Protein Chapter 17 What is Protein? Protein molecules are very large and complex. Made up of: – Hydrogen, carbon, oxygen ,nitrogen and sometimes other elements – Because of protein large size, protein molecules are often called macromolecule Macromolecule = large molecule containing many atoms. – Nitrogen is a crucial part of protein Nitrogen is the site of the chemical bond that gives proteins their variety and versatility Structure of Protein Protein is made of chains of amino acids. – Amino Acids = type of organic acid. – Organic acids are molecules that contain a carboxyl group, plus the amino acids also contain an amine group. – Amine group = composed of two atoms of hydrogen and one atom of nitrogen. – (Look on page 258, figure 17-1, both carboxyl group and the amine group are attached to a central carbon.) – With four bonds needed and one is left open, this is what makes one amino acid different from another. Peptide Bonds Peptide bonds = bonds between the nitrogen of one amino acid and the carbon of a second amino acid. (example is on page 258, figure 17-3) Through peptide bonds, amino acids chain together, creating a polypeptide. Polypeptide = single protein molecule containing then or more amino acids linked in peptide chains – Peptide bonds hold chains of amino acids together, that are called helixes (ex. page 259, figure 17-4) Protein Structure and Function Protein’s shape determine its function Protein molecules that form rope-like fibers are called fibrous protein ex is collagen and elastin Protein molecules that can be compared to a ball of steel wool are called globular proteins – The shape makes them convenient carriers ex hemoglobin (transports oxygen in the blood) Denaturation of Protein Denaturation = this process changes the shape of a protein molecule without breaking its peptide bonds As the molecule unfolds, some of the protein’s original properties are diminished or lost. This change is unique to protein and because each protein is unique the process varies from one molecule to the next. Denaturation in the first step in the process of coagulation Coagulation = changes a liquid into a soft, semisolid clot or solid mass. (look pg. 260, figure 17-6) – Ex is scrambled eggs. Beating the egg denatures its protein. The protein coagulates as the egg cooks. Other Means of Denaturation Heat is the most common agent in denaturing protein. The temperature is significant, the degree of denaturation increase 600 times for every 10°C. Protein molecules may unfold in reaction to the following: Freezing, pressure, and sound waves Mechanical treatment, such as beating eggs and kneading bread Very high or very low pH. (adding lemon juice for example can sour milk, causing the milk to separate from the liquid – forming curds and whey) Certain metal ions Protein in Eggs Eggs Complex biological system, containing almost every vitamin and mineral you need. All that lacks is vit. C and calcium which is found in the egg shell Look on pg. 262, figure 17-7 for a diagram of an egg – Albumen = egg white – substance makes up 54% of the inner contents of an egg – Chalaza = a twisted, ropelike structure that keeps the egg yolk centered – Egg Yolk – main component of the yolk include the globular protein livetin and both high-and-low-density lipoproteins Storage – Do not exposed eggs to light and temperature changes Eggs lose quality as compounds in the egg white break down, forming water Protein in Meat Muscle tissue, the lean part of meat, is about 15-20% protein – Other components include: water, fat and minerals – Muscle is composed of fibrous proteins called actin and myosin. These proteins form bundles of fiber, which are held together by connective tissue made of collagen and elastin Protein in Fish, Nuts and Legumes Fish – Shorter segmented muscle fibers are layered between thin sheets of connective tissue. This is why some books say fish is cooked when you can “flake with a fork” – Far less connective tissue than meat, it’s also a type that liquefies easily Nuts and Legumes – Cholesterol free, but fat content ranges – Soybeans are excellent source of protein, they’re composed of 40% protein, high in fiber and polyunsaturated fatty acids. Soybeans have also been associated with reducing the risk of coronary heart disease Protein in Cooking Emulsifiers Certain proteins contain amino acids that our polar. One end is attracted to water (forming a hydrogen bond), the other end avoids water and bonds with oil. This is why an egg can emulsify oil and lemon juice Foams Foam = air bubbles incorporated and trapped in a protein film by whipping – Foam begins when a protein-containing liquid (egg whites or cream) are whipped, introducing air and denatures the protein molecules – Fats can be trouble for foams, they keep protein molecules from bonding with each other, making the end product watery Protein in Cooking Continued Gelatin Used to set desserts and thicken meat sauces in animal protein Made by using heat and water to hydrolyze the collagen on the inner layer of hides and bones Gelatin can bind 100 times its weigh in water Gluten Gluten = an elastic substance formed by mixing water with the proteins found in wheat Gluten includes both fibrous and globular proteins, making the reactions between the water and proteins both stretch and springy Gluten is developed as dough is kneaded, denaturing the protein molecules Protein in the Body During digestion, proteins are denatured by hydrochloric acid in the stomach, making the peptide bonds easier for enzymes to break. Most proteins enter the bloodstream as amino acids (the body creates ALL of its proteins from 20 different amino acids) Functions of Body Proteins – Structural protein is needed by every cell in the body – New growth requires a continuous supply of protein to replace and repair cells – Some proteins pick up, deliver and stores nutrients in cells – Proteins called antibodies help you ward off disease Antibodies = very large proteins that weaken or destroy foreign substances in the body. Essential Amino Acids Of the 22 amino acids the body needs to build protein, it manufactures 18 the other 9 needs to be digested. Look on pg 269, figure 17-8 – Complete Protein = a protein that contains all the essential amino acids ex. ground beef – Incomplete protein = lacking one or more essential amino acids, among plants only soybeans provide complete protein High-quality Protein = contains all the essential amino acids in proportion to the body’s need for them Protein Allowances The RDA for a healthy adult is 0.8g of high- quality protein per kilogram of ideal body mass (take your weight in kg and multiply by 0.8 gives you the amount of protein needed in a day (young males under 18 multiply by 0.9) RDA for children ages 11-14 is higher 1.0g of protein per kg of body mass Most people in the US get enough protein Look on page 270 figure 17-9 for a list of protein contents in certain food choices. Questions 1. Why are amine groups important to proteins? 2. Compare peptide bonds and hydrogen bonds in protein formation. 3. Describe the two basic protein shapes. What is the main function of each? 4. What occurs during Denaturation? 5. Can proteins coagulate without denaturation? Why or why not? 6. How does the egg yolk compare in composition to the egg white? Questions Continued 7. A friend refrigerates eggs still in their carton immediately after returning form the supermarket. Explain whether this is a good idea. 8. Compare the protein structure of meat to that in fish. 9. Suppose your egg whites aren’t making a very impressive foam. What might be some reasons? 10. Why is kneading the dough an important step in making bread? 11. Why might injury and illness be more serious for someone with a long-term protein deficiency? 12. Should you be concerned if your diet lacks just one essential amino acid? Explain. 13. Can a diet of fruits, vegetables, and grains supply all of the essential amino acids? Explain.
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