Dr Rob Wildman answers questions on protein and different types and turnover in the body.
The name protein is derived from the Greek term proteos, which means “primary” or “to take place first.” Protein was first identified in a laboratory about a century ago at which time scientists described it as a nitrogen-containing part of food that is essential to human life. While protein has long been the darling of the weightlifting and sport community, over the past few years there has been more attention focused on the importance of protein during weight loss, aging and general health.
Much of structure and function of our body is based on proteins. Thus, protein and individual amino acids must function in our body in several ways. For instance, proteins can function as:
- enzymes (regulate chemical reactions)
- structural proteins (yield form to cells and tissue)
- contractile proteins (provide basis for muscle contraction)
- antibodies (help protect us from foreign entities)
- transport proteins (help transport substances in our blood)
- protein hormones (e.g., insulin, glucagon, and growth hormone)
- clotting factors (allow our blood to clot to stop a hemorrhage)
- receptors on cells (allow hormones and neurotransmitters to function)
As mentioned above, peptides and proteins are comprised of links of amino acids. Some smaller proteins will exist as a somewhat straight chain of amino acids; however, most proteins will exist in a complex ¬three-dimensional design. Links of amino acids will contort themselves based upon the specific sequencing of the amino acids.
How links of amino acids contort depends on the interaction between the side groups “R” group on the different amino acids. For instance, some amino acids are attracted to other amino acids in the chain while others are repulsed. This is due to either opposing or similar charges. An analogy would be children holding hands to form a chain. As you can imagine, within a short period of time the chain would bend in a manner specific to the children. Some children would want to be closer (or further away) from others. As amino acid chain bends, twists, and warps about three dimensionally, some amino acids will form bonds with other amino acids. This helps stabilize the three-dimensional design.
It will be the final structure that determines the functional properties of a protein. It is interesting that many proteins are all globbed up, somewhat like crumpled paper or loosely packed yarn. In fact, the names of some proteins, such as hemoglobin and immunoglobins, reflect their globbed (globular) nature. On the contrary, many proteins have more of a filament design, meaning that they are much longer than they are wide. Many of these proteins are like stretched out coils. This is the case with collagen. In fact, numerous collagen proteins come together, side by side, to form a ropelike fibrous super-protein. Further still, it is possible for a protein to demonstrate both globular and filament attributes as is the case with muscle proteins actin and myosin.
During a single day roughly ¼ to 1 pound of our body protein is broken down to amino acids. The lower end of the range would apply more to a smaller woman while the higher end of the range would be more applicable to a larger, more muscular man. Much of the breakdown occurs in the liver and muscle and during the same day an equivalent amount of protein is made (synthesis) in a generally healthy adult. Protein breakdown and production considered together is called “protein turnover” and even though there is this significant quantity of protein turnover we are mostly the same from one day to the next.
Protein is either broken down or manufactured to allow us to adapt to the most current metabolic situation within cells, tissue and in our body in general. These activities allow cells to make or break down enzymes, which are either involved or not involved in different metabolic states such as fasting, feeding, and exercise as well as to make and break down hormones and neuro¬trans¬mitters. General protein turnover helps maintain the integrity of proteins subjected to daily wear and tear as well as allow for the remodeling and adaptation of tissue such as muscle and bone. It is important to remember that our cells are constantly active. This allows us to grow, heal, remodel, and internally defend ourselves on a continual basis.
All proteins in our body have a certain life expectancy. For instance, when insulin and glucagon are released into our blood an individual molecule of either will circulate for about five to ten minutes before they are removed and broken down. Meanwhile, some enzymes within cells may exist only for a few minutes or so before they are replaced or not remade. This can allow cells to shift metabolic gears, so to speak, when going from a fasting to a fed state, resting to exercise state, and so on.
Contractile proteins in muscle (e.g., myosin and actin) may last only a couple of days, while connective tissue proteins, such as collagen, may last weeks to months before they are broken down and replaced. The rate of turnover or remodeling of skeletal muscle contractile proteins and connective tissue proteins helps us understand why the human body seems to get bigger and stronger in just a couple of weeks or so when lifting weights regularly. Meanwhile, it seems to take months and years for scar tissue, which is largely connective tissue, to change.
For a generally lean man, roughly 16% of his body weight is protein. Furthermore, about two-thirds will be in muscle and the remaining third will be elsewhere. For instance, roughly 40% of the weight of his skeleton will be protein, largely collagen. Protein is found elsewhere in the body as the muscle in the heart and digestive tract wall, basis of connective tissue that holds organs and blood vessels together, functional protein like transporters, receptors, skin, hair, nails, etc. In essence, protein is the structural and functional basis of the human body. While protein contributes to daily fuel, either directly or indirectly, the role of fuel for our trillions of cells is best served by carbohydrate and fat.
Free amino acids are found in the body because of digestion of food protein liberates amino acids for absorption into the body from the digestive tract. In addition, amino acids are liberated as body proteins are broken down daily. However, free amino acids account for about one percent of the amino acids in our body, while the remaining 99 percent of amino acids are components of body peptides and proteins. Most cells in the body have a small assortment of free amino acids, meaning they are independent and not linked to other amino acids as part of peptides and proteins. In addition, there is a small amount of amino acids circulating in the blood which, although this increases after a protein containing meal and wanes as amino acids enter tissue.
Circulation provides a deliver system for diet derived amino acids to get to all tissue as well as a means for amino acids to be exchanged between tissue such as during fasting and exercise. Free amino acids in cells and in the blood are collectively referred to as the “amino acid pool” and these amino acids are available to make new body protein or amino acid-derived substances (e.g. neurotransmitters, hormones, metabolic factors, etc.).