Lecture 4: Proteins PDF

Lecture 4: Proteins Dr.Fatma Naser Ahmed Lecturer of Nutrition, Nutrition and Food Sciences Dept., Faculty of Home Economics, Helwan University.  Introduction Protein makes up approximately 20 percent of the human body and is present in every single cell. The wo...

Lecture 4: Proteins Dr.Fatma Naser Ahmed Lecturer of Nutrition, Nutrition and Food Sciences Dept., Faculty of Home Economics, Helwan University.  Introduction Protein makes up approximately 20 percent of the human body and is present in every single cell. The word protein is a Greek word, meaning “of utmost importance.” Protein is necessary for proper immune system function, digestion, and hair and nail growth, and is involved in numerous other body functions. In fact, it is estimated that more than one hundred thousand different proteins exist within the human body. 2  Definition of proteins  Proteins, simply put, are macromolecules composed of amino acids. Amino acids are commonly called protein’s building blocks. Proteins contain the elements carbon, hydrogen, and oxygen just as carbohydrates and lipids, but proteins are the only macronutrient that contains nitrogen.  Each amino acid consists of a central carbon atom connected to : Side chain Hydrogen Nitrogen -containing amino group Carboxylic acid 3  Protein’s Functions in the Body Proteins are needed in our diets for growth (especially important for children, teens and pregnant women) and to improve immune functions. They also play an important role in making essential hormones and enzymes, in tissue repair, preserving lean muscle mass, and supplying energy in times when carbohydrates are not available. 4  Sources of proteins The main sources of proteins are meats, chicken, eggs, breast milk, beans, ground nuts, lentils, fish, cheese and milk. All animal foods contain more protein than plants and are therefore usually better sources of body building foods. However, even though plant proteins are usually not as good for body-building as animal proteins, they can become more effective nutritionally when both are mixed with each other. 8  Protein Digestion and Absorption When you eat food the body’s digestive system breaks down the protein into the individual amino acids, which are absorbed and used by cells to build other proteins and a few other macromolecules, such as DNA. 9  Protein Digestion and Absorption From the Mouth to the Stomach The first step in protein digestion involves chewing. The teeth begin the mechanical breakdown of the large pieces of proteins into smaller pieces that can be swallowed. The stomach releases gastric juices containing hydrochloric acid (HCL) and the enzyme, pepsin, which initiate the breakdown of the protein. Pepsin, which is secreted by the cells that line the stomach, dismantles the protein chains into smaller and smaller fragments. The powerful mechanical stomach contractions churn the partially digested protein into a more uniform mixture called chyme. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach and food remains in the stomach longer, making you feel full longer. 11  Protein Digestion and Absorption From the Stomach to the Small Intestine The stomach empties the chyme into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). Once the amino acids are in the blood, they are transported to the liver because amino acids contain nitrogen, so further catabolism of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. 12 The Role of Proteins in Foods: Cooking and Denaturation  When a cake is baked, the proteins are denatured. Denaturation: refers to the physical changes that take place in a protein exposed to abnormal conditions in the environment. Heat, acid, high salt concentrations, alcohol, and mechanical agitation can cause proteins to denature.  During cooking the applied heat causes proteins to vibrate. This destroys the weak bonds holding proteins in their complex shape (though this does not happen to the stronger peptide bonds). The unraveled protein strands then stick together, forming a network. 13 Denaturation of proteins Examples on denaturation of proteins  Examples on important peptides Glutathione What Is Glutathione? Your cells contain glutathione, which is a substance made from three amino acids: cysteine, glutamate, and glycine. Glutathione acts as an important antioxidant in your body. That means it helps combat free radicals, molecules that can damage your body's cells. Glutathione plays a role in many chemical reactions in your body. It also helps detoxify chemicals, including some that your body creates naturally, as well as pollutants and drugs.  Examples on important peptides Vasopressin (antidiuretic hormone): This peptide hormone is secreted in the hypothalamus—a small area located in the center of your brain. It is responsible for regulating the amount of water present in the fluid space around cells (extracellular fluid). It does this by causing the kidneys to absorb water. Vasopressin is inhibited by alcohol consumption, which causes people to urinate more than usual. In high quantities. Oxytocin: This peptide hormone is produced by the pituitary gland (located in the brain) and is made up of nine amino acids. It causes the uterus to contract during childbirth. Oxytocin also plays a pivotal role in the milk ejection reflex ("let down") during breastfeeding. Oxytocin is sometimes known as the "cuddle hormone" or the "love hormone" because it is released when people snuggle up together or bond socially.