The role and importance of proteins
Protein (albumin) is digested into smaller molecular components , amino acids and peptides, which give protein it’s nutritional impact. Individuals may consume adequate amounts of total protein BUT still be deficient in specific amino acids, because of the quality of the protein. Therefore the PROTEIN QUALITY or AMINO ACID COMPOSITION is more important, than the total protein intake. The total protein intake does not account for the TYPE OF PROTEIN and how it relates to food-allergy conditions and the immune system. Therefore it is necessary to understand the metabolic role of protein!.
MOLECULAR STRUCTURE OF PROTEINS
Proteins are normal constituents of every living cell and therefore indispensable. They are essential components of the nucleus and cell protoplasm and found in most animal tissue fluids. The building blocks of proteins, amino acids, vary in structure and size, but all have an amino group (NH2) and a carboxyl group (COOH) linked to the same terminal carbon of the molecule. The side chains consist of a combination of carbon, hydrogen, sulfur, nitrogen and/or oxygen – the configuration of these differentiates one amino acid from another. The properties of the different proteins vary, depending on the amino acid makeup and on the order and structural arrangement. There are 22 different amino acids. In the protein molecule, hundreds of amino acids are joined together by peptide bonds; when the amino group of one amino acid is linked to the carboxyl group of the other, one molecule of water is being split off.
The branched-chain amino acids are isoleucine, leucine and valine, which are responsible for muscle structure. Tyrosine, phenylalanine and tryptophan are the aromatic amino acids (side chain with a ring-shaped formation) and are necessary for the production of the neurotransmitters serotonin and melatonin. Serotonin is important for healthy and restful sleep, elevating mood and modulate metabolism, appetite and sexuality. Melatonin regulates the interior body clock (circadian rhythm) and is a powerful antioxidant- it protects mitochondrial and nuclear DNA. Lysine is important in absorbing and conserving calcium and in the formation of collagen. Too little lysine in the diet can lead to kidney stones and other health problems: dizziness, fatigue, nausea, loss of appetite, agitation, anemia, slow growth, bloodshot eyes, reproductive disorders.
Proteins cannot cross semipermeable membranes. Extreme heat will coagulate them (come together) them and heavy metal salts will precipitate (separate) them out of a solution. Proteins form true salts with cations, and combine with metals in nonionic complexes. Proteins also form more complex compounds: glucoproteins (protein-carbohydrate complexes e.g. mucin from mucous glands of gastrointestinal tract); lipoproteins (protein-fatty compounds e.g. LDL cholesterol); nucleoproteins (protein-nucleic acids e.g. chromatin materials in nuclear cell); hemoglobin ( iron pyrrole group (heme), conjugated with globin protein); enzymes (protein “carrier” molecule).
ABSORPTION OF PROTEINS
Amino acids obtained from dietary protein enter the amino acid pool, together with the amino acids catabolized and anabolized from the tissue protein (called protein turnover). At all times, a balance exists between dietary protein, plasma protein and tissue protein: during anabolism, the dietary amino acids contribute to the synthesis of body protein, while during starvation, the plasma and tissue proteins are catabolized and released to the amino acid pool. This rate of protein turnover varies – higher during periods of rapid growth. The daily turnover of protein is much greater than the dietary intake, which testifies to the importance of re-utilization of amino acids in the body. This recycling of amino acids are not entirely efficient – some amino acids are lost during cellular metabolism and have to be replaced from dietary sources.
Proteins must be digested and broken down into amino acids for absorption. They are broken down by the hydrolytic enzymes of the gastrointestinal tract. Occasionally some polypeptides may pass the mucosal barrier to be absorbed directly into the bloodstream; these undigested proteins are responsible for the development of allergy to specific food proteins. The absorption of the individual amino acids is a selective process, where individual amino acids compete for a place in the transport system. There are wide differences in the rate of absorption of the various amino acids.
The stomach produces enzymes (proteases) to break down the protein. Protein digestion occurs in the first part of the small intestine (duodenum). These digestive enzymes are produced by the liver and pancreas (primary organ of digestion). The cells in the pancreas produce enzymes for breaking down carbohydrates (amylase), fats (lipase) and proteins (proteases).
The production of digestive enzymes depends on many micronutrients, especially vitamin B6. Undigested food in the small intestine, encourages bad bacteria to flourish with symptoms like bloating, flatulence and abdominal pain. A broad-spectrum, digestive enzyme supplement, with each meal, will correct this problem. When food protein is ingested, it stimulates the digestive tract to secrete gastro-intestinal mucoproteins and digestive enzymes, which dilute the food amino mixture in the intestinal lumen sufficiently to be available for absorption. When large quantities of an incomplete amino acid mixture are presented to the gastrointestinal tract, they are not well utilized in the liver for protein synthesis. After absorption, most of the amino acids are carried to the liver via the portal vein (some pass into the lymphatics). Some of the amino acids from the portal circulation are retained by the liver for it’s functioning; the rest enter the blood to be circulated to the various tissues. The liver forms a wide spectrum of human plasma proteins; All tissues utilize the available amino acids for the synthesis of specific cell proteins needed for the formation of new cells or the repair and maintenance of existing ones or for specialized secretions of secretory cells. After the anabolic needs have been met, the excess amino acids are catabolized (used) for energy and heat or converted to carbohydrate and stored as fat (if the energy needs are exceeded). During starvation or shortage of calories, amino acids are utilized as sources of heat and energy, since maintenance of essential body functions has a priority over anabolic activities.