Proteins are very complex substances that are present in every living organism. Proteins have high nutritional values which play a direct role in the chemical process which is necessary for life. For example, muscles, for instance, contain about 30% protein, the liver 20 to 30%, and red blood cells 30%. Proteins are the most important element of the striated skeleton muscle. Hair, bone, organs, and tissues contain a higher percentage of protein.
The movement of the human body is dependent on the skeletal muscles, that are connected to the bones. Proteins are the fundamental material of tissue structure and constitute the most important element of the striated skeletal muscle. Muscle proteins are a major part of nutrition. The protein supplies essential color, texture, and nutritive value. In humans, the total quantity of muscular protein is greater than that of other proteins. A healthy human body has approximately 5 to 6 kg of muscular protein.
The muscle protein can be classified into,
Myofibrillar proteins
Contractile proteins
Regulatory proteins
Structural proteins
Stromal proteins
The myofibrillar proteins are long fibrillar proteins, and these are the major component of skeletal muscle, which represent 60 to 70% of all the muscles.
The myofibrillar protein mainly consists of myosin and actin.
The myofibrils contain light and dark bands alternatively. That light band indicates actin and it is called the isotropic band (I band). The dark bands contain myosin and are known as anisotropic bands (A band).
Actin filaments are thin so it is called thin filaments. Whereas, myosins are thick and are known as thick filaments.
Myosin makes up 35% of the total amount of protein in skeletal muscles.
Myofibril proteins primarily involve in muscle contraction and relaxation.
These are the proteins that cause the slippage of contractile fibers in the cytoskeleton of cells as well as the heart and skeletal muscles. Within a muscle fiber, there are two kinds of myofilaments for muscle contraction i.e. thin or actin filaments and thick or myosin filaments.
These are formed by a polymeric protein called myosin protein.
The myosin molecule is formed by six polypeptide chains. The monomer units are called meromyosin.
Every meromyosin has two major parts such as a globular head with a short arm and a tail.
The part globular head with the arm is called heavy meromyosin (HMM) and the tail part is called light meromyosin (LMM).
The globular head with an arm projecting outward at a uniform distance and angle from each other concerning the surface of a polymerized myosin filament is called a cross arm.
The globular head part is an active ATPase enzyme with the binding site ATP as well as a site for actin.
Each actin filament consists of two helically coiled filamentary actin (F). Every filamentary or F actin is a polymer of globular or G actin monomer.
Throughout the length of actin filaments, two other proteins bound around it. These proteins are called tropomyosin.
A complex protein that is distributed periodically on tropomyosin is called troponin which hides the active site of myosin fixation, at rest.
Tropomyosin and troponin act as regulatory proteins and has a major role in muscle contraction.
There are two regulatory proteins in muscles such as tropomyosin and troponin. Which inhibits myosin from binding to the actin when muscles are at rest.
The binding of myosin heads with actin muscle is a highly regulated procedure.
The actin and myosins get separated when the muscles relax or at rest.
The regulating proteins block the molecular binding sites to prevent actin from binding to the active myosin site.
Tropomyosin blocks the sites where myosin binds to actin molecules to prevent the formation of cross-bridges, it will prevent muscle contraction without any nerve input.
The troponin protein complex binds to tropomyosin, assisting in positioning it on the actin molecule.
These proteins provide the basis for the basic contractile unit known as sarcomeres.
The other structural proteins are titin, nebulin, a-actinin, b-actinin, tropomodulin, desmin, filamin, C-protein, H-protein, myomesin, etc.
The main functions of these structural proteins are given below,
| Structural Proteins | Functions |
|---|---|
| Titin | Provides elasticity to the sarcomere during muscular contraction. |
| Nebulin | Controls the length of thin or actin filament. |
| a-actin | Provides support and helps to fix actin to Z-disk |
| b-actin | Inhibits the action of a-actin from the formation of the network. |
| Tropomodulin | Controls the length of thin filament as well as the quantity of G-actin monomer. |
| Desmin and filamin | Maintains muscle structure by connecting myofibrils with sarcolemma. |
| C-protein and H-protein | Maintains the stability of thick or myosin filaments. |
| Myomesin | Prevents the binding of titin and myosin and maintains the structure of myosin filament. |
Table: Structural proteins and functions
The connective tissue consists of an aqueous material that is dispersed into a matrix of stromal protein fibrils.
Stroma proteins mainly consist of collagen, elastin, and reticulin in the muscle.
Collagen − These are the most common type of proteins present in mammals. Which are present in horns, bone, skin, tendons, etc. It is a unique and specialized protein that performs several functions. Major function is it gives strength and support.
Elastin &mius; These are yellow-colored small fibers found in connective tissue. These fibres can stretch back and forth. It may regain its original form when stretched and released. It is formed from a rubber-type protein known as elastin. It gives elasticity to the tissue.
Reticulin − It is present in smaller quantities than collagen and elastin. It gives structural support.
Contractile, structural and regulatory proteins comprise myofibrillary proteins.
Myofibrillar protein comprises myosin and actin.
Thin filaments and thick filaments are the two types of myofilaments that control muscle contraction and relaxation.
Tropomyosin and troponin are the regulatory proteins that have an important role in muscle contraction.
Titin, nebulin, a-actinin, b-actinin, tropomodulin, desmin, filamin, C-protein, Hprotein, myomesin, etc., are some of the structural proteins.
Stromal proteins are composed primarily of collagen, elastin, and reticuline in the muscle.
Q1. What is the role of the sarcoplasmic reticulum?
Ans. It helps in the excitation and contraction of muscles. It is the main store for calcium and controls the concentration of calcium during contraction and excitation in muscles.
Q2. What do you mean by meromyosin?
Ans. Myosin filament is made up of several monomeric protein units called meromyosin. It has two important parts, the globular head with the arm is called heavy meromyosin and the tail part is known as light meromyosin.
Q3. What is the difference between collagen and elastin?
Ans. These two proteins work together and with different functions. Collagen brings rigidity and strength while elastin provides flexibility to skin and rebounds to its original shape.
Q4. What is muscle cramping?
Ans. When a muscle is overused during any physical activity, it results in loss of blood supply. This causes cramping of muscle leading to shortening of muscle which causes pain.
Q5. What is muscular dystrophy
Ans. It is a disorder related to weakening and loss of muscle mass. It is a genetic disorder where protein production is reduced thereby degrading muscle activity.
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