Lecture 1. Proteins.-1.doc
- 1. Lecture 2 PROTEIN STRUCTURE AND FUNCTIONS Proteins are polymers made of 20 α – amino acids. Proteins are a very important class of food molecules because they are the most abundant macromolecules in the cell, and they carry out most of the work in a cell. Proteins have many biological functions which are very important for human organism. Functions of proteins 1. Structural proteins provide mechanical support: keratin of hair, nail, collagen of bone. 2. Enzymes and catalytic proteins are protein in nature. Enzymes accelerate rate of biochemical reaction in the human body. 3. Transport proteins carry materials from one place to another in the body. The proteins hemoglobin and myoglobin are responsible for transport and storage of oxygen. Plasma albumin transports free fatty acids, bilirubin, steroid hormones, calcium. 4. Regulatory proteins or hormones control many aspects of cell function, including metabolism and cell reproduction. Insulin and glucagon regulate blood glucose level. Insulin lowers blood glucose level, while glucagon elevates blood glucose level. 5. Contractile proteins: myosin and actin of the muscle proteins take part in the muscle contraction. 6. Storage proteins: Ovalbumin of eggs and casein of milk source of amino acids for development of fetus. 7. Genetic proteins: nucleoproteins take part in transmission of genetic information. 8. Defense proteins or antibodies which are produced by the immune system in response to antigens. Antigens are bacteria and viruses. Structural organization of protein molecule Proteins are the polymers of L-α-aminoacids. The structure of proteins can be divided into 4 levels of organization. Primary structure is the linear sequence of amino acids in the chain are joined by the peptide bond. The peptide bond is formed between the α – carboxyle group of one amino acid and the α-amino group of another amino acid. The primary structure of proteins is dictated by the genetic information in the DNAs.
- 2. Secondary structure results from folding of the covalently linked amino acids into regularly repeating structure. The secondary structure is maintained by numerous hydrogen bonds between the amide hydrogens (− NH) and the carboxyl oxygen (− CO) of the peptide chain background. The most common types of secondary structures are α- helix and ß-pleated sheet. α- helix is a right-handed helical conformation and ß-pleated sheet in proteins resembles the pleated folds of drapery. Tertiary structure is the further folding of a peptide to a globular structure. The polypeptide chain with its regions of secondary structure, α-helix and ß- pleated sheet, further folds on itself to achieve the tertiary structure. The tertiary structure is maintained by the mоre attractive forces between side chains of amino acids. These forces include hydrophobic interactions, hydrogen bonds, ionic bridges, and disulfide bonds.
- 3. Quaternary structure is the association of two or more peptide to form the functional protein. It is maintained by the same noncovalent interactions that are responsible for tertiary structure. For example: Insulin consists of two polypeptide chains: A – and B – chains and hemoglobin (Hb) is a tetramer composed of four polypeptide subunits: 2α-subunits and 2β-subunits. Properties of proteins 1. Solubility: Proteins form colloidal solution due to huge size of protein molecule. 2. Molecular weight: Proteins has high molecular weight. 3. Proteins are ampholytes due to presence of NH2 – and - COOH groups. 4. Isoelectric PH. At isoelecrtic pH, the proteins exist as zwitterions or dipolar ions. They are electrically neutral (do not migrate in electrical field) with minimum solubility, maximum precipitability and least buffering capacity. The isoelectric pH for some proteins: Pepsine-1.1; Casein -4.6; Albumin – 4.7; hemoglobin 6.7.
- 4. When net electric charge of the proteins is zero they no longer repeal one another. As a result, they clump together and precipitate out of solution. In this case proteins my coagulated even though they aren’t denatured. The proteins in general are least soluble at isoelectric pH. Casein easily precipitated when pH 4.6. Formation of curd from milk, precipitation of milk protein, casein in pl occurs due to the lactic acid produced by fermentation of bacteria which lowers the pH to the pl of casein. Denaturation Denaturation is the phenomen of disorganization of native protein structure. Denaturation results in the loss of secondary, tertiary and quaternary structure of proteins. However it doesn’t alter the primary structure of the protein. This involves a change in physical, chemical and biological properties of protein molecules. If a protein is denaturated under mild conditions, its original structure can often be regenerated by removing the denaturant or by retoring the temperature to normal conditions. This process is called reversible denaturation or renaturation. AGENTS OF DENATURATION Physical agents Temperature. Increasing the temperature simply increases the rate of molecular movement, the movement of the individual molecules within the solution. Then, as the temperature continues to increase, the bonds within the proteins begin to vibrate more violently. Eventually, the weak interactions, like hydrogen bonds and hydrophobic interactions, that maintain the protein structure are disrupted. The protein molecules are denatured as they lose their characteristic three dimensional conformation and become completely disorganized. Many of the proteins of our cells, for instance, the enzymes, are in the same kind of viscous solution within the cytoplasm. To continue to function properly, they must remain in solution and maintain the correct three-dimensional configuration. If the body temperature becomes too high, or if local regions of the body are subjected to very high temperatures, as when you touch a hot cookie sheet, cellular proteins become denatured. They lose their function, and the cell or the organism dies.
- 5. Mechanical Stress. Stirring, whipping, or shaking can disrupt the weak interactions that maintain protein conformation. This is the reason that whipping egg whites produces a stiff meringue. Chemical agents pH. Because of the R groups of the amino acids, all proteins have a characteristic electric charge. Because every protein has a different amino acid composition, each will have a characteristic net electric charge on its surface. The positively and negatively charged R groups on the surface of the molecule interact with ions and water molecules, and these interactions keep the protein in solution within the cytoplasm. Changing pH alter the net charge on the protein, causing electrostatic repulsion and the disruption of some hydrogen bonding. When the blood pH drops too low, blood proteins become polycations. Similarly, when the blood pH rises too high, the proteins become polyanions. In either case, the proteins will unfold because of charge repulsion and loss of stabilizing ionic interactions. Under these extreme conditions, the denatured blood proteins would no longer be able to carry out their required functions. The blood cells would also die as their critical enzymes were denatured. The hemoglobin in the red blood cells would become denatured and would no longer be able to transport oxygen. Fortunately, the body has a number of mechanisms, such as the carbonate buffer system to avoid the radical changes in the blood pH that can occur as a result of metabolic or respiratory difficulties. Organic Solvents. Polar organic solvent, denature proteins by disrupting hydrogen bonds within the protein, in addition to forming hydrogen bonds with the solvent, water. The nonpolar regions of these solvents interfere with hydrophobic interactions in the interior of the protein molecule, thereby disrupting the conformation. Heavy Metals. Heavy metal ions (for example, Pb2+ , Hg2+ , and Cd2+ ) also denature protein by attacking the ̶ SH groups. They form salt bridges, as in S- ̶ Hg2+ ̶ S- . This very feature is taken advantage of in the antidote for heavy metal poisoning: raw egg whites and milk. The egg and milk proteins are denatured by the metal ions, forming insoluble precipitates in the stomach. These must be pumped out or removed by inducing vomiting. In this way, the poisonous metal ions are removed from the body. If the antidote is not pumped out of the stomach, the digestive enzymes would degrade the proteins and release the poisonous heavy metal ions, which would then be absorbed into the bloodstream.