Muscle is unique in a number of ways. The fibers of muscle are made up of a number of different proteins arranged in a very specific manner. The appearance of the majority of muscle used for food is characterized by the presence of a number of relatively light and dark filaments. The relationship between muscle cells, fibers abd myofibrils is shown in figure 1.
This muscle is called striated muscle. Figure 2 is an electron micrograph of striated muscle.
Figure 2. Electron micrograph of striated muscle.
The striations are clearly visible in this micrograph. This can be seen at a higher magnification in figure 3.
The filaments are better visible in this figure and can be seen to be composed of a series of thick and thin lines. This structure of muscle can be related to the arrangement of individual muscle proteins. Muscle is unique in a number of ways. The fibers of muscle are made up of a number of different proteins arranged in a very specific manner. These fibers represent classic examples of Quarternary structure as well as super molecular organization. Muscle is also responsible for the transduction of chemical energy into mechanical energy. While all of the mechanisms responsible for motion are not understood, much is. An understanding of how the muscle components interact with small molecules and ions can lead to a better understanding of the quality of meat products. The final quality of meat is dependent on the conditions of slaughter and storage of the muscle proteins as well as the post mortem processing. As much as is possible, these changes in quality will be related to the biochemistry of the tissue.
Lean beef muscle contains approximately 70 to 75% water and 20 to 22 % protein. There is essentially no carbohydrate, 4 to 8% lipid and about 1% ash. The protein component can be divided into the myofibrillar proteins(salt soluble) which make up about 50 to 55% of the total and are responsible for the actual contraction of the muscle; the sacroplasmic or soluble proteins make up about 25 to 35% of the total. These proteins are almost all enzymes. The glycolytic enzymes are the major proteins of this fraction. The remainder of the protein fraction is the insoluble or stroma fraction. It contains membrane proteins, connective tissue proteins and some large specialized structural proteins that will be discussed later.
The contractile proteins of muscle are listed in table 1.
Table 1. The contractile proteins of muscle*.
|Protein||Muscle Type||Molecular Wt.|
|Bl||Chicken fast twitch||35,000|
|a-tropomyosin||Chicken fast twitch||33,000|
|b-tropomyosin||Chicken fast twitch||36,000|
|C||Chicken fast twitch||18,000|
|T||Chicken fast twitch||30,500|
|Heavy chain||Chicken fast twitch||220,000|
|Alkali light chain||16,000 - 22,000|
|DTNB light chain||18,000|
|C-Protein||Human fast twitch||128,000|
|a-actinin||Rabbit fast twitch||100,000|
|Nebulin||Rabbit||700,000 - 900,000|
|I-Protein||Chicken fast twitch||50,000|
*Adapted from Foegeding et al and other sources.
Also contains collagen.
Myosin makes up approximately 45 to 50% of the contractile proteins It properties include:
The tail regions of myosin provide structure while an ATPase activity as well as an actin binding reside in the head region of the molecule. A portion of the tail region is responsible for the association of myosin molecules with themselves. Hundreds of associated myosin molecules make up the thick filaments found in muscle.
In the native state, actin is a roughly globular protein with a molecular weight of approximately 46,000 called G actin. In the presence of ATP, G actin causes hydrolysis of the ATP and aggregates into a double helical structure called F-actin having a molecular weight of approximately 14,000,000. F actin can interact with troponin and tropomyosin (see figure 4). The bound ADP can interact with the head units of myosin. The interaction of actin with myosin can lead to motion in muscle.
Figure 4. The structure of thin filaments. Adapted from Cheftel et. al.
Tropomyosin and Troponin
Two other proteins associate with actin to from the thin fliaments. Tropomyosin is compose of two polypeptides (a- and b-tropomyosin). Together they make up from 5 to 8% of the myofibrillar protein. These polypeptides aggregate to form long filaments that fit within the grove formed by the two chains of actin. Each molecule spans seven actin molecules and controls the reactivity of these actin molecules. Troponin is made up of three subunits. Troponin C conatins a calcium binding domain, troponin, T interacts with tropomyosin and tropinin I can block the actin binding site for myosin. One set of three troponin subunits is associated with each molecule of tropomyosin and is involved with the activity of seven actin molecules.
Thick Filament Proteins.
The role of myosin in thick filaments has already been discussed. Associated with the thick filaments are the C, H and X proteins. These proteins appear to form rings around the thick filament at roughtly even intervals. It has been suggested that they aid in the structure of thick filaments, possibly preventing helping in the maintanance of thick filament structure during contaction.
The structure of sarcomeres.
The arrangement of muscle proteins into a contractile unit known as a sarcomere is represented diagramatically in figure 5.
The elongated ovals are meant to represent the thick filaments. The upper filament demonstrtaes the loacation of the globular myosin head groups in two dimensions. In an actual muscle, these head groups would be situated around the entire thick filament. The bottom filament shows the location of the C, H and X proteins in bands encirlcing the entire filament. The line through the center of the sacromere is called the M line. The Z shaped line at each end are called the Z lines. Above and below each thick filament are the think filaments composed of actin, tropomyosin and troponin.
Other Structural Proteins
Alpha actinin is compsoed of two subunits and is found in the Z line. Alpha actinin can bind to F actin and it may provide structure to the Z line and an anchor for actin.
Beta actinin is made up of two proteins and is found at the end of the thin filaments ( represented as dark circles in figure 4). It has been suggested that b-actinin helps to control the length of the thin filaments.
Myomesmin is the major component of the M-line. It has been suggested that its role is to ensure proper alignemtn of the thick filaments.
Desmin is a normal component of the cytoskelleton of the cell and is capable of assuming a filamentous structure. Some of of the filaments appear to be attached to the Z-lines and they may serve to connect adjacent sarcomeres.
Two vey large proteins are found associated with the sacromere. Titan, with a molecular weight of 1,000,000 makes up 10% of the total sacromere protein. It can interact with myosin, a-actinin and M-line proteins and is thought to have a role in determining the length of the sarcomere. Nebulin makes up about 5% of the total protein and has a molecular weight of about 800,000. It has many actin binding sites and is thought to help determine the length of thin filaments.
The sarcoplasmic reticulum is a netwrok of membranes that passes through the muscle cells. It is in communication with a netwrok of membranes called the T tubule system. Upon receiving the appropriate message from the T tubules it can change its permeability to calcium. An important transmembrane protein of the sacroplasmic reticulum is a calcium ATPase. This membrane functions in the transport of calcium into and out of the saromere. In the normal resting state, this ATPase functions as a calcium pump. It binds to and pumps calcium from the sacroplasm into the sacroplasmic reticulum where the calcium is bound by a calcium binding protein. A signal for contraction alters the permeability of the membrane to calcium and allows calcium to enter the tissue. In this case, the calcium ATPase functions as an ATP synthetase and uses the energy from the chemical potential to synthesize ATP.
The sarcoplasmic reticulum is able to control contraction in a muscle cell by controling the concentration of calcium in the cell. This control is coordinated through the T tubule assembly and is coordinated by nerve impulses.
The contraction of muscle requires coordination of the thick and thin filaments, ATP and calcium. A nerve impulse changes the polarity of the membrane surrounding the muscle cell. This change is transmitted though the cell through the T tubular system. The signal is further transmitted to the sarcoplasmic reticulum and calcium is released from the membrane. The concentration of calcium in the cell increases by a factor of about 100.
The calcium binds to the calcium binding site of troponin (troponin C). This binding causes a change in the inhibitory site (TnI) which causes ta change in the position of tropomyosin. The entire troponin-tropomyosin complex moves into the groove created by the twisting of the two actin chains. This removes the portion of tropomyosin that was blocking the myosin binding site. The binding of one calcium moves a complex covering seven actins. These actins are now capable of interacting with the head portion of the myosin molecule.
The release of myosin heads with actin requires ATP. An ATP molecule will bind o a myosin head and cause it to release from one of its actin binding sites. It will still be bound weakly to another site. The myosin head will hydrolyze the ATP to ADP and phosphate which will remain bound to the myosin head. If the troponin-tropomyosin complex is in the actin groove (calcium is present), The myosin will bind to actin. The energy from the hydrolysis will be used to change the myosin conformation and contraction will occur, accompanied by the release of ADP and phosphate. Thus cycle will be repeated as long as calcium and ATP are present. The change in structure of the myosin head is thought to cause a swivel like motion that slides the thin filament past the thick filament in what is lnown as the sliding filament model. This can be compared to the action of an oar in a boat.
When the calcium concentration falls (a nerve impulse for relaxation), ATP will bind to myosin and cause it to release from actin. It will not be able to reattach because of the change in position of the troponin-tropomyosin complex and the filaments will slide back to their original position.
In a living animal, there is never total relaxation or contraction. A portion of muscle is partially contracted to result in normal muscle tone. Contraction of one muscle leads to a relaxation of the muscle that controls the opposite motion. These conditions change upon death.
Post mortem changes
Loss of oxygen
Lactic acid accumulation
Mitochondria cease to function
Lipid metabolism ceases
ATP depleted by ATPases
Creatine PO4 converted to ATP and creatine
Glycolysis yields ATP and lactic acid
Glycolysis controlled by PFK
PFK inactive around pH 5.1-5.5
pH of inactivation is higher at higher temperatures
These conditions lead to a loss of ATP and a decrease of pH in muscle. As long as ATP is present, the muscle can relax and calcium can be continually be pumped into the sacroplasmic reticulum. As ATP is depleted, relaxation can no longer occur. In addition, the calcium content of the cell increases. This causes contraction of opposing muscles and results in a state known as rigor mortis. The rate and extent of the pH decline associated with rigor can have a large impact on the final quality of the muscle. In general, a slow decline to a low pH is desirable.
In a stressed animal (high adrenaline) the rate of depletion of ATP will be rapid. This causes the muscle to be exposed to a low pH at a relatively high temperature. The combination of low pH and high temperature can result in protein denaturation. A major affect of this denaturation is a loss of water holding capacity. The release of moisture results in a condition known as pale-soft exudate (PSE) and is a quality defect.
In an exhausted animal, a different problem may occur. The low energy state will not result in protein denaturation, but will yield a muscle with a high ultimate pH. This can lower the time that the muscle may be kept before microbial spoilage becomes an issue. The high pH also results in a dark colored meat that tends to be dry known as dark, firm and dry (DFD).
If the rate of cooling of the carcass is too fast, cold shortening may result. This is especially a problem with small animals such as lamb. This is thought to result from a change in the crystalline state of the lipids of the sacroplasmic reticulum. If they begin to solidity, membrane integrity can be lost and there will be a sudden increase in the calcium content of the tissue. This results in a rapid contraction that is difficult to relax and causes the resulting meat to be tough. If rigor has occurred before excessive cooling, cold shortening will be avoided.