Energy sources of muscle activity. “BIOLOGICAL CHEMISTRY”, Berezov T. T, Korovkin B. F
It is considered that the process is directly related to the working mechanism of striated muscle fibers, is the disintegration of ATP with the formation of ADP and inorganic phosphate. The question arises: how does a muscle cell can provide the contractile apparatus sufficient amount of energy in the form of ATP. i.e. how does the process of muscle activity there is a continuous resynthesis of the compound?
First of all, the resynthesis of ATP is provided by transphosphorylation
ADP with creatine. This reaction is catalyzed by the enzyme creatine kinase:
Creatinkinase path resynthesis of ATP is an extremely fast and efficient as possible (at the expense of each molecule of creatine phosphate molecule of ATP is formed ). That is why for a long time failed to set reduced ATP concentration and thus an increase in ADP concentration even at a sufficiently prolonged tetanus. Using the specific inhibitor of creatine kinase (1-fluoro-2,4-dinitro-phenol), as well as with agents that prevent oxidative phosphorylation of ADP to ATP. T. Kane et al. (1962) were able to demonstrate the direct decay of ATP with a simultaneous increase in inorganic phosphate and ADP with a single contraction of isolated frog muscle. A certain amount of ATP can resentatives during adenocarcinomas (myokinase) reactions :
The reserves of creatine phosphate in the muscle are low and the availability of energy is creatine phosphate has value to working muscles only if its consumption is constantly compensated by the synthesis of ATP in the process of metabolism. For any fabric. including muscle, there are two fundamental biochemical process, during which regenerated energy-rich phosphorus compounds. One of these processes – glycolysis. another oxidative phosphorylation. The most important and effective of these is the last. When a sufficient supply of oxygen to the muscle, despite the anaerobic reduction mechanism, ultimately works due to the energy produced in the oxidation (in the Krebs cycle ) as degradation products of carbohydrates. and a number of other substrates of tissue respiration. in particular fatty acids. as well as acetate and acetoacetate.
Fig. 20.7. The energy transfer from mitochondria to the cytoplasm of myocardial cells (scheme V. A. Sachs, etc.). The explanation in the text.
a – outer membrane ; inner membrane ; CR – creatine; KRF – creatine phosphate ; QC-creatine kinase; T – translocase.
Recently, there is evidence proving that createinfo-fat in muscle tissue (particularly in the heart muscle) is able to perform not only the role as depot legkovushki macrorhyncha phosphate groups, but also the role of the transport form macrorhyncha phosphate bonds forming in the process of tissue respiration and the associated oxidative phosphorylation. The proposed scheme of energy transfer from mitochondria to the cytoplasm of myocardial cells (Fig. 20.7). ATP. synthesized in the matrix of mitochondria. transported across the inner membrane involving a specific ATP–ADP-translocase at the active site of mitochondrial isoenzyme of creatine kinase, which is located on the outer side of the inner membrane ; intermembrane space (in the presence of the ions Mg 2+ ) in the presence in the environment of creatine equilibrium ternary enzyme-substrate complex creatine–creatine kinase–ATP–Mg 2+. which then decomposes with the formation of creatine phosphate and ADP–Mg 2+. Creatine phosphate diffuses into the cytoplasm. where used in myofibrillar creatinkinase reaction to repositoryservice ADP. formed during the reduction. It has been suggested that not only in heart muscle, but also in skeletal muscles have a similar path of transport of energy from the mitochondria to the myofibrils.
When moderate-intensity muscle can cover their energy costs through aerobic metabolism. However, at high loads when the oxygen supply is lagging behind demand, the muscle is forced to use the glycolytic pathway of energy supply. During intensive muscular work the rate of breakdown of glycogen or glucose with the formation of lactic acid is increased hundreds of times. Accordingly, the content of lactic acid in muscle tissue may be increased to 1.0–1.2 g/kg and more. With the blood flow a significant amount of lactic acid enters the liver. where resynthesized into glucose and glycogen (gluconeogenesis ) by the energy of oxidative processes (see Chapter 16). The above mechanisms of resin-thesis of ATP during muscular activity are included in a specific sequence. The most urgent is creatinkinase mechanism, and only after about 20 with the most intensive work begins strengthening of glycolysis. the intensity of which reaches a maximum at 40-80 C. for longer, and hence less intense work is becoming increasingly important aerobic pathway for the resynthesis of ATP .
The content of ATP and creatine phosphate in heart muscle are lower than in skeletal muscles, and the consumption of ATP is high. In this regard, the resynthesis of ATP in the myocardium should be much more intense than in skeletal muscles. For the cardiac muscle of warm-blooded animals and humans, the main route of formation of energy-rich phosphate compounds is the way of oxidative phosphorylation. associated with the absorption of oxygen. Regeneration of ATP in the anaerobic breakdown of carbohydrates (glycolysis ) in the heart of man is practical does not matter. That is why the heart muscle is very sensitive to lack of oxygen. A characteristic feature of metabolism in cardiac muscle compared to skeletal is the fact that aerobic oxidation of substances neugeborne nature when the heart muscle is more important than the reduction in skeletal muscle. Only 30-35% oxygen. absorbed by the heart is normal, is consumed for oxidation of carbohydrates and products of their transformation. The main substrate of respiration in cardiac muscle are fatty acids. Oxidation of non-carbohydrate substances provides about 65-70% of the demand of the myocardium in energy. Of free fatty acids in the heart muscle is particularly susceptible to oxidation of oleic acid .