developed on the basis of thermodynamics or on the basis of thermal fluctuation, and
the experimental evaluation of the theories are described. In the last section
microtubule-based, depolymerization-driven motility is briefly explained.
7.1 Biological Movements
7.1.1 Muscle Contraction and Mechano-Enzyme
Voluntary movements are remarkable features of life. Most notable biological
movement is skeletal muscle contraction (Fig. 7.1). Skeletal muscle is an organ
highly specialized for generating force and movement. It is consisted of multinuclear
muscle cells, each of which contains a number of contractile units each called
sarcomere. Sarcomere is regularly aligned in a muscle cell both in series and parallel.
Each sarcomere is consisted of a parallel array of thick, bipolar filaments consisted of
myosin and thin actin filaments. The unipolar actin filament is tagged at one end by a
structure called Z-line, resulting in the bipolar structure. The thin and the thick
filaments overlap each other. Because of the polarities of two filament systems, if
individual myosin filaments pull actin filaments, the spacing of the sarcomere is
narrowed. This is the essence of muscular contraction. The sarcomere-based highly
regular structure in the skeletal muscle makes the fast contraction and the generation
of the contractile force possible [1].
Myosin is a hexamer consisted of four small (~20 kDa each) polypeptides and
two large (~200 kDa each) polypeptides (Fig. 7.2). The large polypeptides (each
called heavy chain) form a globular structure on the side of N-terminus and a long
alpha-helix on the side of C-terminus, and the two heavy chains are held together by
the coiled-coil structure of the alpha-helical portion. Hence, as a whole, myosin
looks like a double-headed snake. This type of myosin is called myosin II. Two
small polypeptides (each called light chain) are reversibly bound to individual
junctions of globular domain and the helical domain. The bipolar myosin filament
is formed as a result of reversible association of the tails of many myosin molecules.
Sticking out from the shaft of the filament is the structure called cross-bridge. Cross
bridge was first recognized as a structure bridging the myosin filament and actin
filament in muscle and is consisted of the head portion of myosin [2, 3]. The head has
ability to reversibly bind to actin filament. The head portion binds and hydrolyzes
ATP. Myosin is thus an ATP-hydrolyzing enzyme and also produces the contractile
force (mechanical work). Hence, myosin is called a mechano-enzyme.
Actin filament is a polymer of a globular protein called G-actin (described in
detail in Sect. 7.3). Tropomyosin-troponin complex binds along the filament axis
(Fig. 7.3). When Ca
2+ ions are bound to the troponin complex, the position of the
tropomyosin shifts on the thin filament and each actin molecule becomes capable of
interacting with the myosin heads. As a result of this, the ATP-hydrolyzing activity
(ATPase activity) of myosin head is greatly enhanced. The binding of ATP and actin
to myosin head is reciprocally coupled. Thus, when ATP is bound to the ATPase site
of myosin head, the affinity of the myosin head to actin drastically lowered so that
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7 Moving Life
the experimental evaluation of the theories are described. In the last section
microtubule-based, depolymerization-driven motility is briefly explained.
7.1 Biological Movements
7.1.1 Muscle Contraction and Mechano-Enzyme
Voluntary movements are remarkable features of life. Most notable biological
movement is skeletal muscle contraction (Fig. 7.1). Skeletal muscle is an organ
highly specialized for generating force and movement. It is consisted of multinuclear
muscle cells, each of which contains a number of contractile units each called
sarcomere. Sarcomere is regularly aligned in a muscle cell both in series and parallel.
Each sarcomere is consisted of a parallel array of thick, bipolar filaments consisted of
myosin and thin actin filaments. The unipolar actin filament is tagged at one end by a
structure called Z-line, resulting in the bipolar structure. The thin and the thick
filaments overlap each other. Because of the polarities of two filament systems, if
individual myosin filaments pull actin filaments, the spacing of the sarcomere is
narrowed. This is the essence of muscular contraction. The sarcomere-based highly
regular structure in the skeletal muscle makes the fast contraction and the generation
of the contractile force possible [1].
Myosin is a hexamer consisted of four small (~20 kDa each) polypeptides and
two large (~200 kDa each) polypeptides (Fig. 7.2). The large polypeptides (each
called heavy chain) form a globular structure on the side of N-terminus and a long
alpha-helix on the side of C-terminus, and the two heavy chains are held together by
the coiled-coil structure of the alpha-helical portion. Hence, as a whole, myosin
looks like a double-headed snake. This type of myosin is called myosin II. Two
small polypeptides (each called light chain) are reversibly bound to individual
junctions of globular domain and the helical domain. The bipolar myosin filament
is formed as a result of reversible association of the tails of many myosin molecules.
Sticking out from the shaft of the filament is the structure called cross-bridge. Cross
bridge was first recognized as a structure bridging the myosin filament and actin
filament in muscle and is consisted of the head portion of myosin [2, 3]. The head has
ability to reversibly bind to actin filament. The head portion binds and hydrolyzes
ATP. Myosin is thus an ATP-hydrolyzing enzyme and also produces the contractile
force (mechanical work). Hence, myosin is called a mechano-enzyme.
Actin filament is a polymer of a globular protein called G-actin (described in
detail in Sect. 7.3). Tropomyosin-troponin complex binds along the filament axis
(Fig. 7.3). When Ca
2+ ions are bound to the troponin complex, the position of the
tropomyosin shifts on the thin filament and each actin molecule becomes capable of
interacting with the myosin heads. As a result of this, the ATP-hydrolyzing activity
(ATPase activity) of myosin head is greatly enhanced. The binding of ATP and actin
to myosin head is reciprocally coupled. Thus, when ATP is bound to the ATPase site
of myosin head, the affinity of the myosin head to actin drastically lowered so that
96
7 Moving Life