Proteins and Their Importance for Nutrition PDF

11/13/2024 Proteins in your life The versatility of the proteins in our body is impressive. They ensure that you...

11/13/2024 Proteins in your life The versatility of the proteins in our body is impressive. They ensure that your muscles contract, that your blood coagulates and that your eyes can see. They keep you alive and healthy by enabling chemical reactions and warding off infections. Without them, your bones, Proteins and Their Importance for Nutrition skin and hair would have no structure. as structural materials The building blocks of muscles, blood, and skin as enzymes Some proteins act as enzymes. as regulators of fluid balance Proteins help to maintain the body’s fluid balance. as acid-base regulators Proteins also help to maintain the balance between acids and bases Assoc. Prof. Dr. Şebnem Şimşek within the body fluids. as transporters Some proteins move about in the body fluids, carrying nutrients and other molecules. as antibodies Proteins also defend the body against disease. 2024 as a source of energy and glucose 1. Definition 2. Structure of Proteins Chemically, proteins contain the same atoms as carbohydrates and lipids — carbon (C), hydrogen (H) and oxygen (O) - as well as nitrogen atoms (N). A protein consists of about 20 different amino acids, each with a different side group. The simplest amino acid, glycine, has a hydrogen atom as a side group. A slightly more All amino acids have the same basic structure — a central carbon atom (C) to which a hydrogen atom (H), an amino group (NH2) and an acid group (COOH) are bonded. Another complex amino acid, alanine, has an additional carbon with three hydrogen atoms. Other amino acids have even more complex side groups. So although all amino acids atom or a group of atoms is bonded to the central carbon atom at the fourth bond, which is have a common structure, they differ in size, shape, electrical charge and other referred to as a side group or side chain. characteristics due to the differences in these side groups. More than half of the amino acids are nonessential, which means that the body can synthesize them itself. The body can produce all of them if it receives nitrogen to form the amino group and fragments from carbohydrates or fat to form the rest of the structure. There are nine amino acids that the human body either cannot synthesize or cannot synthesize in sufficient quantities to meet its needs. These nine amino acids must be obtained from food; they are essential. 1  11/13/2024 Cells combine amino acids in a variety of sequences to form thousands of different proteins. A peptide Essential amino acids Nonessential amino acids bond connects each amino acid to the next. Two amino acids joined together form a dipeptide. Through another such reaction, a third amino acid can be added to the chain to form a tripeptide. When more Histidine Alanine amino acids are added to the chain, a polypeptide is formed. Most proteins are a few dozen to several Isoleucine Arginine hundred amino acids long. Leucine Asparagine Lysine Aspartic acid Methionine Cysteine Phenylalanine Glutamic acid Threonine Glutamine Tyrptophan Glycine Valine Proline Serine Tyrosine 3. Protein Denaturation Conjugated Proteins Large protein molecules are fragile. Their conformations depend on their conditions. The globular proteins are sensitive to changes in chemical and physical conditions. Conjugated proteins are proteins that are formed by combining amino acids as well as compounds that are called prosthetic groups. When proteins are subjected to heat, acid, or other conditions that disturb their stability, they undergo denaturation—that is, they uncoil and lose their shapes and, consequently, Nucleoproteins: contain nucleic acid, example DNA, RNA loose their ability to function. In brief protein denaturation affects the solubility of protein under certain conditions, and changes in chemical, physical and biological Lipoproteins: contain lipids, example LDL, HDL properties. Heat, acid, alkali, other chemical and physical effects can make significant changes in the structure of the proteins, their biological activities are destroyed, their Chromoproteins: contain a color-giving metal, example hemoglobin (blood) solubility decreases and their viscosity increases. At a certain point, denaturation is irreversible. Well-known examples of denaturation are Glycoproteins: contain carbohydrates, example egg white the hardening of an egg when it is boiled, the curdling of milk when acid is added and the stiffening of egg whites when they are whipped. During denaturation, the protein Phosphoproteins: contain phosphate, example milk protein casein structure is damaged, but the nutritional value is retained. This is because the primary and secondary structure of the protein is not affected by denaturation. 2  11/13/2024 4. Digestion and Absorption of Proteins In the mouth: Proteins are crushed and moistened in the mouth. In the stomach: The actual action begins in the stomach. The most important event in the stomach is the partial breakdown (hydrolysis) of the proteins. The hydrochloric acid unravels (denatures) the tangled strands of the individual proteins so that the digestive enzymes can attack the peptide bonds. Hydrochloric acid also converts the inactive form of the enzyme pepsinogen into its active form, pepsin.* Pepsin breaks down proteins — large polypeptides — into smaller polypeptides and some amino acids. In the small intestine: When polypeptides enter the small intestine, they are further cleaved by several pancreatic and intestinal proteases (such as trypsin and chymotripsin) into short peptide chains, tripeptides, dipeptides and amino acids.** Peptidase enzymes on the membrane surfaces of intestinal cells then cleave most dipeptides and tripeptides into individual amino acids. Only a few peptides escape digestion and enter the blood intact. The knowledge that protein is broken down into amino acids before it is absorbed, i.e. before it enters the bloodstream through digestion, is sufficient to explain certain incorrect approaches. Proteins are used directly for protein synthesis in the body, and a diet containing sufficient carbohydrates and fats prevents the use of proteins for energy. Proteins, like carbohydrates, provide 4 kcal/g of energy. 5. Protein Synthesis The protein molecule is much more complex than the carbohydrate and fat molecule. If a person Essential amino acids could walk along a carbohydrate molecule such as starch, the first step would be glucose. The next step would also be a glucose, followed by a glucose and another glucose. However, if you were to The function of protein in food is to supply protein directly to the human body. The body walk along a polypeptide chain, each stepping stone would be one of 20 different amino acids. The utilises the amino acids in the protein structure of food to produce its own protein. Protein first stepping stone might be the amino acid methionine. The second could be an alanine. The third synthesis depends on a diet that contains sufficient protein and all essential amino acids. could be a glycine, the fourth a tryptophan and so on. On another polypeptide pathway, a person Some amino acids can be synthesized by the body itself, while others cannot be synthesized by might step on a phenylalanine, then a valine and then a glutamine. In other words, the amino acid the body, either because the necessary enzyme gene is missing or because synthesis is very sequences in proteins vary. slow. They must therefore be supplied to the body with proteins from food. These amino acids The amino acids can behave much like the letters in an alphabet. If you only had the letter G, you are known as essential amino acids. Arginine, for example, is only synthesized very slowly, could only write a sequence of Gs:G–G–G–G–G–G–. But with 20 different letters, you can write meaning that the body's requirements cannot be fully met and must be obtained from food. novels. Similarly, the 20 amino acids can be linked together in a variety of sequences — even more Cystine and tyrosine are not normally essential as they can be synthesized from methionine and than are possible for letters in a word or words in a sentence. The variety of possible sequences phenylalanine. However, if these amino acids are not ingested in sufficient quantities with food, for polypeptide chains is therefore enormous, between 10,000 and 50,000 parts of the human cystine and tyrosine cannot be synthesized. body. Only about 1000 of these have been identified in the body. 3  11/13/2024 In order to produce proteins, a cell must have all the required amino acids available at the same time. The liver can produce any non-essential amino acid that is in short supply so that the cells can continue to link amino acids to form protein strands. However, if an essential amino acid is Some limiting amino acids of foods missing, a cell must break down its own proteins to obtain it. To prevent the breakdown of proteins in the body, dietary protein must therefore provide at least the nine essential amino acids as well as enough nitrogenous amino groups and energy to synthesize the non-essential amino acids. If the diet provides too little of an essential amino acid, protein synthesis is Animal sources Limiting amino Plant sources Limiting amino restricted. The body only makes whole proteins; if one amino acid is missing, the others cannot acid acid make “partial” protein. This situation leads to the 'all-or-nothing rule' in protein synthesis. An Milk - Wheat Lisin essential amino acid that is supplied in a smaller quantity than is required to support protein Egg - Corn Tryptophan synthesis is called a limiting amino acid. Example: If we think of protein synthesis like the sentence «Lock the door» and 'If we want to Meat - Legumes Methionine write this sentence 50 times, we need 50 of the letters l, c, k, t, h, e, d, r and 150 of the letter Cheese Methionine Soybean Methionine o. If we have 40 letters l, the sentence can be written 40 times even if all the other letters are Gelatine Tyrptophan Hazelnut Methionine complete. The letter «y» becomes your limiting letter. In protein synthesis, the smaller amount of the amino acid that limits synthesis is the limiting amino acid. In general, lysine, methionine, threonine and tryptophan are the limiting amino acids. The body cannot store any other amino acids until the limiting amino acids are replenished through the diet. These amino acids are deaminated. During deamination, the nitrogen-containing amino groups are removed by enzymes and converted into urea and excreted. The carbon remaining in the amino acid structure is used for glucose or fat synthesis or metabolised to generate energy. High-quality protein contains all the essential amino acids in relatively the same amounts and proportions that human beings require; it may or may not contain all the 6. Protein Quality nonessential amino acids. Proteins that are low in an essential amino acid cannot, by themselves, support protein synthesis. In general, animal foods (meat, seafood, poultry, The quality of proteins in food is of great importance for nutritional recommendations. eggs and milk and dairy products) provide high-quality proteins, with gelatine being an Nutritional disorders, especially those caused by protein deficiency, are quite common. Poor exception. Gelatine does not contain tryptophan and, as the only protein in the diet, quality protein does not provide the necessary essential amino acids, which leads to cannot support growth and health. Plant proteins (vegetables, nuts, seeds, grains and developmental delays, especially in children. pulses) have a more diverse amino acid pattern and are usually one or more essential The quality of a protein in the diet can be discussed in terms of the amount of protein, the amino acids short. Some plant proteins are notoriously inferior (e.g. maize protein). number of essential amino acids and the rate of digestion and absorption. As all these characteristics determine protein synthesis in the body, foods are categorised according to the The biological value (BV) is a measure of the proportion of absorbed protein from a composition of the amino acids they contain. food that is incorporated into the proteins of the organism. It summarizes how easily the degraded protein can be used for protein synthesis in the organism's cells. The biological value of a protein goes beyond its amino acid composition and digestibility. The best example is egg protein. It is almost 100% converted into body protein. 4  11/13/2024 Biological value of some food proteins A normal diet contains a mixture of proteins. In this case, it does not matter whether the proteins are of good or poor quality. What matters is the total daily amount of proteins from Foods BV different sources that contain all the essential amino acids in the quantity required by the human Egg protein 100 body. If you eat wheat, for example, this diet is limiting for protein synthesis due to the amino acid lysine. It is supplemented by consuming lysine-rich milk on the same day. Another example is Milk protein 92 eating fish and rice on the same day. In a good nutritional plan, these protein sources interact Fish 92 with each other in terms of their quality. Cheese 85 Soybean 85 Rice 1 Bovine meat 78 Casein 72 Potato 69 Wheat flour 58 To summarise, the diet should contain proteins so that the body can utilise them with maximum Egg (%36) + potato (%64) 136 efficiency; Milk (%75) + wheat flour (%25) 125 -contain adequate amounts of amino acids, - be easily digestible, Egg (%76) + milk (%24) 119 - provide sufficient energy (so that the amino acids are not utilised as a source of energy) Milk (%51) + potato (+49) 114 and Milk (%78) + potato (%22) 114 -contain sufficient vitamins and minerals. Protein and Amino Acid Supplements 7. Recommendations for protein intake In recent years, protein and amino acid supplements have been sold with many health claims and are There are three types of recommendations for protein intake for healthy people. used by consumers for many purposes, e.g. against depression, herpes viruses, weight loss and muscle building. There is no magical substance or supplement that can provide solutions to health problems in 1. As a percentage of total energy: in a way that ensures that 10-15% of total daily energy this way. They can also be harmful to the body. Because the body builds muscle protein from amino comes from protein acids, many athletes take protein powders in the false hope of stimulating muscle growth. Muscle work 2. Per kg of ideal body weight: 0.8 g / kg / day, builds muscle, protein supplements do not, and most athletes do not need them. It is certainly 3. As a single daily value: 50 g per day. important to consume enough protein to support protein synthesis in the muscles, but consuming “more than enough” protein does not promote muscle growth or muscle function. Research suggests that protein intake should not be more than double this. The Maasai Protein powders can provide the body with amino acids, but natural sources of protein — lean meat, tribe living in Africa consumes 300 g of protein per day and is considered healthy and has a milk, eggs and pulses — provide all these amino acids and more. Normal and healthy people do not need good physique. However, people living in developing countries also consume 45 g of protein protein supplements. per day and are extremely healthy. These results indicate that the body can adapt to a wide range of protein intake. However, excessive protein intake is harmful and causes bone Amino acid supplements are not necessary either. Impure amino acids are not found in nature and demineralisation. It certainly damages kidney function through the excretion of excess provide no benefit to the body and can even be harmful. The body is not designed to process the high nitrogen. concentrations and unusual combinations of amino acids contained in supplements. Large doses of amino acids cause diarrhea. An excess of one amino acid can create such a demand for a carrier that it limits the absorption of another amino acid, creating the possibility of a deficiency. The amino acids that win the competition are taken in excess, creating the possibility of toxicity. Anyone considering taking amino acid supplements should be careful not to exceed the amounts normally found in foods. 5