Dominic Stone
Muscles are made 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. Muscle proteins are made up of three specific groups of proteins: myofibrillar proteins, sarcoplasmic proteins, and stromal or connective tissue proteins. The most abundant proteins in muscle are actin and myosin, which are responsible for the ability of muscles to contract and relax. These proteins are built into cells, along with fats, DNA, and RNA, and when assembled in a particular way, these cells can be muscle cells.
| Characteristics | Values |
|---|---|
| Are muscles made of protein? | Yes, muscles are made of proteins called actin and myosin. |
| What percentage of muscle is protein? | About 20% of muscle is protein. |
| What are actin and myosin? | Actin and myosin are the proteins that allow muscles to contract and relax. |
| What happens when you lift weights? | Lifting weights triggers a signal for the muscle cell to replicate and grow, which causes the muscle to build more myosin. |
| What is myosin? | Myosin is a contractile protein that occurs in muscle and blood platelets. |
| What is actin? | Actin is another contractile protein that works with myosin to allow muscles to contract and relax. |
| What are some other proteins found in muscle? | Troponin, myoglobin, collagen, elastin, and reticulin. |
| How do proteins build muscle? | Proteins are the building blocks for all life and can be used to build muscle cells, which then bind together to form muscles. |
| What is the role of diet and exercise? | Diet and exercise play a crucial role in building and maintaining muscle mass. A moderate amount of high-quality protein with each meal and habitual exercise are recommended. |
| What happens to muscle with age? | As people age, the muscle unit may contain fat, leading to age-related muscle loss and weakness. |
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What You'll Learn
Skeletal muscle proteins
Muscle proteins are indeed the basic material of tissue structure and are the most important component of striated skeletal muscle. Skeletal muscle proteins are made up of three specific groups of proteins: myofibrillar proteins, sarcoplasmic proteins, and stromal or connective tissue proteins.
Myofibrillar proteins, including actin and myosin, make up between 50% and 60% of the total skeletal muscle protein content. These proteins are responsible for muscle contraction and relaxation. Myosin constitutes as much as 35% of the total protein and is particularly important for muscle function. Actin, on the other hand, works together with myosin to enable muscle contraction and relaxation.
Sarcoplasmic proteins, such as glycolytic enzymes, hemoglobin, and myoglobin, comprise approximately 30-34% of skeletal muscle proteins. Hemoglobin plays a crucial role in carrying oxygen from the lungs to the muscles and other tissues. Myoglobin, on the other hand, stores the oxygen transported by hemoglobin until it is utilized in metabolism. These pigments also contribute to the red color of skeletal muscles.
Stromal proteins, including collagen, elastin, and reticulin, make up the remaining 10-20% of skeletal muscle proteins. These proteins form the connective tissue framework within which the myofibrillar proteins function. Collagen, in particular, is essential for the structure and strength of skeletal muscles.
The understanding of the structure and functionality of these skeletal muscle proteins is crucial for optimizing muscle growth and maintenance. For instance, resistance exercise and protein ingestion work synergistically to stimulate muscle protein synthesis (MPS), leading to muscle growth and improved adaptive responses to exercise. Additionally, a sufficient daily protein intake, ranging from 1.4 to 2.0 g/kg body weight/day, is recommended for building and maintaining skeletal muscle mass.
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Myofibrillar proteins
Muscle fibres are composed of myofibrils, which are made up of myofibrillar proteins. These proteins include actin, myosin, and titin, as well as other proteins that hold them together. The actin-myosin complex is sometimes referred to as actomyosin. Myofibrillar proteins make up 65-75% of the total protein in fish muscles and 50-55% of the total meat protein content. They are the most abundant proteins in muscle and play a crucial role in muscle contraction and relaxation.
The structure of myofibrils consists of thick, thin, and elastic myofilaments that repeat along the length of the myofibril in sections or units of contraction called sarcomeres. These sarcomeres are around 3 micrometres in length and give the muscle cell its striped or striated appearance. The thick myosin and thin actin myofilaments slide along each other during muscle contraction.
The contractile properties of myofibrillar proteins are influenced by factors such as temperature, pH, and ionic strength. For example, extreme pH and high temperatures can lead to protein denaturation, resulting in low solubility. Additionally, the pressure-induced aggregation of myofibrillar proteins involves the dissociation of myosin heavy and light chains, followed by the aggregation of the heavy chains.
The synthesis of myofibrillar proteins can be influenced by exercise and protein ingestion, particularly resistance exercise, which can stimulate muscle protein synthesis (MPS). Additionally, the regulation of myofibrillar protein synthesis in skeletal muscles can be affected by neural and hormonal influences, as well as the origin of myogenic cells and fibre generations.
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Sarcoplasmic proteins
Muscle proteins are a valuable part of the human diet and are composed of all the essential amino acids. They are the most important component of striated skeletal muscle. Muscle tissues have a complex assembly of bundles of fibrous proteins embedded within connective tissue. Lean muscle is composed of about 17%–23% of the protein that can be classified into myofibrillar, sarcoplasmic, and stromal proteins.
The sarcoplasm is the cytoplasm of a muscle cell. It contains unusually large amounts of glycogen, myoglobin, and mitochondria. The calcium ion concentration in sarcoplasm is a critical element of the muscle fiber; it is the means by which muscle contractions take place and are regulated. The sarcoplasm plays a vital role in muscle contraction as an increase in Ca2+ concentration in the sarcoplasm begins the process of filament sliding, which ceases when the Ca2+ concentration decreases.
The functional behaviour of proteins is dictated by their structural and physicochemical characteristics. The physicochemical properties of sarcoplasmic proteins can be altered by modifying the pH and NaCl concentration. For example, sarcoplasmic proteins from jumbo squid exhibited better emulsifying activity at pH 11 in the absence of NaCl.
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Stromal proteins
Muscle proteins can be divided into myofibrillar, regulatory, sarcoplasmic, and stromal proteins. Stromal proteins are collagen, elastin, and reticulin. Collagen is a connective tissue protein that forms bundles of fibrous proteins embedded within connective tissue made from triple helices. Elastin is a key protein of the extracellular matrix, and reticulin is a protein substance similar to collagen. Stromal proteins are also known as connective tissue proteins and make up 10-15% of proteins in meat.
Stromal cells, or mesenchymal stromal cells (MSCs), are differentiating cells found in abundance in the bone marrow and all around the body. They can become connective tissue cells of any organ, such as the uterine mucosa (endometrium), prostate, bone marrow, lymph node, and the ovary. Stromal cells support the function of the parenchymal cells of the organ. They are also an important part of the body's immune response and modulate inflammation through multiple pathways. Stromal cells aid in the differentiation of hematopoietic cells and the formation of necessary blood elements.
The interaction between stromal cells and tumor cells plays a major role in cancer growth and progression. Stromal cells secrete increased levels of proteins and matrix metalloproteinases (MMPs) such as fibroblast activating protein and alpha-smooth muscle actin. They also secrete pro-tumorigenic factors such as vascular endothelial growth factor (VEGF), stromal-derived factor-1 alpha, IL-6, IL-8, tenascin-C, and others. These factors are known to recruit additional tumor and pro-tumorigenic cells. The cross-talk between the host stroma and tumor cells is essential for tumor growth and progression.
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Contraction and protein synthesis
Muscle proteins are made up of three specific groups of proteins: myofibrillar proteins, sarcoplasmic proteins, and stromal or connective tissue proteins. Myofibrillar proteins, such as actin and myosin, are the most abundant proteins in muscle and are directly involved in the ability of muscles to contract and relax. The contractile structure of muscle cells is made of filaments of these proteins, which, when stimulated, pull along each other, causing the muscle cells to contract.
The process of muscle protein synthesis (MPS) is triggered by resistance exercise and protein ingestion, with the greatest effect when protein consumption occurs before or after exercise. MPS is the driving force behind the body's adaptive responses to exercise and nutrition. Skeletal muscle proteins have a daily turnover rate of about 1.2% in healthy, recreationally active individuals, and protein synthesis is increased to meet the secondary metabolic demands of recovery from intense exercise.
The body can only utilize a finite amount of the essential amino acids (EAAs) it receives; anything in excess will be broken down and excreted by the liver. However, the anabolic effect of exercise lasts for at least 24 hours, and likely diminishes with time post-exercise. To build muscle, it is recommended to incorporate habitual exercise in close temporal proximity to protein-containing meals, with a daily protein intake in the range of 1.4–2.0 g protein/kg body weight/day.
The functional behaviour of proteins is dictated by their structural and physicochemical characteristics, as well as processing parameters. The structure of proteins is determined by the exact order of the amino acids in the polypeptide chain. Contractile proteins have been isolated and characterized in non-muscle cells such as neurons and chromaffin cells, which share a common embryological origin, the neural crest.
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Frequently asked questions
Yes, muscles are made of proteins called actin and myosin.
Actin and myosin are filaments of proteins that, when stimulated, pull along each other, causing the muscle cells/fibres to contract.
Troponin, myoglobin, and collagen are some other proteins that are important for muscle function and structure.
Proteins are built into cells, along with fats, DNA, and RNA. When assembled in a particular way, these cells can be muscle cells. These cells then bind together with other cells and ligamentous tissue to form muscles.
When you lift heavy weights, the muscle spends time under tension, which triggers a signal for the muscle cell to replicate and grow. This process is fuelled by the amino acids obtained from dietary protein.
