Molecular Composition ofFilaments

The thick filaments are aggregates ofa protein called myosin, which consists of a long fibrous "tail" connected to a globular "head" region, as shown schematically in Figure 10-3. The fibrous tails tend to aggregate into long filaments, with the heads projecting off to the side. Figure 10-4 shows a generally accepted view of how myosin molecules are arranged in the thick filaments of a sarcom-ere. The aggregated tails form the backbone of the thick filament, and the globular heads form the cross-bridges that connect with adjacent thin filaments.

The globular head of myosin contains a region that can bind ATP and split one of the high-energy phosphate bonds of the ATP, releasing the stored chemical energy. That is, myosin acts as an ATPase. The energy provided by the ATP is transferred to the myosin molecule, which is transformed into an "energized" state. This sequence can be summarized as follows:

Myosin + ATP ^ Myosin ATP ^ Energized Myosin ADP + P;

Figure 10-5 Myosin is an ATPase. ATP binds to the globular head of myosin, which catalyzes hydrolysis of ATP to ADP + inorganic phosphate (Pi). Energy released by ATP hydrolysis is stored in myosin, which is transformed into an energized form. The transition from the resting to the energized state of myosin involves rotation of the globular head around its flexible attachment site to the fibrous tail.

Here, the dot indicates that two molecules are bound together, as in an enzyme-substrate complex. To make a mechanical analogy, the globular head behaves as though it is attached to the fibrous tail at a hinged connection point. The energy released by ATP causes the head to pivot about the hinge into the energized state, as drawn schematically in Figure 10-5. To continue with mechanical analogies, this can be thought of as cocking the spring-loaded hammer of a cap pistol. As we will see shortly, the energy stored in this energized form of myosin is the energy that fuels the sliding of the filaments during contraction.

The thin filaments within a myofibril are largely made up of the protein actin. Thin filaments also contain two other kinds of protein molecule called troponin and tropomyosin, whose roles in contraction will be discussed a little later; for the present we will concentrate on the actin molecules. Actin is a globular protein that polymerizes to form long chains; thus, the thin filament can be thought of as a long string of actin molecules, like a pearl necklace. (Actually, each thin filament consists of two actin chains entwined about each other in a helix, but for a conceptual understanding of the sliding filament hypothesis it is not necessary to keep this in mind.) Each actin molecule in the chain contains a binding site that can combine with a specific site on the globular head of a myosin molecule. This is the site of attachment of the crossbridge on the thin filament.

0 0

Post a comment