Protein
Protein is critical for growth and development, including growth of muscle tissue, so it places as an important nutrient. But its role can be overstated if it is elevated above other nutrients. No one nutrient, by itself, ensures proper nutrition. Protein functions optimally only when energy intake from carbohydrate and fat is sufficient.
A primary function of protein is to build and maintain tissues. To increase muscle mass, an athlete must engage in resistance exercise, consume a sufficient amount of energy (kcal), and be in positive nitrogen (protein) balance. Because so much emphasis is put on protein and muscle growth, it can easily be forgotten that protein is also the basis of enzymes and many hormones, and of structural, transport, and immune system proteins. Although it is not the primary function of protein, the amino acids that make up protein can be used to provide energy. In prolonged endurance exercise, amino acids are an important energy source even though carbohydrates and fats supply the majority of the energy.
The basic component of all proteins is the amino acid, a nitrogen-containing compound. Proteins found in food are broken down into amino acids through the processes of digestion and absorption. Once absorbed, the amino acids are transported to the liver, which plays a major role in amino acid metabolism. Body proteins are constantly being manufactured and broken down. Dietary protein and degraded body proteins provide a steady stream of amino acids for protein synthesis.
In a prolonged starvation state the body’s ability to maintain nitrogen balance is compromised and muscle tissue will be sacrificed to ensure survival. Proteins are found in both plant and animal foods. Although they differ in quality (i.e., amount and type of amino acids), in industrialized countries it is easy to consume an adequate amount of protein with sufficient quality.
Proteins are made up of amino acids, which contain carbon, hydrogen, oxygen, and nitrogen. It is the nitrogen that distinguishes them from the composition of carbohydrates, fats, and alcohol, which are made up of only carbon, hydrogen, and oxygen. To understand their functions one must understand the structures of proteins. The basic structural component is an amino acid.
Protein quality is determined based on the amounts and types of amino acids and the extent to which the amino acids are absorbed. Protein quality is a critical issue in human growth and development. Humans must obtain through diet all of the indispensable amino acids, which are found in lower concentrations in plant proteins than in animal proteins. Animal proteins are termed complete proteins because they contain all the indispensable amino acids in the proper amounts and proportions to each other to prevent amino acid deficiencies and to support growth.
In contrast, plant proteins may lack one or more of the indispensable amino acids or the proper concentrations and are termed incomplete proteins. The indispensable amino acids that are of greatest concern are lysine, threonine, and the sulfur-containing amino acids, cysteine and methionine. If the intake of these specific amino acids is limited, then protein deficiencies could occur. It is possible to pair different plant proteins with each other and bring the total concentration of all the indispensable amino acids to an adequate level. This is the concept of complementary proteins or combining two incomplete proteins. When consumed together (i.e., during the same day), the complementary proteins can be nutritionally equal to a complete (animal) protein.
Basic Structure of Polypeptides
Peptide refers to two or more amino acids that are combined. Specifically, dipeptide refers to two amino acids, tripeptide to three amino acids, and polypeptide to four or more amino acids. Most proteins are polypeptides and are made up of many amino acids, often numbering in the hundreds or thousands. Protein and polypeptide are terms that are used interchangeably. Dipeptide and tripeptide are terms that are typically used when discussing digestion and absorption. Polypeptides are synthesized on ribosomes, organelles found in large numbers in the cytoplasm of cells.
The primary structure of the protein is determined at its creation based on information contained in DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). The RNA acts like a blueprint for the type, number, and sequence of amino acids to be included in a particular polypeptide. The differences in amino acids influence the bonding abilities of the polypeptide, which affect the shape of the protein. For example, proteins can be straight, coiled, or folded based on the type, number, and sequence of amino acids in the polypeptide. The primary structure of the polypeptide determines how a protein functions.
Functions of Polypeptides
As explained above, the structure of the protein determines its function. Body proteins are often classified in five major categories: enzymes, hormones, structural proteins, transport proteins, and immune system proteins. Enzymes are polypeptides that are necessary to catalyze reactions. It is the structure of the enzyme, particularly the quaternary structure, which allows the protein-based enzyme to interact with other compounds. The unique structure of each enzyme interacts with its substrate much like a key fits into and opens a specific lock.
Hormones are compounds that act as chemical messengers to regulate metabolic reactions. Many hormones are protein based, although some hormones are made from cholesterol (e.g., steroid hormones). Insulin, glucagon, and human growth hormone are just three examples of the hundreds of hormones made from amino acids. Insulin is a relatively small polypeptide, made up of only 51 amino acids, yet it is one of the body’s most essential hormones. Part of the polypeptide chain folds back on itself (due to its secondary structure) and the two protein chains that make up its quaternary structure are linked by disulfide bonds. Because insulin is small, it can move through the blood quickly and the folded chain and the disulfide bonds give it great stability.
Structural proteins include the proteins of muscle and connective tissue (e.g., actin, myosin, and collagen), as well as proteins found in skin, hair, and nails. Structural proteins can be constructed into long polypeptide strands, similar to a long chain. These strands can be twisted and folded into a wide variety of three dimensional shapes (secondary structure). Elements of the constituent amino acids in the polypeptide chains, such as sulfide groups, may be brought close together by the twisting and folding and may form interconnected bonds (tertiary structure). The secondary and tertiary structures of the polypeptide are responsible for the differences in rigidity and the durability of these polypeptides.
Examples of transport proteins include lipoproteins (lipid carriers) and hemoglobin, which carries oxygen and carbon dioxide in the blood. Without its particular quaternary structure, hemoglobin would not be as efficient. Four polypeptide chains are bonded in such a way that they can work in concert and can change shape slightly when necessary. This structure allows hemoglobin to be flexible and capable of changing its ability to bind and release oxygen. For example, hemoglobin needs a high affinity to bind oxygen in the lungs and carry it throughout the body, but it must reduce its affinity for oxygen so oxygen can be released for use by the tissues.
The immune system is a protein-based system that protects the body from the invasion of foreign particles, including viruses and bacteria. One immune system response is the activation of lymphocytes, cells that produce antibodies. All antibodies are compounds that are made of polypeptide chains (usually four) in the shape of a Y. The antibody fits the virus or bacteria like a key in a lock, aiding in their destruction. The shape of the “key” is due to disulfide bonds and the sequence of the amino acids.
All of the compounds described above are proteins, but none of these compounds are provided directly from proteins found in foods. Enzymes, hormones, and the other protein-based compounds are manufactured in the body from indispensable and dispensable amino acids. To understand how food proteins become body proteins one must know how amino acids found in food are digested, absorbed, transported, and metabolized.
Protein Anabolism
One of the major functions of the liver is protein anabolism. Some amino acids will be incorporated into liver enzymes. Others will be used to make plasma proteins. For example, the liver manufactures albumin, a protein that circulates in the blood and helps to transport nutrients to tissues. Many of the proteins made in the liver are synthesized and released in response to infection or injury. As mentioned earlier, the liver continually monitors the body’s amino acid and protein needs and responds to changing conditions.
In addition to protein synthesis, the liver uses amino acids to manufacture compounds such as creatine. The creatine synthesis process begins in the kidney but is completed in the liver. Of particular interest to athletes is the synthesis and breakdown of skeletal muscle proteins.
Protein Catabolism
Amino acids that are not used for building proteins are catabolized. In other words, excess amino acids are not “stored” for future use in the same way that carbohydrates and fats are stored. Carbohydrate can be stored in liver or muscle as glycogen and fat can be stored in adipocytes (fat cells) and at a later time removed easily from storage and used as energy.
In contrast, the so-called “storage” site for protein is skeletal muscle. Under relatively extreme circumstances, protein can be removed from the skeletal muscle, but the removal of a large amount of amino acids has a very negative effect on the muscle’s ability to function. Amino acids can provide energy. In fact, the source of approximately half of the ATP used by the liver comes from amino acids. The term “oxidation of amino acids” is understood to mean that after the nitrogen is removed, the carbon skeleton of the amino acid is oxidized for energy and the nitrogen goes through the urea cycle in the liver. Similar to carbohydrate, protein yields approximately 4 kcal/g.
Energy is best supplied for exercise by carbohydrate and fat rather than protein. Sufficient caloric intake in the form of carbohydrate and fat is referred to as having a protein-sparing effect. In other words, the carbohydrate and fat provide the energy that the body needs and the protein is “spared” from this function. The protein is available for other important functions that can only be provided by protein. When people consume sufficient energy, all the important protein- related functions can be met and, in fact, some of the protein consumed will be metabolized for energy. Problems can result if caloric intake is too low and some protein must be used to meet energy needs.
Protein Balance and Turnover
The body is in a constant state of protein turnover. In other words, every day the body simultaneously degrades and synthesizes proteins. It is estimated that 1 to 2 percent of the total protein in the body is degraded each day (i.e., proteins are broken down to the amino acids that formed them). The source of most of the degraded protein is skeletal muscle, and these amino acids become part of the amino acid pool. About 80 percent of the amino acids that result from protein degradation are resynthesized into new proteins.
In a starvation state, protein degradation outpaces protein synthesis. Each protein in the body has its own turnover rate. Some turn over very quickly, in a matter of minutes, while others take several months to turn over. Protein metabolism is never static; rather, it is always changing. One way to measure and describe the changes is to determine nitrogen balance. Nitrogen balance is the difference between total nitrogen (protein) intake and total nitrogen loss (via the urine and feces), usually determined over several weeks. When intake is equal to loss, a state of nitrogen balance exists. When intake is greater than loss, a person is in positive nitrogen balance. Conversely, when loss is greater than intake, a state of negative nitrogen balance is present.