Muscle: Structure, Proteins and Phosphagens | Biochemistry

In this article we will discuss about Muscle:- 1. Muscle Structure 2. Proteins in Muscle 3. Phosphagens 4. Inorganic Constituents.

Muscle Structure:

1. Skeletal muscle is composed of fibrils sur­rounded by electrically excitable mem­brane, the sarcolemma.

2. The individual muscle fibre consists of a bundle of many myofibrils arranged in parallel. These are embedded by intramus­cular fluid termed the sarcoplasm.

3. The fluid contains glycogen, the high en­ergy compounds ATP and phosphocreatine, and the enzymes of glycolysis.

4. The functional unit of muscle is sarcom­ere. It exists along the axis of a fibril at distances of 2.5 µm.

5. Alternating dark and light bands (A bands and I bands) are present in the myofibrils. The central region of the A band (the H zone) is less dense than the rest of the band. The I band is bisected by a very dense and narrow Z line.

6. The cross-section of myofibril under elec­tron microscope shows that myofibrils are constructed of two types of longitudinal filaments. One type confined to the A band (the thick filament) contains mainly the protein myosin.

These filaments are about 16 nm in diameter. The other filament (the thin filament) lies in the I band and ex­tends also into the A band. The thin fila­ments are smaller than those of myosin (about 6 nm in diameter).

7. The thin filaments contain the proteins actin, tropomyosin and troponin. Each thin filament lies between 3 thick filaments.

8. When the muscle contracts, the length of the A bands remains the same but the I bands disappear.

Proteins in Muscle:

The muscle fibrils are composed of:

Proteins—20%.

Water—75%.

Inorganic materials, certain organic extractives and carbohydrates (glycogen and its derivatives)—5%.

Muscle proteins are characterized by their elasticity.

Myosin:

1. Myosin is the richly abundant muscle pro­tein; it is a globulin.

2. It is soluble in dilute salt solutions and insoluble in water.

3. Its molecular weight is 500,000 contain­ing 2 major chains and 4 light chains.

4. The enzyme trypsin cleaves myosin into two components—meromyosins—of un­equal size. Therefore, they are termed light and heavy meromyosins.

5. It has adenosine triphosphatase (ATPase) activity.

6. Myosin binds actin forming actomyosin. Light meromyosin does not combine with actin. But heavy meromyosin combines with actin.

7. Heavy meromyosin is a rod-shaped pro­tein attached to the two globular compo­nents of myosin. The rod portion can be split off of the globular region by the ac­tion of papain and the resulting portions are termed as HMM (heavy meromyosin) S-2 (the rod) or HMM S-1 (the globular portion). Each HMM S-1 possesses an ac­tive site for ATPase activity and a bind­ing site for actin.

8. The 4 light chains of myosin molecule are bound to the HMM S-1 fragments. The light chains function as modulators of ATPase activity.

9. Actomyosin is formed by 3 myosin mol­ecules with 1 actin molecule.

Actin:

1. Actin is a globulin of molecular weight 60,000.

2. It is the major constituent of thin filaments in striated muscle.

3. During the preparation of actin by extrac­tion with solution of low ionic strength it is obtained as a molecular weight of 42,000 in a globular configuration called G-actia.

4. G-actin polymerizes to the fibrous form F- actin in the presence of Mg⁺⁺ and ATP is hydrolyzed to ADP and Pi is released. Hence, ATP must be added to get de-polymerization of F-actin to G-actin.

5. Actomyosin is formed by actin and myosin.

But ATP dissociates it into actin and myosin:

Tropomyosin and Troponin:

1. Tropomyosin and troponin complex are proteins present in the thin filaments of muscle.

2. Calcium ion has the effect on the interac­tion of actin and myosin which, in turn, is mediated by tropomyosin and troponin.

3. Tropomyosin is a double-stranded a-helical rod of molecular weight 70,000 and present between the 2 strands of F-actin.

4. Troponin is a complex of 3 polypeptide chains—TPC, TPI, TPT.

5. The troponin complex is present in the thin actin filaments at intervals of 38.5 nm.

6. A troponin complex with tropomyosin molecule regulates the activity of 7 actin monomers.

Muscle Phosphagens:

1. ATP is the immediate source of energy for muscular contraction. But the amount of it in muscle is very small. It is just enough to cause contraction for a fraction of a sec­ond.

2. In vertebrate muscle, the source of high- energy phosphate is phosphocreatine which can transfer higher phosphate groups than ATP, and can donate a high-energy phosphate group to ADP to reform ATP.

Phosphocreatine is the vertebrate phosphagen; whereas phosphoarginine is the invertebrate phosphagen.

3. In the resting state, mammalian muscle contains 4-6 times phosphocreatine than ATP. The enzyme creatine kinase (creat­ine phosphokinase, CPK) catalyzes the hydrolysis of creatine phosphate transfer­ring high-energy phosphate from creatine phosphate to ADP (the Lohmann reac­tion).

The reaction is reversible and the re-synthesis of creatine phosphate takes place when ATP becomes available dur­ing the recovery period following a pe­riod of muscular contraction. The transfer of phosphate from ATP to creatine to form creatine phosphate is catalysed by the enzyme ATP-creatine trans-phosphorylase (shown in Fig. 14.6).

4. ATP is also available in the muscle from ADP by the enzyme myokinase (adenylate kinase) which catalyzes the transfer of a high-energy phosphate from ADP; AMP is also formed.

5. Iodoacetate can block re-synthesis of ATP and phosphocreatine by preventing gly­colysis.

6. ATP is also formed in the muscle from gly­cogen under anaerobic conditions in which glycogen is converted to pyruvate or lactate by glycolysis. The initial break­down of glycogen takes place by the en­zyme phosphorylase which is activated by the calcium ions.

Lactate thus formed is reconverted to glucose in liver by Cori cycle and re-enters the muscle. ATP is also formed from the oxidation of other fuels such as free fatty acids, ketone bodies and glucose. Muscle contraction generally takes place under aerobic con­ditions.

7. Fat is the ultimate source of energy for long periods of muscular exertion as in the case of migratory birds, whose total carbohydrate stores are quite inadequate to act as the sole source of energy and whose fat stores become depleted during migration.

The flight muscles of birds (red meat) are particularly well-developed for aerobic oxidation of pyruvate, free fatty acids and ketone bodies. These muscles also have a well-developed vasculature to increase oxygenation and a high content of enzymes of the respiratory chain as well as of cytochromes and myoglobin.

8. Heart muscle is also similar to the flight muscles of birds. It can utilize free fatty acids, ketone bodies and even lactate which are oxidized under aerobic condi­tions.

Inorganic Constituents of Muscle:

1. The cations (potassium, sodium, magne­sium and calcium) of muscle are the same as in extracellular fluids excepting potas­sium which predominates in muscle.

2. The anions are phosphate, chloride and small amounts of sulphates.

3. A good amount of potassium is incorpo­rated into the tissue during deposition of glycogen in the muscle and synthesis of protein. Muscle weakness is a sign of po­tassium deficiency.

4. Muscle calcium and magnesium act as ac­tivators or inhibitors of intramuscular en­zyme systems.