LPT 321: Meat Science (1+1)

PROTEINS OF THE MUSCLE

• Proteins of the muscle are classified based on their solubility characteristics as Sarcoplasmic proteins, which are soluble in water; Myofibrillar proteins, which are soluble in high ionic strength solutions and Connective tissue or Stromal proteins, which are insoluble in high ionic strength solutions, at low temperatures.
• Myofibrillar proteins, as the name indicates are associated with the myofibrils.
• Myofibrillar proteins are further classified into Contractile proteins, Regulatory Proteins and Cytoskeletal proteins
• Contractile proteins as called so  as they are involved in muscle contraction and the contractile proteins include actin and myosin.
• The proteins actin and myosin constitute approximately 65 percent of the protein in the myofibril.
• Regulatory proteins are so named because of their direct or indirect regulatory functions on the adenosine triphosphate-actin-myosin complex.
• The regulatory proteins include tropomyosin, troponin, two M proteins α-actinin, and β-actinin – listed in the decreasing order of concentration in the myofibril.
• The cytoskeletal proteins are involved maintaining the myofibrillar proteins in register and include titin, nebulin, C protein, M protein, desmin, filamentin, synemin and vinculin.
• Actin is a globular molecule about 5.5 µm in diameter. This is referred to as G-actin (for globular actin) and it constitutes the monomeric (single molecule) form of actin.
  • The fibrous nature of the actin filament is due to the longitudinal polymerization (linking) of G-actin monomers to form F-actin (fibrous actin).
  • In F-actin, the G-actin monomers are linked together in strands, similar to beads on a string.
  • Two strands of F-actin are spirally coiled around one another to form a “super helix “that is characteristic of the actin filament.
  • The isoelectric pH (pH of minimum electrical charge and solubility) of actin is approximately 4.7.
  • Actin possesses a relatively low charge.
• Myosin constitutes approximately 50-55 percent of the myofibrillar protein and is characterized by a high proportion of basic and acidic amino acids, making it a highly charged molecule.
  • The isoelectric pH of myosin is approximately 5.4.
  • Myosin, with lower proline content than actin, has a more fibrous nature.
  • The structure of the myosin molecule is an elongated rod shape, with a thickened portion at one end.
  • The thickened end of the myosin molecule is usually referred to as the head region and the long rod-like portion that forms the backbone of the thick filament is called the tail region.
  • The portion of the molecule between the head and the tail regions is called the neck.
  • The head region of the molecule is double headed and it projects laterally from the long axis of the filament.
  • When myosin is subjected to the proteolytic (protein breakdown) action of the enzyme trypsin, it is split near the neck into two fractions that differ in molecular weight; light meromyosin and heavy meromyosin.

• In the centre of the ‘A’ band, on either side of the M line, the myosin filament contains the tail portion of the myosin molecules without any of the globular heads. This region within the H zone, on either side of the M line, is called the pseudo H zone.
• The polarity of the myosin filaments is such that the heads on either side of the bare central region of the A band are oriented at an oblique angle away from the M line.
• The protruding heads are the functionally active sites of the thick filaments during muscle contraction, since the myosin heads form cross bridges with actin filaments.
• During muscle contractions each myosin head attaches to a G-actin molecule of the actin filament.
• The formation of cross bridges through this interaction of actin and myosin filaments produces the chemical complex known as actomyosin.
  • The formation of actomyosin results in a rigid and relatively inextensible condition in the muscle.
  • Actomyosin is the major form of the myofibrillar proteins that is found in postmortem muscle and the rigidity associated with rigor mortis is largely due to this complex.
  • It is a transient compound in the living animal, since the cross bridges between the actin and myosin filaments are broken during the relaxation phase of the contraction cycle. (Cross bridges are almost nonexistent in muscle when it is at rest.)
• Tropomyosin constitutes 8-10 percent of the myofibrillar protein and like myosin, is highly charged molecule with a high content of acidic and basic amino acids.
  • The isoelectric point of tropomyosin occurs at a pH of about 5.1.
  • Tropomyosin has a very low proline content, that contributes to its fibrous nature.
  • Tropomyosin molecules, consisting of two coiled peptide chains, attach end to end to one another and thus form long, thin filamentous strands.
  • In the actin filament, one such tropomyosin strand lies on the surface of each of the two-coiled chains of F-actin.
  • The tropomyosin strands lie alongside each groove of the actin super helix.
  • A single tropomyosin molecule extends the length of 7 G-actin molecules in the actin filament.
• Troponin, a globular protein with relatively high proline content, also constitutes 8-10 percent of the myofibrillar protein.
  • Like tropomyosin, troponin is present in the grooves of the actin filament where it lies astride the tropomyosin strands.
  • It is also probably present near the end of the tropomyosin molecules.
  • The troponin units shows a periodic repetitiveness along the length of the actin filament.
  • There is one molecule of troponin for every 7 or 8 G-actin molecules along the actin filament.
  • Troponin is a calcium-ion-receptive protein and calcium ion (Ca²⁺) sensitivity is its major function in the actomyosin-tropomyosin complex.

• α-actinin has proline content comparable to that of actin and it too is a globular molecule.
  • α-actinin is present in the Z line and constitutes about 2-2.5 percent of the myofibrillar protein.
  • It has been suggested that α-actinin functions as the cementing substance in the Z line.
• Β-actinin, which is also a globular protein, is located at the ends of actin filaments and is believed to regulate their length by maintaining a constant length of about 1µm in each half sarcomere.
  • In the absence of β-actinin, actin filaments in vitro attain lengths of 3-4 µm or more.
• Sarcoplasmic proteins include all the enzymes involved in glycolysis, TCA cycle and also myoglobin, which is the pigment responsible for the colour of meat.
• Connective tissue proteins include collagen, reticulin and elastin.