Transcript Protein
Protein Proteins: Complex molecules made of hydrogen, carbon, oxygen, nitrogen and sometimes other elements. – Because of their size, proteins are often called macromolecules Macromolecule: A large molecule containing many atoms Nitrogen plays an important part in proteins. – It sets proteins apart from carbohydrates or lipids, which do not contain nitrogen. Structure of Proteins Protein is made of chains of substances called amino acids: a type of organic acid. – Organic acids are molecules that contain a carboxyl group (COOH). – They also contain an amine group: two atoms of hydrogen and one atom of nitrogen (-NH2). The carboxyl group and the amine group are attached to a central carbon in the amino acid. A third bond to this carbon is to a single hydrogen. One bond is left open for a variable side chain, making each amino acid different than the others. 20 Amino Acids Orange: Hydrophobic Green: Hydrophilic Pink: Acidic Turquoise: Basic Peptide Bonds Peptide bonds: The bonds between the nitrogen of one amino acid and the carbon of a second amino acid. – The peptide bonds link to form a protein molecule. When one hydrogen in one amino acid joins with the –OH of another amino acid, water is formed, and released, leaving a peptide bond. Polypeptides Through peptide bonds, amino acids chain together, creating a polypeptide: a single protein molecule containing ten or more amino acids linked in peptide chains. – Some protein chains contain only a few amino acids. Most molecules contain 100 to 500 amino acids, and some have thousands. The polypeptide chain is not straight; it coils, folds and tangles. Protein Structures Structure and Function of Proteins A protein’s shape determines its function. – Protein molecules that form rope-like fibers are called fibrous protein. This structure strengthens the fiber to serve as connective tissue in the body. – Ex: Collagen and elastin are fibrous proteins. – Protein molecules that form a structure that can be compared to steel wool are called globular proteins. Their rounded shape makes them convenient carriers. – Ex: The protein, hemoglobin transports oxygen to the blood. Fibrous Protein: Connective tissue, tendons, bone matrix, muscle fiber NOT SOLUBLE IN WATER Globular Protein: Hemoglobin, enzymatic catalysis, hormones (messengers), transporters through membranes SOLUBLE IN WATER Denaturation of Protein Denaturation: The process of changing the shape of a protein molecule without breaking its peptide bonds. – Denaturation breaks the hydrogen bonds that create the twists and turns of a protein molecule. The result is a looser, less compact structure, changing the original properties of the protein. Coagulation Denaturation is the first step in the process of coagulation: Changing a liquid into a soft, semisolid clot or solid mass. – It occurs when polypeptides unfold during denaturation, then collide and clump together to form a solid. Ex: Scrambled eggs; beating the eggs denatures the protein, then the protein coagulates as the egg cooks. Denaturation by Heat Heat is the most common agent used to denature protein. – Temperature is a significant factor. The rate and the degree of denaturation increase 600 times for every 10 degrees C rise in temperature. – The structure of the protein affects the process also. Most proteins denature at temps between 47 and 67 degrees C, as in eggs and milk. – The denaturation of beef takes a much higher temp. Other Means of Denaturation Proteins may denature due to the reaction of: – Freezing, pressure, sound waves, and the addition of certain compounds. – Mechanical treatment, as in beating eggs and kneading bread. – Very high and very low pH. Adding lemon juice can sour milk; milk proteins denature, coagulate and separate from the liquid. – Certain metal ions. Sodium and potassium ions are most commonly used, but copper and iron will have the same effect. Denaturation of proteins involves the disruption and possible destruction of both the secondary and tertiary structures. Since denaturation reactions are not strong enough to break the peptide bonds, the primary structure (sequence of amino acids) remains the same after a denaturation process. Denaturation disrupts the normal alpha-helix and beta sheets in a protein and uncoils it into a random shape. Protein in Foods Protein in the diet can come from a variety of sources. Some of the more common ones are: – Eggs – Meat – Fish – Poultry – legumes Eggs Eggs are a complex biological system, containing almost every vitamin and mineral the body needs. – Yolk: Rich in iron, phosphorus, vitamin A and several B vitamins The only vitamin lacking is Vitamin C. Although all these vitamins and minerals are present, eggs are mainly considered a protein food. Structure Albumen (Egg White) – Thick and thin Yolk Vitelline membrane Chalaza Germ spot Air pocket Proteins in Eggs Shell: Protein is interwoven with calcium carbonate. Shells are porous, so the developing chick can breathe. Albumen: Egg white, composes 54% of the egg, and is named for ovalbumin, the major protein in egg white. Yolk: Contain the globular protein livetin, and both high- and low- density lioproteins. – Thus, the yolk contains all of the fat, and most of the other nutrients found in eggs. The Chalaza Chalaza: A twisted, ropelike structure that keeps the egg yolk centered. – Without it, the vitelline membrane could break, and the yolk (a fluid) would spread. Due to its thickness, the chalaza is the last part of the egg to coagulate, and may remain slightly watery. When the egg is freshly laid, the shell is completely filled. The air cell is formed by contraction of the contents during cooling and by the loss of moisture. A high-quality egg has only a small air cell. The shell contains several thousand pores that permit the egg to "breathe." Effects of Improper Storage To retain their quality, do not store eggs in the egg tray of a refrigerator. – It exposes eggs to light and temperature change every time the door is opened. This speeds up chemical changes that cause physical changes that lead to deterioration. Loss of Egg Quality Eggs lose quality as compounds in the egg white break down, forming water. – The white becomes less viscous Some of the water enters the yolk, making it thinner, too. Carbon dioxide and more water escape through the porous shell. – This changes the pH from its normal 7.6 to over 9. Proteins begin to break down Odors and flavors from other foods can enter through the shell, giving it an off taste Protecting Eggs To delay quality loss, a thin film of natural oil in the shell protects the egg. – Shells may also be lightly coated with mineral oil immediately after the eggs are laid. Storing eggs closed in their carton helps preserve their quality after purchase. The best temperature for storing eggs is -1 degree C. This temp. is below the freezing point of water but well above the freezing point of the liquids found in eggs.