Myosin Filaments Are Composed of Multiple Myosin Molecules. Each of the myosin molecules, shown in Figure 1A, has a molecular weight of about 480,000. Figure 1B shows the organization of many molecules to form a myosin filament, as well as interaction of this filament on one side with the ends of two actin filaments.

Fig1. A, Myosin molecule. B, Combination of many myosin  molecules to form a myosin filament. Also shown are thousands of  myosin cross-bridges and interaction between the heads of the cross bridges with adjacent actin filaments. 

The myosin molecule (see Figure 1A) is composed of six polypeptide chains—two heavy chains, each with a molecular weight of about 200,000, and four light chains with molecular weights of about 20,000 each. The two heavy chains wrap spirally around each other to form a double helix, which is called the tail of the myosin molecule. One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head. Thus, there are two free heads at one end of the double-helix myosin molecule. The four light chains are also part of the myosin head, two to each head. These light chains help control the function of the head during muscle contraction.

The myosin filament is made up of 200 or more individual myosin molecules. The central portion of one of these filaments is shown in Figure 1B, displaying the tails of the myosin molecules bundled together to form the body of the filament, while many heads of the molecules hang outward to the sides of the body. Also, part of the body of each myosin molecule hangs to the side along with the head, thus providing an arm that extends the head outward from the body, as shown in the figure. The protruding arms and heads together are called cross-bridges. Each cross-bridge is flexible at two points called hinges—one where the arm leaves the body of the myosin filament, and the other where the head attaches to the arm. The hinged arms allow the heads to be either extended far outward from the body of the myosin filament or brought close to the body. The hinged heads in turn participate in the actual contraction process, as dis cussed in the following sections.

The total length of each myosin filament is uniform— almost exactly 1.6 micrometers. Note, however, that there are no cross-bridge heads in the center of the myosin filament for a distance of about 0.2 micrometer because the hinged arms extend away from the center.

Now, to complete the picture, the myosin filament is twisted so that each successive pair of cross-bridges is axially displaced from the previous pair by 120 degrees. This twisting ensures that the cross-bridges extend in all directions around the filament.

Adenosine Triphosphatase Activity of the Myosin Head. Another feature of the myosin head that is essential for muscle contraction is that it functions as an adenosine triphosphatase (ATPase) enzyme. As explained later, this property allows the head to cleave ATP and use the energy derived from the ATP’s high-energy phosphate bond to energize the contraction process.

Actin Filaments Are Composed of Actin, Tropomyosin, and Troponin. The backbone of the actin filament is a double-stranded F-actin protein molecule, represented by the two lighter-colored strands in Figure 2. The two strands are wound in a helix in the same manner as the myosin molecule.

Fig2. Actin filament, composed of two helical strands of  F-actin molecules and two strands of tropomyosin molecules that  f it in the grooves between the actin strands. Attached to one end  of each tropomyosin molecule is a troponin complex that initiates  contraction. 

Each strand of the double F-actin helix is composed of polymerized G-actin molecules, each having a molecular weight of about 42,000. Attached to each one of the G-actin molecules is one molecule of ADP. These ADP molecules are believed to be the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction. The active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament about every 2.7 nanometers.

Each actin filament is about 1 micrometer long. The bases of the actin filaments are inserted strongly into the Z disks; the ends of the filaments protrude in both directions to lie in the spaces between the myosin molecules.

Tropomyosin Molecules. The actin filament also contains another protein, tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.

Troponin and Its Role in Muscle Contraction. Attached intermittently along the sides of the tropomyosin molecules are additional protein molecules called troponin. These protein molecules are actually complexes of three loosely bound protein subunits, each of which plays a specific role in controlling muscle contraction. One of the subunits (troponin I) has a strong affinity for actin, another (troponin T) for tropomyosin, and a third (troponin C) for calcium ions. This complex is believed to attach the tropomyosin to the actin. The strong affinity of the troponin for calcium ions is believed to initiate the contraction process, as explained in the next section.

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قسم حفظ النظام ينهي استعداداته الخاصة بحفل تخرج طالبات الجامعات العاشر

العتبة العباسية المقدسة تنشر أكاليل الزهور ومظاهر الفرح بمناسبة ذكرى ولادة أمير المؤمنين (عليه السلام)

مجمع الشيخ الكليني يواصل تحضيراته لاستقبال المشاركات في حفل تخرج طالبات الجامعات العاشر

وصول المشاركات في فعاليات مهرجان روح النبوة الثقافي النسوي العالمي الثامن إلى كربلاء

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الآخبار التكنلوجية

دراسة تكشف قدرة الحيتان على سماع الأصوات منخفضة وعالية التردد من مسافة بعيدة

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المزيد

مواضيع ذات صلة

Secretion of Acetylcholine by The Nerve Terminals

Mechanics of Skeletal Muscle Contraction

Energetics of Muscle Contraction

Interaction of One Myosin Filament, Two Actin Filaments, and Calcium Ions to Cause Contraction

General Mechanism of Muscle Contraction

Physiological Anatomy of Skeletal Muscle

المفاصل Joints

العضلة القلبية (Cardiac muscle)

العضلة الناعمة متعددة الوحدات (multi-unit smooth muscle)

العضلة الناعمة المفردة (single-unit smooth muscle)

العضلة الناعمة (smooth muscle)

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