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Muscle proteins are an important part of the human diet and are found in meat and meat products. They are also the most abundant protein in the human body, comprising about 20% of muscle weight. The two main types of muscle protein are myofibrillar and sarcoplasmic, with the former being responsible for muscle contraction and relaxation, and the latter including haemoglobin, which carries oxygen from the lungs to the muscles. Stromal proteins are also found in muscle tissue and include collagen, elastin, and reticulin. Muscle protein synthesis (MPS) is stimulated by exercise and protein ingestion, particularly when protein is consumed before or after resistance exercise.
| Characteristics | Values |
|---|---|
| Muscle protein composition in the human body | About 40% of the body weight of a healthy human adult weighing about 70 kilograms is muscle, which is composed of about 20% muscle protein. |
| Muscle protein weight in the human body | The human body contains about 5 to 6 kilograms of muscle protein. |
| Muscle protein types | Myofibrillar, regulatory, sarcoplasmic and stromal proteins. |
| Myofibrillar protein composition | Actin, myosin and several other proteins. Actin and myosin are the most abundant proteins in muscle and directly impact its ability to contract and relax. |
| Myosin composition | Myosin contains many amino acids with positively and negatively charged side chains, forming 18% and 16% of the total number of amino acids, respectively. |
| Sarcoplasmic protein composition | Hemoglobin, myoglobin pigments, and a wide variety of enzymes, including myogen, myoalbumin, and x-globulin. |
| Stromal protein composition | Collagen, elastin, and reticulin. |
| Muscle protein function in meat | Muscle proteins contribute to the texture, appearance, mouthfeel, juiciness, and physical stability of meat and meat products. |
| Muscle protein function in the body | Muscle proteins provide motive power to animals. |
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What You'll Learn
Muscle proteins are the most abundant protein in the human body
Muscle proteins are the most abundant type of protein in the human body. They are the basic material of tissue structure and the most important component of striated skeletal muscle. Muscle fibres are made up of myofibrils, which are composed of proteins including actin and myosin. These two proteins are the most abundant in muscles and are directly involved in the ability of muscles to contract and relax. Myosin constitutes as much as 35% of the total protein. Actin filaments are linked to each other lengthwise by fine threads called S filaments. During contraction, the S filaments shorten, causing the actin filaments to slide towards each other and past the myosin filaments, resulting in muscle shortening.
Muscle proteins can be categorised into four groups based on their solubility at varying salt concentrations: myofibrillar, regulatory, sarcoplasmic, and stromal. Myofibrillar proteins, which include actin and myosin, make up 50–60% of muscle proteins. Sarcoplasmic proteins include haemoglobin and myoglobin pigments, as well as enzymes such as myogen, myoalbumin, and x-globulin. Haemoglobin carries oxygen from the lungs to the tissues, including muscles, while myoglobin stores the oxygen transported by haemoglobin until it is utilised in metabolism. Stromal proteins, which make up 10–20% of muscle proteins, include collagen, elastin (a key extracellular matrix protein), and reticulin (similar to collagen).
The total amount of muscle proteins in a healthy 70-kilogram human adult exceeds that of any other protein. Muscle makes up about 40% of body weight, and about 20% of muscle is protein. This means that the human body contains approximately 5 to 6 kilograms of muscle protein.
Muscle proteins are an important part of the human diet as they have high digestibility and bioavailability compared to plant proteins. They contain all the essential amino acids, closely resembling the composition profile of the human body, making them highly nutritious. Muscle proteins also play a crucial role in meat production, contributing to the texture, appearance, mouthfeel, juiciness, and physical stability of meat and meat products.
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Myofibrillar, regulatory, sarcoplasmic and stromal proteins
Muscle proteins are the basic material of tissue structure and are the most important component of striated skeletal muscle. Muscle fibres are composed of myofibrils, which are in turn composed of myofibrillar proteins. Myofibrillar proteins include actin and myosin, which are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and relax. Myosin constitutes as much as 35% of the total protein. Myofibrillar proteins make up 50-60% of skeletal muscle proteins.
Regulatory proteins are accessory proteins that interact with RNA polymerase and affect its role in transcription. Troponin and tropomyosin are regulatory proteins that form a complex and regulate muscle contraction. They are also responsible for transducing (converting to an electrical form) the effect of calcium on contractile protein activation and for inhibiting actin and myosin interaction when calcium is absent. Regulatory proteins make up part of the myofibrillar proteins.
Sarcoplasmic proteins include haemoglobin and myoglobin pigments and a wide variety of enzymes. Haemoglobin carries oxygen from the lungs to the tissues, including muscle. Myoglobin is present in muscle and stores the oxygen transported by haemoglobin until it is used in metabolism. Sarcoplasmic proteins make up 30% of skeletal muscle proteins.
Stromal proteins make up the connective tissue that provides mechanical support and protection to the muscle. Stromal proteins include collagen, elastin, and reticulin. Collagen makes up 90% of connective tissue. Stromal proteins make up 10-20% of skeletal muscle proteins.
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Muscle contraction and relaxation
Muscle proteins are the basic material of tissue structure and are the most important component of striated skeletal muscle. Muscle fibres are composed of myofibrils, which are proteins that include actin and myosin. Actin and myosin are the most abundant proteins in muscle and are directly involved in the ability of muscles to contract and relax.
For the thin filaments to continue sliding past the thick filaments, myosin heads must pull actin at the binding sites, detach, and then re-bind to new binding sites. This process requires ATP, which is initially available in small amounts in resting muscles. As contraction starts, this ATP is quickly used up, and more is generated from creatine phosphate for about 15 seconds. As this ATP is depleted, muscles switch to glycolysis, an anaerobic process that breaks down glucose to produce ATP.
Muscle contraction stops when signalling from the motor neuron ends, which causes repolarisation of the sarcolemma and T-tubules, closing the calcium channels in the SR. The Ca++ ions are then pumped back into the SR, causing tropomyosin to re-cover the binding sites on the actin strands, preventing actin from binding to myosin.
Relaxing skeletal muscles involves the motor neuron stopping the release of its chemical signal, acetylcholine (ACh), into the synapse at the NMJ. This results in the muscle fibre repolarising and closing the gates in the SR where Ca++ was being released. ATP-driven pumps then move Ca++ out of the sarcoplasm back into the SR, causing the "reshielding" of the actin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fibre loses its tension and relaxes.
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Muscle proteins in food processing
Muscle proteins are a valuable part of the human diet, constituting about 40% of the body weight of a healthy human adult weighing about 70 kilograms. They are highly digestible and bioavailable, and contain all the essential amino acids. They are also a good source of iron.
In food processing, muscle proteins are used to enhance the nutritional, textural, and sensory qualities of food products. They are particularly important in meat and meat products, where they provide texture, colour, and nutritive value. Myofibrillar proteins, which are soluble in salt solutions, are the most abundant protein fraction in meat and are responsible for many of the functional characteristics of fresh and processed muscle foods. Myosin and actin, the two most abundant constituents in the myofibril, account for more than 70% of total myofibrillar protein. Myosin is a contractile protein that combines easily with actin to enable muscle contraction and relaxation.
The functionality of muscle proteins is sensitive to various processing factors, including temperature, heating rates, pH, salts, ionic strength, oxidizing agents, and non-meat ingredients. For example, protein denaturation induced by oxidative reaction can increase damage to the three-dimensional structure of meat, leading to a decline in texture properties. Similarly, freezing may decrease the texture properties of meat. Therefore, a clear understanding of the structure-functionality relationship and the effects of various meat processing conditions on protein structure and interaction is essential for the maximum utility of muscle proteins in food processing.
In addition to meat and meat products, muscle proteins are also used in plant-based meat alternatives and as functional food ingredients. For example, fish proteins derived from seafood processing by-products can be hydrolyzed enzymatically to recover protein, and muscle proteins such as fish surimi, collagen, plasma protein, and other meat by-product proteins have been utilized as functional food ingredients.
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Muscle protein synthesis (MPS)
MPS is influenced by various factors, including exercise, nutrition, and biological variables. Resistance or endurance exercises can increase MPS, leading to muscle hypertrophy and improved muscle function. However, the duration of MPS sensitisation may differ between exercise modes. Endurance exercises like running or cycling can also lead to increased MPS, but without significant changes in muscle mass.
Nutrition plays a crucial role in MPS as well. While increased protein intake can trigger MPS, it is only effective up to a certain point. The body can only utilise a limited amount of essential amino acids, and excess intake may be excreted by the liver. Therefore, a balanced diet that includes high-quality protein and habitual exercise is recommended for optimising MPS.
The measurement of MPS involves various methods, with the precursor-product method being the most common. This method uses stable isotope-labelled amino acids to trace the incorporation of free amino acids into newly synthesised muscle proteins over a few hours after an exercise or nutrition stimulus. MPS rates are typically higher in healthy, recreationally active individuals, with skeletal muscle proteins exhibiting turnover rates of about 1.2% per day.
MPS is an essential process in muscle growth and maintenance, and understanding its dynamics can help optimise training programmes and nutritional interventions for individuals seeking to improve their muscle health and performance.
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Frequently asked questions
Muscle proteins are fibrous proteins embedded within connective tissue. They are the major constituents of muscle, comprising approximately 20% of total muscle weight.
Actin and myosin are the most abundant proteins in muscle. They are directly involved in the ability of muscles to contract and relax. Other examples include troponin-tropomyosin, myoglobin, and collagen.
Muscle proteins are essential for muscle function, providing motive power to the body. They also play a crucial role in maintaining muscle mass and promoting muscle growth through muscle protein synthesis (MPS). Additionally, muscle proteins are a valuable part of the human diet, as they have high digestibility and contain all the essential amino acids.
