Meat Proteins - Structure and Classification

Meat in true sense refers to the flesh of animals used as food. It includes muscles (musculature), organs such as liver, kidney, brains and other edible tissues. The term carcass is used in meat industry and represent the portion of body left after removal of the blood, head, feet, hides, internal organs (digestive tract, intestine, bladder, heart, trachea, lungs, kidneys, spleen etc.) and adhering fatty tissues.

Chemical and Biochemical Constituents of Muscle

The approximate composition of meat is: 75 per cent water, 19 per cent protein, 3.5 per cent of soluble non-protein substances and 2.5 per cent fat. It must be remembered that meat is the resultant product of a complicated postmortem changes of a tissue.

Chemical composition of a typical adult mammalian muscle

Muscle proteins can be broadly classified into three types depending on their solubility properties:

i) Sarcoplasmic proteins: These are readily extracted in aqueous solution of low ionic (0.15 or less) strength i.e., soluble in water or very dilute salt solutions. It constitutes about 5.5 per cent of total muscle mass. There are about 50 sarcoplasmic proteins. It includes myoglobin, hemoglobin, enzymes associated with glycolysis, the tricarboxylic acid cycle and the electron transport chain, flavour proteins.

ii) Myofibrillar proteins: These are soluble in concentrated salt solutions and require higher (0.3 or greater) ionic strength solutions of sodium or potassium salts for their extraction. Since they are extracted by salt solutions, they are called salt-soluble proteins. They constitute about 11.5 per cent of muscle mass. The myofibrillar proteins are hrther classified into three categories.

a) Contractile Proteins: Actin and myosin constitute the major contractile proteins. They are named so because of their role in muscle cot traction. Actin constitutes approximately 20 per cent of the myofibrillar proteins whereas myosin, fibrous in nature constitutes 45 per cent of the myofibrillar protein. Actin forms the thin filament whereas myosin forms thick filament. The monomeric unit for actin is globular actin or G action and it links to form fibrous actin or F actin. In F action, the G action monomers are linked together in strands, much like beads on a string of pearls. Super helix, a characteristic of actin filament is formed as result of spiral coiling of two strands of F actin around each other. As far as myosin is concerned, proteolytic enzyme degradation reflects two fractions - light meromyosin and heavy meromyosin. The structure 01 the myosin molecule is an elongated rod shape with a thickened portion at one end called head, thin backbone called tail and a connecting between two is neck. Myosin has six subunits (polypeptides): two heavy chains and four light chains. The heavy chains are extended and wrapped around each other in a coiled manner. Molecules of myosin aggregate in muscles to form thick filaments which act as the basic contractile unit.

b) Regulatory proteins: These are involved in regulation of actin-myosin interaction during muscle contraction and in maintenance of myofibril integrity. The chief regulatory proteins are tropomyosin, troponin. a-actinin and p-actinin. Tropomyosin is approximately five per cent ol myofibrillar protein and lies in close contact with actin filament Tropomysoin exerts inhibition on crossbridge formation between actir and myosin except during contraction. Troponin is another regulator protein, approximately five per cent of myofibrillar protein and also founc in close association to actin. It is responsible for picking up the Ca2- available in sarcoplasm because of action potential. The calcium bind to troponin and this calcium activated troponin relieve inhibition being exerted by tropomyosin to facilitate contraction. The other two protein a and p-actinin constitute approximately two per cent and one per cen respectively and found in Z disc and free end of thin filaments within r band.

c) Cytoskeleton protein: They serve as the template and / or provide th scaffold for the alignment of myofilaments during myofibril an sarcomere formation. In mature muscle, these are responsible for maintenance of overall longitudinal and lateral alignment as well as structural integrity of myofibrils. Cytoskeleton proteins includes titin, nebulin, C-protein, myomesin, M-protein, desmine, filamin, vinculin, synemin, 2-protein, creatinine kinase.

iii) Stromal protein: They are of fibrous nature and not soluble even in high ionic strength salt solutions. These are refered as insoluble protein fraction of muscle. As such in muscle fiber they are approximately two per cent but more in connective tissue. The major stromal proteins are collagen, elastin and reticulin. Collagen is the most abundant protein in  animal body, about 20-25 per cent of total body protein. The tropocollagen is the structural unit of collagen fibril. Collagen is a glycoprotein which is a most abundant amino acid and one third of collagen is glycine. The relative insolubility and high tensile strength of collagen fibers results from intermolecular cross-linkages which influence meat tenderness.

Elastin is rubbery protein present throughout the body in ligament and arterial walls. It comprises of high content (about 90 per cent) of nonpolar amino acids responsible for its extreme insolubility. Being resistant to digestive enzyme and cooking, it contributes little or nothing to nutritive value of meat. Reticulin is another stromal protein which gives black with ammonical silver. It is different from collagen in having intimate association with lipids containing myristic acid.