Dominic Stone
Protein is an essential macronutrient that plays a key role in the function and structure of our cells, tissues, and organs. It is the building block of our muscles, and eating adequate amounts can help maintain muscle mass and promote muscle growth and repair. The human body contains about 5 to 6 kilograms of muscle protein, which is about 20% of our total muscle mass. This makes muscle proteins the most abundant type of protein in the body. Actin and myosin are the most important myofibrillar proteins for muscle fibre structure and are directly involved in the ability of muscles to contract and relax.
| Characteristics | Values |
|---|---|
| Percentage of muscle in a healthy human adult weighing 70 kg | 40% |
| Percentage of muscle protein in the human body | 20% |
| Weight of muscle protein in a healthy human adult weighing 70 kg | 5-6 kg |
| Myofibrillar proteins | Actin, Myosin, and Z-disc |
| Regulatory proteins | Troponin, Tropomyosin, M-protein, Beta-actin, Gamma-actin, and C-protein |
| Sarcoplasmic proteins | Hemoglobin, Myoglobin pigments, and Enzymes |
| Protein degradation | Requires energy |
| Muscle protein synthesis (MPS) | 1.2% day-1 |
| Protein requirement for muscle growth in older adults (above 65 years) | 1.2-1.59 g per kg of body weight |
| Protein requirement for muscle growth in adults (above 40 years) | 1-1.2 g/kg/bw |
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What You'll Learn
- Muscle proteins include actin and myosin, which are involved in muscle contraction and relaxation
- Myosin is insoluble in water and is coiled at both ends to form a terminal globule
- Muscle protein synthesis (MPS) is triggered by exercise and protein ingestion
- A high-protein diet is only harmful to those with kidney dysfunction
- Sarcopenia is a condition where muscle mass is lost due to aging
Muscle proteins include actin and myosin, which are involved in muscle contraction and relaxation
Muscle proteins are the most important component of striated skeletal muscle. Muscle fibres are composed of myofibrils, which include the proteins actin and myosin, as well as several others. Actin and myosin are the most abundant proteins in muscle and are directly involved in the muscle's ability to contract and relax.
Actin and myosin filaments are oriented parallel to each other and to the long axis of the muscle. The 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, past the myosin filaments, resulting in a shortening of the muscle. This is known as the sliding filament theory, which states that the sliding of actin past myosin generates muscle tension.
Actin is the most abundant protein in most eukaryotic cells and forms 12 to 15 percent of muscle proteins. Myosin constitutes as much as 35% of the total protein volume of skeletal muscles. It is a contractile protein, also found in blood platelets, and is insoluble in water. Myosin consists of an elongated, probably double-stranded, peptide chain, which is coiled at both ends in such a way that a terminal globule is formed.
The structure of actin and myosin filaments is important to understand muscle contraction. The sliding filament theory proposes that the interaction of these proteins produces contractile force.
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Myosin is insoluble in water and is coiled at both ends to form a terminal globule
Muscle fibres are composed of myofibrils, which are made up of proteins including actin and myosin. Myosin is a contractile protein that is insoluble in water. It has a unique structure, with a long, probably double-stranded peptide chain that is coiled at both ends to form a terminal globule. This coiled structure is essential to its function in muscle contraction.
Myosin is a prototype of a molecular motor, a protein that converts chemical energy in the form of adenosine triphosphate (ATP) into mechanical energy, generating force and movement. During muscle contraction, the actin and myosin filaments slide past each other through the activity of ATP, causing a shortening of the muscle. The actin filaments are linked by fine threads called S filaments, which shorten during contraction, allowing the actin filaments to slide towards each other past the myosin filaments.
The structure of myosin plays a crucial role in this process. Its elongated peptide chain is coiled at both ends, forming two terminal globules with a combined molecular weight of approximately 60,000. The coiled structure of myosin allows it to bind with actin, forming cross-bridges between the thick and thin filaments. This binding is essential for muscle contraction, as it provides the necessary force and facilitates the sliding movement of the filaments.
Myosin contains a high proportion of amino acids with positively and negatively charged side chains, which make up 18% and 16% of the total amino acids in myosin, respectively. These amino acids contribute to the unique properties of myosin, including its insolubility in water and its ability to catalyse the hydrolytic cleavage of ATP.
In summary, myosin is a key muscle protein that is insoluble in water and has a distinct coiled structure. This structure enables myosin to interact with actin, converting chemical energy into mechanical energy and driving muscle contraction. The coiled ends of myosin, forming terminal globules, are integral to its function and highlight the intricate design of this protein.
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Muscle protein synthesis (MPS) is triggered by exercise and protein ingestion
Muscle protein synthesis (MPS) is the driving force behind adaptive responses to exercise. It is a widely adopted proxy for gauging the chronic efficacy of acute interventions, such as exercise and nutrition. Exercise triggers complex mechanotransduction and physico-chemical sensory mechanisms, activating receptor and non-receptor-mediated intramuscular signalling. This modulates cellular apparatus, regulating short-term post-translational control of protein turnover and gene expression, as well as long-term changes in cellular metabolic capacity.
MPS occurs at a rapid rate when the body is growing and slows significantly after age 20. In healthy, recreationally active individuals, skeletal muscle proteins display turnover rates of about 1.2% per day and exist in dynamic equilibrium. The body constantly degrades and rebuilds muscle proteins, a process that becomes less frequent with age. The balance shifts towards decreased protein synthesis with age, except in certain disease states, such as endotoxin poisoning, where muscle protein breakdown can be rapid at any age.
Exercise, particularly resistance exercise, and protein ingestion synergistically stimulate MPS when protein consumption occurs before or after the workout. The anabolic effect of exercise lasts at least 24 hours but likely diminishes as time passes post-exercise. Nutrient-driven increases in MPS are also finite, lasting about 1.5 hours. The combination of exercise and nutrition has a more anabolic effect than nutrition alone, even beyond 24 hours after a single exercise session.
The ideal protein intake to maximise MPS varies but is typically around 0.25 g of high-quality protein per kg of body weight or an absolute dose of 20-40 g. These doses should be evenly distributed every 3-4 hours throughout the day. The ingestion of 40 g of whey protein after whole-body resistance exercise resulted in 20% higher MPS rates compared to a 20 g dose. This suggests that the amount of protein required to maximise MPS rates is higher after whole-body exercises that recruit more muscle.
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A high-protein diet is only harmful to those with kidney dysfunction
Muscles are composed of proteins, which are the basic material of tissue structure. About 40% of the body weight of a healthy human adult weighing about 70 kilograms is muscle, which is made up of about 20% muscle protein. The human body contains about 5 to 6 kilograms of muscle protein.
Myofibrillar proteins, actin and myosin, are the most abundant proteins in muscle and are directly involved in the ability of muscles to contract and relax. Actin filaments are linked to each other lengthwise by fine threads called S filaments. During contraction, the S filaments shorten, so the actin filaments slide past the myosin filaments, causing a shortening of the muscle.
Protein ingestion and exercise work in synergy, and protein consumption before or after resistance exercise is ideal for building and maintaining muscle mass. For this, a daily protein intake in the range of 1.4–2.0 g protein/kg body weight/day is sufficient. However, higher protein intakes may have positive effects on body composition in resistance-trained individuals.
High-protein diets are popular for weight loss and type 2 diabetes, but evidence suggests that they may cause worsening renal function in individuals with impaired kidney function. High dietary protein intake can cause intraglomerular hypertension, which may result in kidney hyperfiltration, glomerular injury, and proteinuria. It is possible that long-term high protein intake may lead to de novo chronic kidney disease (CKD).
The quality of dietary protein may also play a role in kidney health. Animal protein has been associated with an increased risk of end-stage kidney disease (ESKD) in several observational studies, including the Singapore Chinese Health Study. Potential mediators of kidney damage from animal protein include dietary acid load, phosphate content, gut microbiome dysbiosis, and resultant inflammation.
In summary, a high-protein diet is only harmful to those with kidney dysfunction, and it may even be beneficial for those looking to lose weight or manage type 2 diabetes. However, due to the potential for kidney damage, it is important to consult a healthcare professional before starting a high-protein diet, especially for those at risk of kidney disease.
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Sarcopenia is a condition where muscle mass is lost due to aging
Muscle fibres are composed of proteins, which are the basic material of tissue structure. 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. The most abundant proteins in muscle are actin and myosin, which are directly involved in the ability of muscles to contract and relax.
Sarcopenia is a condition characterised by the degenerative loss of skeletal muscle mass, quality, and strength. It is a type of muscle atrophy primarily caused by the natural ageing process. Sarcopenia can also be caused by physical inactivity, an unhealthy diet, and chronic diseases such as chronic obstructive pulmonary disease (COPD), kidney disease, diabetes, cancer, and HIV. The rate of muscle loss in sarcopenia is dependent on exercise level, co-morbidities, nutrition, and other factors. The hallmark sign of sarcopenia is the loss of lean muscle mass, which can be difficult to detect due to obesity, changes in fat mass, or edema. The condition commonly affects people over the age of 60, with rates increasing with age.
The degree of sarcopenia is determined by two factors: the initial amount of muscle mass and the rate at which muscle mass declines. Immobility dramatically increases the rate of muscle loss, even in younger people. Other factors that can increase the rate of progression of sarcopenia include decreased nutrient intake, low physical activity, and chronic disease. Researchers have also identified early environmental influences as potential long-term factors, such as low birth weight, which is associated with reduced muscle mass and strength in adulthood.
Treatment for sarcopenia typically includes lifestyle changes, such as progressive resistance-based strength training and healthy eating habits. Exercise is the intervention of choice for sarcopenia, as aging muscles retain the ability to synthesize proteins in response to short-term resistance exercise. Additionally, increasing protein intake through food or supplements can help to slow down the rate of muscle loss. Research suggests that a combination of improved resistance training, nutrition changes, and other therapeutic measures can work together to manage and prevent sarcopenia.
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Frequently asked questions
Muscle is made of muscle fibres, which are composed of proteins including actin and myosin. Actin and myosin are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and relax.
The amount of protein needed to build muscle depends on factors such as age, weight, and level of physical activity. For older people, a daily protein intake of 1.2 to 1.59 grams per kilogram of body weight is recommended to increase lean muscle mass. For those engaging in endurance sports or weight training, increasing protein intake can be beneficial. Eating protein within 30 minutes of exercise is particularly helpful as muscles are more receptive at this time.
High-quality sources of protein include animal foods such as meat, dairy products, and eggs, as well as plant-based sources like soy, quinoa, buckwheat, and Quorn.
