Glycoproteins Is a protein that contains carbohydrates or carbohydrate derivatives in addition to amino acids. The carbohydrate content of glycoproteins is variable, but it is fixed for any specific glycoprotein. Glycoproteins include a number of important substances; two of these collagen and immunoglobulins, Collagen-the fibrous protein collagen, whose structure was first considered in section 20.16 qualifies as a glycoprotein because carbohydrate units are present in its structure. This structural feature of collagen, not considered previously, involves the presence of the nonstandard amino acids 4-hydroxyproline (5 %) and 5-hydroxylysine (1%) derivatives of the standard amino acid’s proline and lysine. The presence of carbohydrate units (mostly glucose, galactose, and their disaccharides) attached by glycosidic linkages to collagen and its 5-hydroxylysine residues causes collagen to be classified as a glycoprotein. The function of the carbohydrate groups in collagen is related to cross-linking. they direct the assembly of collagen triple helices into more complex aggregations called collagen fibrils. When collagen is boiled in water, under basic conditions, it is converted to the water-soluble protein gelatin. This process involves both denaturation and hydrolysis. Heat acts as a denaturant, causing rupture of the hydrogen bonds supporting collagens triple-helix structure. Immunoglobulins Are among the most important and interesting of the soluble proteins in the human body. An immunoglobulin is a glycoprotein produced by an organism as a protective response to the invasion of microorganisms or foreign molecules. They serve as antibodies to combat invasion of the body by antigens. An antigen is a foreign substance such as a bacterium or virus, which invades the human body. An antibody is a biochemical molecule that counteracts a specific antigen. The immune system of the human body has the capability to produce immunoglobulins that respond to several million different antigens. The importance of immunoglobulins is amply and tragically demonstrated by the effects of AIDS (acquired immunodeficiency syndrome). The AIDS virus upsets the body’s normal production of immunoglobulins and leaves the body susceptible to what would otherwise not be debilitating and deadly infections. Lipoproteins Is a conjugated protein that contains lipids in addition to amino acids. The major function of such proteins is to help suspend lipids and transport them through the blood stream. Lipids, in general, are insoluble in blood (an aqueous medium) because of their nonpolar nature. A plasma lipoprotein is a lipoprotein that is involved in the transport system for lipids in the bloodstream. These proteins have a spherical structure that involves a central core of lipid material (triacylglycerols and cholesterol esters) surrounded by a shell (membrane structure) of phospholipids, cholesterol, and proteins. In the blood, cholesterol exists primarily in the form of cholesterol esters formed from the esterification of cholesterol hydroxyl group with a fatty acid. Four major classes of plasma lipoproteins Chylomicrons- Their function is to transport dietary triacylglycerols from the intestine to the liver and to adipose tissue. Very low-density lipoproteins (VLDL) Their function is to transport triacylglycerols synthesized in the liver to adipose tissue. Low-density lipoproteins (LDL) Their function is to transport cholesterol synthesized in the liver cells throughout the body. High-density lipoproteins (HDL). Their function is to collect excess cholesterol from body tissues and transport it back to the liver for degradation to bile acids. Denaturation of Protein Normal protein > Denaturation > Renaturation Denaturation is a process in which a protein loses its native shape due to the disruption of weak chemical bonds and interactions, thereby becoming biologically inactive In case of proteins: A loss of three-dimensional structure, sufficient to cause loss of function. Loss of secondary, tertiary, and quaternary structure of proteins Change in physical, chemical, and biological properties of protein molecules Denaturation involves transformation of a well-defined folded structure of a proteins formed under physiological conditions, to an unfolded state under non-physiological conditions. Changing pH denatures proteins because it changes the charges om many of the side chains. This disrupts electrostatic attractions and hydrogen bonds. Certain reagents such as urea and guanidine hydrochloride denature proteins by forming hydrogen bonds to the protein groups that are stronger than the hydrogen bonds formed between the groups Detergents such as sodium dodecyl sulphate denature proteins by associating with the non-polar groups of protein, thus interfering with the normal hydrophobic interactions. Organic solvents such as acetone, alcohols denature proteins by disrupting hydrophobic interactions. Proteins can also be denatured by heat. Heat increases molecular motion which can disrupt the attractive forces None of these agents break the peptide bonds, so the primary structure of proteins remain intact when it is denatured. When protein is denatured, it loses its function. A denatured enzyme ceases/stops its function A denatured antibody no longer binds to its antigen A denatured milk proteins losses its biological activity The denatured state does not necessarily mean that complete unfolding or denaturation of protein and randomization of confirmation. Under some of the conditions, these proteins exhibit both properties such as denaturation and renaturation. Mechanism of protein denaturation: Higher temperature denaturation = heat denaturation or thermal denaturation Lower temperature denaturation = cold denaturation In both cases, there is breakage of hydrogen bonds, disulfide bonds, hydrophobic interactions, Vander Waal’s forces, but there is no breakage of peptide bonds during denaturation Causes of protein denaturation: Denaturation occurs when proteins are exposed to an extreme environment conditions such as high level of salt, higher level of acidity, higher temperature etc. Because of these extreme conditions, the function of the proteins alters due to deformities along their bonds and can be temporarily or permanently denatured. Agents causing denaturation Physical agents Heat Violent shaking or agitation Hydrostatic pressure UV radiation Chemical agents Acids and alkalis Organic solvents Salts of heavy metals Chaotropic agents Detergents Altered pH Denaturation by heat Most proteins can be denatured by heat, which affects the weak interactions in a protein (primarily hydrogen bonds) in complex manner If the temperature is increased slowly, a protein’s conformation generally remains intact until an abrupt loss of structure and function occurs over a narrow temperature range During cooking, this stress causes denaturation which is typically as heat and ultimately proteins gets coagulated. As higher temperatures can cause irreversible denaturation of proteins, and when a cell is exposed to high temperatures, several types of molecular chaperones swing into action for this reason, these chaperones are also called heat-shock proteins (HSPs). Denaturation by violent shaking: Agitation also denatures protein We see this clearly in the whipping of egg whites. The constant churning of milk or cream creates foam from various proteins which also causes denaturation of proteins. Denaturation of milk protein occurs during whipping or beating of ice-cream mix during ice-cream manufacturing. Denaturation of proteins by hydrostatic pressure: Proteins undergo dissociation and unfolding by pressure mostly because the final states are more hydrated, have fewer non-hydrated cavities and therefore, occupy smaller volumes. For a typical case of protein denaturation, pressure will shift the equilibrium of the reactants. The decrease in volume is due to hydration of newly exposed nonpolar and polar residues as well as to the loss of free volume arising from packing defects in the folded structure. High pressure has been used to assess the underlying mechanisms of protein misfolding and aggregation. Denaturation by UV radiation: UV radiation supplies kinetic energy to protein molecules, causing their atoms to vibrate more rapidly and disrupting the relatively weak hydrogen bonding and dispersion forces of protein molecules. Chemical agents: Denatured by Acids and alkalis: Acids and bases disrupt salt bridges held together by ionic charges. Double replacement reaction occurs where the positive and negative ions in the salt change partners with the positive and negative ions in the new acid or base added. This reaction occurs in the digestive system when the acidic gastric juices cause the curdling (coagulating) of milk. Strong Acids or Bases salt formation; disruption of hydrogen bonds. (skin blisters and burns, protein precipitation.) Acidic protein denaturants include: Acetic acid Trichloroacetic acid 12% in water Sulfosalicylic acid Basic protein denaturants include Sodium bicarbonate Sodium sulphate Denaturation by organic solvents: Proteins can also be denatured by using organic solvents such as ether, alcohol, acetone, diethyl ether etc. These added alcohol disrupts the hydrogen bond between protein molecules and new hydrogen bonds are formed instead between the new alcohol molecule and the protein side chains. Denaturation of proteins by salts of heavy metals: The heavy metal salts usually contain Hg+2, Pb+2, Ag+1Ti+1, Cd+2 and other metals with high atomic weights. Since salts are ionic in nature they disrupt salt bridges in proteins. The reaction of a heavy metal salt with a protein usually leads to an insoluble metal protein salt complex This reaction is used for its disinfectant properties in external applications. For examples: AgNO3 is used to prevent gonorrhea infections in the eyes of newborn infants. Silver nitrate is also used in the treatment of nose and throat infections. Heavy metals may also disrupt disulfide bonds because of their high affinity and attraction for sulfur and will also lead to the denaturation of proteins.
