When muscle contracts certain chemical changes of distinct nature take place simultaneously.
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The knowledge about the study of chemical changes involved in the muscle contraction begins with von Helmholtz (1845) idea that in the liberation of energy in the form of mechanical work as it occurs in muscle the energy must come from some pre-existing energy.
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In the year of 1907 Fletcher and Hopkins declared that the lactic acid is formed during anaerobic contraction of vertebrate muscle.
A few years latter Parnas showed that the lactic acid originates from the glycogen. This discovery led to an enormous source of activity, both biophysical and biochemical.
On the biophysical side A.N. Hill began his studies on the energetics of muscular contraction, while on the biochemical side Otto Meyerhof began the studies of glycolysis which ultimately led to our present rather complete understanding of this complex and important process.
The immediate energy for the contraction of the muscle is derived, whether directly or indirectly is uncertain, from the breakdown of the ATP to ADP, while the ultimate source is the combustion of the carbohydrates (glucose, glycogen and others) present in the muscle.
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However, there is strong evidence that lipids are also utilized at times. These lipids exist in the form of free fatty acids which can diffuse rapidly from the blood stream into the muscle or derive from the muscle’s fat stores.
All the enzymes required for the oxidative metabolism of the free fatty acids are found in the muscle. The availability of oxygen determines the degree to which free fatty acids constitute a source of energy.
When ATP is not available in sufficient amount during the muscle contraction, creatine phosphate (phosphocreatine) serves as a supplemental source of energy which through the action of a transferase enzyme restores the depleted ATP.
The sequence of chemical changes that take place during muscle contraction are as follows:
1. Conversion of adenosine triphosphate into adenosine diphosphate:
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The first and the important chemical change, that takes place during muscle contraction, is the conversion of the adenosine triphosphate into the adenosine diphosphate.
This conversion is brought about by the enzyme, adenosine triphosphatase (ATPase) present in the muscle.
During this conversion one molecule of phosphoric acid is removed from the adenosine triphosphate which supplies the immediate energy to the muscle for contraction. This reaction can occur anaerobically.
2. Break down of creatine phosphate (phosphocreatine):
The next step is the breakdown of creatine phosphate present in the muscle to produce creatine ahd phosphoric acid.
The phosphoric acid molecule combines with adenosine disphosphate (ADP) and forms ATP.
Creatine phosphate—–à Creatine + Phosphoric acid
Phosphoric acid + ADP——à ATP.
3. Break down of muscle glycogen:
The glycogen present in the muscle after reacting with phosphoric acid liberated during the break down of ATP into ADP, is converted into glucose phosphate.
Glycogen + Phosphoric acid -à Glucose phosphate
4. Formation of Fructose diphosphate:
The glucose phosphate after undergoing various chemical reactions is converted into fructose diphosphate.
Glucose phosphate —Enzymatic reaction—-à Fructose diphosphate
5. Formation of latic acid:
Fructose diphosphate after undergoing various chemical changes is converted into latic acid. During the formation of latic acid three molecules of ATP are formed.
Fructose diphosphate ——–à Latic acid + 3 ATP.
6. Resynthesis of creatine phosphate:
During periods of inactivity or less intense activity, the creatine is rephosphorylated by ATP (enzymatic reactions are responsible produced in intermediary metabolism.
The energy for the resynthesis of creatine phosphate is derived from the breaking down of glycogen to lactic acid.
1/5 part of the lactic acid is oxidized in the presence of oxygen with the result energy along with carbon dioxide and water is produced, while remaining 4/5 part of the lactic acid is resynthesized to form glycogen in the liver, muscles and other tissues of the body.
The energy for the body and for the resynthesis of glycogen is derived from the breakdown of the 1/5 part of lactic acid produced during the glycolytic process of the glycogen.
Thus, as the muscle contracts due to sliding of the actin filaments in the spaces between the myosin filaments, simultaneously breakdown of ATP into ADP takes place with the result heat is liberated.
The ADP thus formed is regenerated to ATP by an energy rich phosphate bond denoted by creatine phosphate.
Creatine is converted back to creatine phosphate by the ATP molecules generated during anaerobic or aerobic breakdown of carbohydrates.
The foregoing account, therefore, reveals that ATP plays important role in the muscle contraction and it is always found re.idy for muscle contraction.