Back to the Basics: Part 3 Roles of Protein and Protein Metabolism

Whenever the body is growing, repairing, or replacing tissue, proteins are involved in some way. Sometime their role is to facilitate or to regulate, other times it is to become part of a structure. Versatility is a key feature of proteins. Often we think protein as just building blocks but they much more in the human body than most realize. Proteins are used as building blocks, enzymes hormones, regulators of fluid balance, acid base regulators, antibodies, transporters, and also sources of energy and glucose.

From the moment of birth proteins form the building blocks of most body structures. For example, to build a bone or tooth, cells first lay down a matrix of protein collagen and then fill it with crystals of calcium, phosphorus, magnesium, fluoride and other minerals. Collagen also provides the materials of ligaments and tendons and the strengthening glue between cells of the artery walls that enables the arteries to withstand the pressure of the blood surging through them with each heartbeat. Also made of collagen are scars that knit the separated parts of torn tissues together.

Proteins are also needed for replacement. The life span of a skin cell is only about 30 days. As old skin cells shed, new cells made largely of proteins grow from underneath to compensate. Cells in the deeper skin layers synthesize new proteins to go into hair and fingernails. Cells of the GI tract are replaced every three days. Both inside and outside the body continuously deposits proteins into new cells that replace those that have been lost.

Cells can switch their protein machinery on or off in response to the body’s needs. Often hormones do the switching with incredible precision. The body’s many hormones are messenger molecules and some hormones are proteins. Various glands in the body release hormones in response to the change in the internal environment. The blood carries the hormones form these glands to their target tissues where they elicit the appropriate responses to restore normal conditions. Insulin is a perfect example of one such hormone. When blood glucose rises the pancreas releases insulin. Insulin stimulates the transport proteins of the muscles and adipose tissues to pump glucose into the cells faster than it can leak out. After acting on the message, the cells destroy the insulin. As blood glucose falls the pancreas reduces its insulin output. Other proteins that act as hormones are Growth Hormone, Glucagon, Thyroxin, Calcitonin, ADH and so on.

Proteins also help maintain the body’s fluid balance. The body’s fluids are contained within the cell (intracellular) or outside the cell (extracellular). Exctracellular fluids in turn can been found either in spaces between cells (interstitial) or within the blood vessels (intravascular). The fluid within the intravascular spaces of blood vessels is called plasma (essentially blood without its red blood cells). Fluids can flow freely between these compartments but being large, proteins can not. Proteins are trapped primarily within the cells and to a lesser extent in plasma. Wherever proteins are, they attract water. The exchange of materials between the blood and the cells takes place across the capillary walls which allow passage of fluids and a variety of materials but usually not plasma proteins. Still some plasma proteins leak out of the capillaries into the interstitial between the cells. These proteins cannot be reabsorbed back in the plasma and the usually reenter circulation via the lymph system. If plasma proteins enter the interstitial spaces faster than they can be cleared, fluid accumulates and causes swelling. This swelling due to excess fluid is known as edema.

Proteins also help to maintain the balance between acids and bases within the body fluids. Normal body processes continually produce acids and bases which the blood carries to the kidneys and lungs for excretions. The challenge is to do this without upsetting the bloods acid base balance. In an acid solution, the more hydrogen ions the more concentrated the acid. Proteins which have negative charges on their surfaces attract hydrogen ions which have positive charges. By accepting and releasing hydrogen ions proteins maintain the acid-base balance of the blood and body fluids.

As transporters some proteins move about in the bodies fluids carrying nutrients and other molecules. We have already seen this with the protein hemoglobin which carries oxygen form the lungs to the cells. When iron enters an intestinal cell after a meal has been digested and absorbed it is captured by a protein. This protein will not release the iron unless the body needs it. Before leaving the intestinal cell to enter the bloodstream, iron is attached to a carrier protein in the bone marrow or other tissues which will hold the iron until it is needed. When it is needed iron in incorporated into proteins in the red blood cells and muscles that assist in oxygen transport and use.

Within each cell, proteins are constantly being made and broken down a process known as protein turnover. When proteins break down the free amino acids join the general circulation. The amino acids mix with amino acids form dietary protein to form an amino acid pool within the cells and circulating blood. The rate of protein degradation and the amount of protein intake may vary but the pattern of amino acids within the pool remains fairly constant. Regardless of their source any of these amino acids can be used to make body proteins or other nitrogen containing compounds, or they can be stripped of their nitrogen’s and used for energy.

Protein turnover and nitrogen balance go hand in hand. In healthy adults protein synthesis balances with degradation and protein intake from food balances with nitrogen excretion in the urine, feces and sweat. When nitrogen intake equals nitrogen output the person is in nitrogen equilibrium or zero nitrogen balance. The body synthesized more than it degrades and adds protein, nitrogen status becomes positive. Nitrogen status is positive in growing infants and children, pregnant women and people recovering form protein deficiency or illness; their nitrogen intake exceeds their nitrogen output. They are retaining protein in new tissues as they add blood, bone, skin and muscles (THAT’S YOU!) to their bodies. If the body degrades more than it synthesizes and loses protein nitrogen status becomes negative. During these times the body loses nitrogen as it breaks down muscle and other body proteins for energy.

As mentioned, cells can assemble amino acids into the proteins they need to do their work. If a particular nonessential amino acid is not readily available, cells can make it from another amino acid. If an essential amino acid is missing the body may break down some of its proteins to obtain it.

Cells can also use amino acids to make other compounds. For example the amino acid tyrosine is used to make the neurotransmitters norepinephrine and epinephrine which relay nervous system messages throughout the body. Tyrosine can also be made in the pigment melanin which is responsible for brown hair, eye and skin color. It also can be converted into thyroxin which helps to regulate the metabolic rate.

When glucose or fatty acids are limited cells are forces to use amino acids for energy and glucose. The body does not make a specialized storage form of proteins as it does for carbohydrates and fats. Glucose is stored as glycogen in the liver and muscles and fat is stored as triglycerides in adipose tissue, but protein in the body is available only as the working and structural components of the tissues. When the need arises the body dismantles its tissue proteins and uses them for energy. Thus over time energy depravation always incurs wasting of leaning body tissue as well as fat loss. AN adequate supply of carbohydrates and fats spares amino acids from being used for energy and allows them to perform their unique roles.

When amino acids are broken down they first are deaminated or stripped of their nitrogen containing amino groups. Deamination produces ammonia which the cells release into the bloodstream. The liver picks up the ammonia, converts it into urea (a less toxic compound) and returns the urea to the blood. The kidneys filter urea out of the blood thus the amino nitrogen ends up in the urine. The remaining carbon fragments may enter a number of metabolic pathways- for example they may be used to make fat.

If a person eat more protein than the body needs the amino acids are deaminated, the nitrogen excreted and the remaining carbon fragments are converted to fat and stored for alter use. In this way, valuable, expensive protein rich foods can contribute to obesity.

Next up we will look at protein quality and its use in foods and also the helath effects and recommended intakes protein.