Muscle Contraction Proteins

Proteins are the very basic material of tissue structure. They are the most vital component of striated skeletal muscle. The total amount of muscle proteins present in  humans exceeds that of any other protein. About 40% of the total body weight of a healthy human adult weighing around 70 kg is muscle, which is composed of about 20% muscle protein. Thus, the human body comprises about 5 to 6 kg of muscle protein.

Classification of Muscle Proteins

The muscle proteins are divided into myofibrillar, contractile, regulatory, sarcoplasmic and stromal proteins.

Myofibrillar Proteins: muscle fibers are mainly composed of myofibrils. The proteins that comprise the myofibril include actin and myosin and many more.  The myofibrillar protein components which are most important for muscle fibre structure are actin and myosin. These are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and relax. Myosin comprises as much as 35 percent of the total protein volume of skeletal muscles while actin is the most abundant protein in most of the eukaryotic cells. 

Contractile proteins: it represents the main protein of the A-band that is myosin, a filamentous protein which forms thick filaments of muscle cells. The quaternary structure of a myosin molecule is composed of six subunits- two myosin heavy chains (MHC), two critical myosin light chains (MLC1), and two regulatory myosin light chains (MLC2).G-actin (i.e. globular actin) and filamentous actin (i.e. F-actin) are further two types of actin that make up thin filaments (F-actin). G-actin can be  polymerized into double-stranded, coiled filaments to form F-actin. Tropomyosin and troponin mainly bind to F-actin. Actin is seen to bind with myosin during muscle contraction in order to form actomyosin cross-bridges that activate the myosin ATPase, resulting in myosin to pull thin filaments toward the M-line and  shortening the sarcomere.

Regulatory proteins: these include troponin, tropomyosin, M-protein, beta-actin, gamma-actin and C-protein. They are all vital proteins, the troponin-tropomyosin is mainly responsible, not only for transducing (i.e. converting to an electrical form) the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction as and when calcium is absent.

Sarcoplasmic Proteins: it includes hemoglobin and myoglobin pigments and a wide variety of enzymes. E.g myogen, myo albumin and x-globulin. Pigments from hemoglobin and myoglobin usually help to contribute the red colour to muscles. Hemoglobin helps in carrying oxygen from the lungs to the tissues including muscle. Myoglobin is located in muscle and it stores the oxygen that is transported to the muscle through the blood via hemoglobin until it is utilized in metabolism. Together with the mitochondria it has a dominant effect on the aerobic potential of muscles.

Stromal Proteins: the connective tissue is composed of a watery substance to which is dispersed in a matrix of stromal- protein fibrils. These stromal proteins include collagen, elastin (which is a key protein of the extracellular matrix) and reticulin (it is a protein substance similar to collagen).

The remaining proteins involve myofibrillar proteins of the Z-disc as well as small amounts of other proteins.

Synthesis of Muscle Proteins

Muscle protein synthesis (MPS) is the main driving force behind all adaptive responses to exercise. In healthy, recreationally active individuals, skeletal muscle proteins represent a turnover rate of approximately 1.2% per day and exist in dynamic equilibrium.

It is seen that muscle protein breakdown (MPB) exceeds muscle protein synthesis (MPS) in the fasted state, and MPS exceeds MPB in the fed state. MPS can be exceeded via increasing one’s protein intake immediately after exercise. The amino acids derived from protein will then be accumulated into muscles, replacing any loss to exercise. Learning how MPS is regulated through exercise and diet can help accelerate muscle growth, improve recovery and athletic performance, and will increase overall endurance.

Conclusion

Muscle contraction helps in providing animals with great flexibility and allowing them to move in exquisite ways. The molecular changes have resulted in muscle contraction and is being conserved across evolution in the majority of animals. After studying sarcomeres, the basic unit controlling changes in muscle length, scientists have proposed the sliding filament theory in order to explain the molecular mechanisms behind muscle contraction. Within the sarcomere, myosin slides over actin to contract the muscle fibre in a process that requires ATP. Scientists also identified many of the molecules that are involved in regulating muscle contractions and motor behaviours, involving calcium, troponin, and tropomyosin. This research has helped us learn muscle contraction and how it can change their shapes to produce movements.