Muscle Proteins

Tenderness, juiciness, and mouthfeel are all quality attributes of cooked meat that are influenced by proteins found in the muscle matrix. The textural properties of processed meats are primarily determined by salt- and phosphate-extracted myofibrillar proteins, which are superior gelling, fat-emulsifying, and water-binding agents. 

Myosin is the most essential functional protein. The enhanced meat tenderness during postmortem ageing of intact muscle cuts is attributed to the proteolytic activities of various endogenous proteases.

The muscle proteins, such as fish surimi, collagen, plasma protein, and other meat by-product proteins, have been prepared and used as functional food additives in addition to their in situ functionality.

What are the Muscle Proteins Types Found in Muscles?

Based on their solubility at different salt concentrations, skeletal muscle proteins can be divided into three categories: myofibrillar (50–60%), sarcoplasmic (30%), and stromal (10–20%) proteins.

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Myofibrillar Proteins

• Contractile, structural, and regulatory proteins make up the myofibrillar proteins. 

• Myosin and actin are contractile proteins that form thin and thick filaments that regulate skeletal muscle contraction and relaxation.

• Troponin and tropomyosin are two regulatory proteins. 

• Titin, nebulin, a-actinin, b-actinin, tropomodulin, desmin, filamin, C-protein, H-protein, and myomesin are among the structural proteins.

Contractile Proteins-

• The main protein of the A-band is myosin, a filamentous protein that forms the thick filaments of muscle cells. The quaternary structure of a myosin molecule is made up of six subunits: two myosin heavy chains (MHC), two critical myosin light chains (MLC1), and two regulatory myosin light chains (MLC2).

• G-actin (globular actin) and filamentous actin (F-actin) are the two types of actin that make up thin filaments (F-actin). G-actin is polymerized into double-stranded, coiled filaments to form F-actin. Tropomyosin and troponin bind to F-actin.

• Actin binds to myosin during muscle contraction to form actomyosin cross-bridges, which activate the myosin ATPase, causing myosin to pull thin filaments toward the M-line, shortening the sarcomere.

Regulatory Proteins-

• Tropomyosin and troponin are two of the most important proteins that control muscle contraction and relaxation. They interact with actin filaments to block the myosin binding site, preventing actomyosin ATPase activation in the absence of calcium ions.

• Tropomyosin is a coiled protein made up of two a-helix polypeptide subunits known as a- and b-tropomyosin. Tropomyosin molecules bind to F-actin filaments head-to-tail. Troponin complexes, which include troponin, troponin I, and troponin T, are bound to each tropomyosin molecule.

• Troponin C serves as a calcium binding site; troponin T binds the troponin complex to tropomyosin; and troponin I, when bound to actin, inhibits actomyosin ATPase activity.

• Calcium ions bind to Troponin C at high calcium ion concentrations, causing a conformation shift in the tropomyosin-troponin complex and dislocating troponin I, enabling actomyosin ATPase to act on muscle contraction.

Structural Proteins-

• Myofibril filamentous structure and integrity are regulated by structural proteins. Titin, also known as connectin, is the backbone of the A-thick band's filaments. It also serves as a molecular spring in the I-band, giving the sarcomere elasticity during muscle contraction.

• The structural protein nebulin controls the length of thin filaments.

• The main constituent of the Z-disk is a-Actinin, which supports and attaches actin to the Z-disk.

• The heterodimer b-Actinin, also known as CapZ protein, is made up of a- and b-subunits. It binds to a-actinin in the Z-disk and prevents actin filaments from forming networks.

• Tropomodulin binds tropomyosin and actin while also regulating the length of thin filaments by regulating the amount of G-actin monomers present.

• Desmin and filamin are essential for connecting myofibrils to the sarcolemma and stabilising muscle structure.

• Myosin-binding proteins C-protein and H-protein are contained in the A-band of thick filaments. The alignment and stability of the thick filaments are thought to be aided by these proteins.

• The M-main line's protein is myomesin. It is in control of titin and myosin binding as well as preserving the structure of the thick filaments.

Stromal Proteins

• In the form of tendons, epimysium, perimysium, and endomysium, stromal proteins make up the connective tissue that provides mechanical support and protection to the muscle. 

• Collagen makes up 90% of connective tissue, along with other fibrous proteins including elastin, laminin, and fibronectin, as well as proteoglycans. 

• Fibroblasts, macrophages, lymphoid cells, mast cells, and eosinophils are among the cells found in connective tissue. 

• For the reinforcement of the connective tissue network, collagen and elastin are connected to an amorphous ground substance created by proteoglycans and glycoproteins.

1. Collagen-

• Fibroblasts synthesise collagen, which is the most common stromal protein in skeletal muscles. 

• Based on aggregation properties, it is divided into four major types: striated and fibrous (Types I, II, III, V, and XI); non-fibrous and network forming (Type IV); microfibrillar or filamentous (Type VI); and fibril-associated collagen (Type VII).

• Tropocollagen is made up of three polypeptide alpha chains with -Gly-X-Y- repeating units, where X is usually proline and Y can be any amino acid (except tryptophan), but is usually hydroxyproline, that coil to form a triple helix structure.

• Tropocollagen molecules are polymerized into collagen fibres through covalent intermolecular cross-links, which give collagen fibres significant tensile strength. 

• The divalent reducible cross-linkages in collagen fibres connect as they age to form mature trivalent, non-reducible, more heat-stable cross-links, which improves their stability and mechanical strength.

2. Elastin-

• Elastin is a minor component of connective tissue that gives blood vessels and ligaments in the muscle elasticity. 

• In the presence of water, elastin is an insoluble, hydrophobic, heat-stable, and cross-linked protein fibre that acts in a highly elastic manner. The amount of elastin in various muscle types varies. 

• The epimysium and perimysium of the Semitendinosus muscle are rich in coarse elastin fibres, which are thought to contribute to meat toughness. 

• Longissimus dorsi, on the other hand, has only a few coarse elastin fibres in the epimysium and even less in the perimysium.

Sarcoplasmic Proteins

• Sarcoplasmic proteins are found in the sarcoplasm that surrounds myofibrils. Protein metabolism, fatty acid oxidation, electron transport, glycolysis, glycogenesis, and glycogenolysis are just a few of the metabolic roles they play.

• Heme Pigments (myoglobin), glycolytic enzymes (glyceraldehyde phosphate dehydrogenase), mitochondrial oxidative enzymes (such as succinate dehydrogenase, cytochrome), lysosomal enzymes (notably cathepsin), nucleoproteins, and others are among the sarcoplasmic proteins.

• Proteolytic enzymes, which are involved in post-mortem muscle tenderization, and myoglobin, which is responsible for meat colour, are among them.

1. Calplains-

• Calpains are a group of calcium-activated cysteine proteases that perform best at neutral pH. During protein turnover for muscle development, calpains degrade myofibrillar proteins. 

• M-calpain and m-calpain are the two forms of calpains that are responsible for post-mortem proteolysis. 

• Calcium ion concentrations for m-calpain and m-calpain activation are in the micromolar and millimolar ranges, respectively. 

• Along the Z-disk, calpains are found and function. Calpastatin is a natural m-calpain and m-calpain inhibitor.

2. Cathepsins-

• Cathepsins are sarcoplasmic proteins that are released from post-mortem muscle lysosomes and are active at an acidic pH. 

• The most abundant cathepsins in muscles are cysteine cathepsin B, H, and L, as well as aspartic cathepsin D. 

• MHC, troponin T, troponin I, tropomyosin, and collagen are all broken down by cathepsins. 

• Cystatins and pepstatin can inhibit the proteolytic activity of cysteine cathepsins and cathepsin D, respectively.

3. Myoglobin-

• Myoglobin is a heme protein that serves as an oxygen carrier in muscle cells and gives meat its colour, both raw and cooked. 

• The amount of myoglobin in various muscle types varies. Myoglobin content is higher in oxidative muscle fibres type I and IIA than in glycolytic muscle fibres type IIB and IIX.

• Owing to a higher content of quick glycolytic fibres and, as a result, a lower amount of myoglobin, poultry has a paler appearance than beef.

• Deoxymyoglobin (purplish red), oxymyoglobin (cherry red), metmyoglobin (brown), and carboxymyoglobin (carboxymyoglobin) are the four types of myoglobin found in muscles, depending on the condition of the heme community (cherry red).

• The ratio of these types of myoglobin determines the colour of the muscle.

Additional Questions/Information

Q. Name the Muscle Protein Which Plays an Important Role in Our Body.

Contractile, regulatory, sarcoplasmic, and extracellular types of muscle proteins exist. The contractile proteins actin and myosin are the most essential. Troponin, tropomyosin, M-protein, beta-actin, gamma-actin, and C-protein are all essential regulatory proteins.

Q. Name the Muscle Protein Involved in Structural Maintenance of a Sarcomere.

Since it covers half of a sarcomere and acts as a "molecular monarch" for the integration of most sarcomeric proteins, many signalling proteins, and even metabolic enzymes, titin (also known as connectin) is considered the most important structural protein of the sarcomere.