortens and that interact
through cross‐connections (Huxley, 1957, 1969). Since then the muscle
story has progressed in leaps and bounds so that in some ways it is now
one of the best understood tissues. But this progress has been a story of
using all of the techniques mentioned above in a correlated way; each
method has illuminated the application of the other techniques and
without any single one of them, our knowledge of muscle would be much
the poorer.
This article summarizes current knowledge about the major muscle com-
ponents, the actin and myosin filaments. We also briefly discuss a third set
of filaments, the titin filaments, which have remarkable properties and play
a central role in integrating sarcomere structure (see Granzier and Labeit,
2005) We further describe how these filaments are organized in the muscle
repeating unit, the sarcomere, through the cross‐linking M‐band and Z‐
band structures. Finally, we discuss the filament arrangements in inverte-
brate muscles. This articles serves as an introduction to the detailed articles
on titin (Granzier and Labeit, 2005), on muscle regulation (Brown and
Cohen, 2005), and two articles on the contractile mechanism (Geeves
and Holmes, 2005; Squire and Knupp, 2005).
II. HIerarcHY
Anyone eating a steak or a slice carved from roast beef knows that meat
is fibrous in texture. These fibers, 20 to 100 mm in diameter and very
~
long, are the multinucleate muscle cells of which skeletal muscles are
composed (Fig. 1A, B). Such fibers in the light microscope appear cross‐
striated and the muscles from which they are derived are known as striated
muscles. The term striated also covers the muscles in animal hearts (Fig.
1C), but here the cells (the myocytes) are much shorter, they contain a
single nucleus, and they are linked end to end by special structures
known as
muscles. Striations are from repeating units, the sarcomeres, with A‐band and I‐
band regions. Each sarcomere extends between successive Z‐bands and is about 2.2 to
2.3 mm long in a resting muscle (Bloom and Fawcett, 1975). (B) Groups of muscle
fibers (F), showing they are multinucleated (N ), and composed of the myofibrils (MF).
Myofibrils may be about 2 to 5 mm in diameter. (C) Representation of the muscle cell
arrangement in animal hearts, showing similar striations to those seen in skeletal
muscles, but here the cells (the myocytes) are much shorter, they contain a single
nucleus (N ), and they are linked end‐to‐end by special structures known as intercalated
disks (D), which provide mechanical and electrical continuity between cells. (D) A typical
smooth muscle that can be found surrounding the blood vessels and various hollow
organs apart from the heart. These visceral muscles do not have cross‐striations. M,
mitochondria, Z, Z‐band, N, Nucleus.
, 20 SQUIRE ET AL.
intercalated disks, which provide mechanical and electrical continuity
between cells. Other muscles in animals surround the blood vessels
and various hollow organs apart from the heart. In vertebrates these
visceral muscles are smooth muscles; they do not have cross‐striations
(Fig. 1D).
A closer look at striated muscle fibers shows that they themselves are
assemblies of fine, hairlike structures known as myofibrils (Fig. 1A, B).
Myofibrils may be about 2 to 5 mm in diameter, with cell organelles such
as mitochondria and membranous systems called T‐tubules and the
sarcoplasmic reticulum (SR) sandwiched between them (Fig. 2B).
Vertebrate skeletal and cardiac muscles have a striated appearance
because the myofibrils themselves are cross‐striated; they have a repeating
unit along them known as the muscle sarcomere (Fig. 1A). In vertebrate
muscles this repeat is approximately 2.2 to 2.3 mm long in a resting muscle
but varies in length as the muscle stretches or shortens. The sarcomere,
taken to extend between successive Z‐bands (Z‐discs) along the myofibril
(Fig. 1A), is really the ‘‘business part’’ of the muscle where force is gener-
ated. Large muscles are assemblies of millions of sarcomeres all working
together. Understanding the basic contractile mechanism in muscle there-
fore requires an understand how the sarcomere itself works. Sarcomere
structure and function are the main topics of this article and those by
Granzier and Labeit, Brown and Cohen, Geeves and Holmes, and Squire
and Knupp in this volume. Before going into such detail, a description of
some of the properties of muscle as a whole is warranted. Muscle innerva-
tion mechanisms and the kinds of contractile response that are produced
are the topics of the rest of this section.
Muscle contraction in the so‐called voluntary muscles, of which most
skeletal muscles are examples, is initiated when a nerve action potential
arrives at the nerve‐muscle (neuromuscular) junction (Fig. 2A). This in
turn causes a depolarization of the muscle outer membrane, the sarcolem-
ma, which causes the release of calcium ions into the interior of the
muscle. However, muscle fibers are relatively large in diameter and it
would take far too long for calcium to diffuse to the muscle interior to
activate the centrally located myofibrils. For this reason, there are invagina-
tions of the sarcolemma into the interior of the fiber. These
invaginations are the T‐tubules (Fig. 2B). Thus, when the sarcolemma is
depolarized, the depolarization is also propagated down the T‐tubules,
which in turn interact with the terminal cisternae of the SR (Fig. 2B) to
trigger the release of calcium ions locally into the adjacent myofibrils.
The induced calcium release is mediated through the ryanodine
receptor in the SR and subsequent sequestration of calcium when the
muscle relaxes is accomplished by calcium pumps (Franzini‐Armstrong,
1999).
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