The Role of Actin and Myosin
Figure 7.5 shows the placement of two other proteins associated with an actin filament, which you will recall is composed of a double row of twisted actin molecules. Threads of tropomyosin wind about an actin filament, and troponin occurs at intervals along the threads. Calcium ions (Ca2+) that have been released from the sarcoplasmic reticulum combine with troponin. After binding occurs, the tropomyosin threads shift their position, and myosin binding sites are exposed. The double globular heads of a myosin filament have ATP binding sites.
The heads function as ATPase enzymes, splitting ATP into ADP and P. This reaction activates the head so that it will bind to actin. The ADP and P remain on the myosin heads until the heads attach to actin, forming a cross-bridge. Now, ADP and P are released, and this causes the cross-bridges to change their positions. This is the power stroke that pulls the thin filaments toward the middle of the sarcomere. When another ATP molecule binds to a myosin head, the cross-bridge is broken as the head detaches from actin. The cycle begins again; the actin filaments move nearer the center of the sarcomere each time the cycle is repeated. Contraction continues until nerve impulses cease and calcium ions are returned to their storage sites. The membranes of the sarcoplasmic reticulum contain active transport proteins that pump calcium ions back into the sarcoplasmic reticulum.
Figure 7.5 The role of calcium and myosin in muscle contraction. a. Upon release, calcium binds to troponin, exposing myosin binding sites. b. After breaking down ATP, myosin heads bind to an actin filament, and later, a power stroke causes the actin filament to move.
Energy for Muscle Contraction
ATP produced previous to strenuous exercise lasts a few seconds, and then muscles acquire new ATP in three different ways: creatine phosphate breakdown, cellular respiration, and fermentation (Fig. 7.6). Creatine phosphate breakdown and fermentation are anaerobic, meaning that they do not require oxygen.
Creatine Phosphate Breakdown
Creatine phosphate is a high-energy compound built up when a muscle is resting. Creatine phosphate cannot participate directly in muscle contraction. Instead, it can regenerate ATP by the following reaction:
This reaction occurs in the midst of sliding filaments, and therefore is the speediest way to make ATP available to muscles. Creatine phosphate provides enough energy for only about eight seconds of intense activity, and then it is spent. Creatine phosphate is rebuilt when a muscle is resting by transferring a phosphate group from ATP to creatine.
Cellular respiration completed in mitochondria usually provides most of a muscle’s ATP. Glycogen and fat are stored in muscle cells. Therefore, a muscle cell can use glucose from glycogen and fatty acids from fat as fuel to produce ATP if oxygen is available:
Myoglobin, an oxygen carrier similar to hemoglobin, is synthesized in muscle cells, and its presence accounts for the reddish-brown color of skeletal muscle fibers. Myoglobin has a higher affinity for oxygen than does hemoglobin. Therefore, myoglobin can pull oxygen out of blood and make it available to muscle mitochondria that are carrying on cellular respiration. Then, too, the ability of myoglobin to temporarily store oxygen reduces a muscle’s immediate need for oxygen when cellular respiration begins.
The end products (carbon dioxide and water) are usually no problem. Carbon dioxide leaves the body at the lungs, and water simply enters the extracellular space. The by-product, heat, keeps the entire body warm.
Fermentation, like creatine phosphate breakdown, supplies ATP without consuming oxygen. During fermentation, glucose is broken down to lactate (lactic acid):
The accumulation of lactate in a muscle fiber makes the cytoplasm more acidic, and eventually enzymes cease to function well. If fermentation continues longer than two or three minutes, cramping and fatigue set in. Cramping seems to be due to lack of the ATP needed to pump calcium ions back into the sarcoplasmic reticulum and to break the linkages between the actin and myosin filaments so that muscle fibers can relax.
Figure 7.6 Energy sources for muscle contraction.
When a muscle uses fermentation to supply its energy needs, it incurs an oxygen deficit. Oxygen deficit is obvious when a person continues to breathe heavily after exercising.
The ability to run up an oxygen deficit is one of muscle tissue’s greatest assets. Brain tissue cannot last nearly as long without oxygen as muscles can. Repaying an oxygen deficit requires replenishing creatine phosphate supplies and disposing of lactic acid.
Lactic acid can be changed back to pyruvic acid and metabolized completely in mitochondria, or it can be sent to the liver to reconstruct glycogen. A marathon runner who has just crossed the finish line is not exhausted due to oxygen deficit. Instead, the runner has used up all the muscles’, and probably the liver’s, glycogen supply. It takes about two days to replace glycogen stores on a high-carbohydrate diet.
People who train rely more heavily on cellular respiration than do people who do not train. In people who train, the number of muscle mitochondria increases, and so fermentation is not needed to produce ATP.
Their mitochondria can start consuming oxygen as soon as the ADP concentration starts rising during muscle contraction. Because mitochondria can break down fatty acid, instead of glucose, blood glucose is spared for the activity of the brain. (The brain, unlike other organs, can only utilize glucose to produce ATP.) Because less lactate is produced in people who train, the pH of the blood remains steady, and there is less of an oxygen deficit.