PROTEIN CONCENTRATE

a human or animal dietary supplement that has a very high protein content and is extracted or prepared from vegetable or animal matter. The most common of such substances are leaf protein concentrate (LPC) and fish protein concentrate (FPC). LPC is prepared by grinding young leaves to a pulp, pressing the paste, then isolating a liquid fraction containing protein by filter or centrifuge. Herbaceous plants and legumes, such as clover and lucerne, produce higher yields of protein concentrate than perennial grasses. The protein quality of some LPCs has been found to approach that of the soybean, the most protein-rich of the oilseeds; all LPCs require supplements, however, because they are deficient in two of the nutritionally essential amino acids, lysine and methionine. FPC, processed directly from fish, is most commonly incorporated in cereal or wheat-based foods as a source of lysine. FPC flour is made by grinding the fish and adding to it an isopropanol solvent, which separates liquids and solids; the solid material is then extracted by centrifuge, and the process may be repeated several times. After the final centrifuging, the solid material is dried and ground. Enzymes Practically all of the numerous and complex biochemical reactions that take place in animals, plants, and microorganisms are regulated by enzymes. These catalytic proteins are efficient and specificthat is, they accelerate the rate of one kind of chemical reaction of one type of compound, and they do so in a far more efficient manner than man-made catalysts. They are controlled by activators and inhibitors that initiate or block reactions. All cells contain enzymes, which usually vary in number and composition, depending on the cell type; an average mammalian cell, for example, is approximately one one-billionth (10-9) the size of a drop of water and generally contains about 3,000 enzymes. The existence of enzymes was established in the middle of the 19th century by scientists studying the process of fermentation. Their role as catalysts of all living things followed rapidly. Developments before 1850 included (in 1833) the separation from malt of the enzyme amylase, which converts starch into sugar, and (in 1836) the isolation from the stomach wall of animals of a component of gastric juice that could partially digest food in a test tube, the enzyme pepsin. Enzymes were known for many years as ferments, a term derived from the Latin word for yeast. In 1878 the name enzyme, from the Greek words meaning in yeast, was introduced; since the late 19th century it has been universally used. Role of enzymes in metabolism Some enzymes help to break down large nutrient molecules, such as proteins, fats, and carbohydrates, into smaller molecules. This process occurs during the digestion of foodstuffs in the stomach and intestines of animals. Other enzymes guide the smaller, broken-down molecules through the intestinal wall into the bloodstream. Still other enzymes promote the formation of large, complex molecules from the small, simple ones to produce cellular constituents. Enzymes are also responsible for numerous other functions, which include the storage and release of energy, the course of reproduction, the processes of respiration, and vision. They are indispensable to life. Each enzyme is able to promote only one type of chemical reaction. The compounds on which the enzyme acts are called substrates. Enzymes operate in tightly organized metabolic systems called pathways. A seemingly simple biological phenomenonthe contraction of a muscle, for example, or the transmission of a nerve impulseactually involves a large number of chemical steps in which one or more chemical compounds (substrates) are converted to substances called products; the product of one step in a metabolic pathway serves as the substrate for the succeeding step in the pathway. The role of enzymes in metabolic pathways can be illustrated diagrammatically. The chemical compound represented by A (see diagram) is converted to product E in a series of enzyme-catalyzed steps, in which intermediate compounds represented by B, C, and D are formed in succession. They act as substrates for enzymes represented by 2, 3, and 4. Compound A may also be converted by another series of steps, some of which are the same as those in the pathway for the formation of E, to products represented by G and H. The letters represent chemical compounds; numbers represent enzymes that catalyze individual reactions. The relative heights represent the thermodynamic energy of the compounds; e.g., compound A is more energy-rich than B, B more energy-rich than C. Compounds A, B, etc., change very slowly in the absence of a catalyst but do so rapidly in the presence of catalysts 1, 2, 3, etc. The regulatory role of enzymes in metabolic pathways can be clarified by using a simple analogy: that between the compounds, represented by letters in the diagram, and a series of connected water reservoirs on a slope. Similarly, the enzymes represented by the numbers are analogous to the valves of the reservoir system. The valves control the flow of water in the reservoir; that is, if only valves 1, 2, 3, and 4 are open, the water in A flows only to E, but, if valves 1, 2, 5, and 6 are open, the water in A flows to G. In a similar manner, if enzymes 1, 2, 3, and 4 in the metabolic pathway are active, product E is formed, and, if enzymes 1, 2, 5, and 6 are active, product G is formed. The activity or lack of activity of the enzymes in the pathway therefore determines the fate of compound A; i.e., it either remains unchanged or is converted to one or more products. In addition, if products are formed, the activity of enzymes 3 and 4 relative to that of enzymes 5 and 6 determines the quantity of product E formed compared with product G. Both the flow of water and the activity of enzymes obey the laws of thermodynamics; hence, water in reservoir F cannot flow freely to H by opening valve 7, because water cannot flow uphill. If, however, valves 1, 2, 5, and 7 are open, water flows from F to H, because the energy conserved during the downhill flow of water through valves 1, 2, and 5 is sufficient to allow it to force the water up through valve 7. In a similar way, enzymes in the metabolic pathway cannot convert compound F directly to H unless energy is available; enzymes are able to utilize energy from energy-conserving reactions in order to catalyze reactions that require energy. During the enzyme-catalyzed oxidation of carbohydrates to carbon dioxide and water, energy is conserved in the form of an energy-rich compound, adenosine triphosphate (ATP). The energy in ATP is utilized during an energy-consuming process such as the enzyme-catalyzed contraction of muscle. Because the needs of cells and organisms vary, not only the activity but also the synthesis of enzymes must be regulated; e.g., the enzymes responsible for muscular activity in a leg muscle must be activated and inhibited at appropriate times. Some cells do not need certain enzymes; a liver cell, for example, does not need a muscle enzyme. A bacterium does not need enzymes to metabolize substances that are not present in its growth medium. Some enzymes, therefore, are not formed in certain cells, others are synthesized only when required, and still others are found in all cells. The formation and activity of enzymes are regulated not only by genetic mechanisms but also by organic secretions (hormones) from endocrine glands and by nerve impulses. Small molecules also play an important role (see below Enzyme flexibility and allosteric control). If an enzyme is defective in some respect, disease may occur. The enzymes represented by the numbers 1 to 4 in the diagram must function during the conversion of the starting substance A to the product E. If one step is blocked because an enzyme is unable to function, product E may not be formed; if E is necessary for some vital function, disease results. Many inherited diseases of man result from a deficiency of one enzyme. Some of these are listed in Table 3. The disease called albinism, for example, results from an inherited lack of ability to synthesize the enzyme tyrosinase, which catalyzes one step in the pathway by which the pigment for hair and eye colour is formed. Special structure and function of proteins Conjugated proteins Combination of proteins with prosthetic groups The link between a protein molecule and its prosthetic group is a covalent (electron-sharing) bond in the glycoproteins, the biliproteins, and some of the heme proteins. In lipoproteins, nucleoproteins, and some heme proteins, the two components are linked by noncovalent bonds; the bonding results from the same forces that are responsible for the tertiary structure of proteins: hydrogen bonds, salt bridges between positively and negatively charged groups, disulfide bonds, and mutual interaction of hydrophobic groups. In the metalloproteins (proteins with a metal element as a prosthetic group), the metal ion usually forms a centre to which various groups are bound. Some of the conjugated proteins have been mentioned in preceding sections because they occur in the blood serum, in milk, and in eggs; others are discussed below in sections dealing with respiratory proteins and enzymes. Mucoproteins and glycoproteins The prosthetic groups in mucoproteins and glycoproteins are oligosaccharides (carbohydrates consisting of a small number of simple sugar molecules) usually containing from four to 12 sugar molecules; the most common sugars are galactose, mannose, glucosamine, and galactosamine. Xylose, fucose, glucuronic acid, sialic acid, and other simple sugars sometimes also occur. Some mucoproteins contain 20 percent or more of carbohydrate, usually in several oligosaccharides attached to different parts of the peptide chain. The designation mucoprotein is used for proteins with more than 3 to 4 percent carbohydrate; if the carbohydrate content is less than 3 percent, the protein is sometimes called a glycoprotein or simply a protein. Mucoproteins, highly viscous proteins originally called mucins, are found in saliva, in gastric juice, and in other animal secretions. Mucoproteins occur in large amounts in cartilage, synovial fluid (the lubricating fluid of joints and tendons), and egg white. The mucoprotein of cartilage is formed by the combination of collagen with chondroitinsulfuric acid, which is a polymer of either glucuronic or iduronic acid and acetylhexosamine or acetylgalactosamine. It is not yet clear whether or not chondroitinsulfate is bound to collagen by covalent bonds. Special structure and function of proteins Albumins, globulins, and other soluble proteins The blood plasma, the lymph, and other animal fluids usually contain one to seven grams of protein per 100 millilitres of fluid, which includes small amounts of hundreds of enzymes and a large number of protein hormones. The discussion below is limited largely to the proteins that occur in large amounts and can be easily isolated from the body fluids. Proteins of the blood serum Human blood serum contains about 7 percent protein, two-thirds of which is in the albumin fraction; the other third is in the globulin fraction. Electrophoresis of serum reveals a large albumin peak and three smaller globulin peaks, the alpha-, beta-, and gamma-globulins. The amounts of alpha-, beta-, and gamma-globulin in normal human serum are approximately 1.5, 1.9, and 1.1 percent, respectively. Each globulin fraction is a mixture of many different proteins, as has been demonstrated by immuno-electrophoresis. In this method, the serum of a rabbit injected with human serum is allowed to diffuse into the four protein bandsalbumin, alpha-, beta-, and gamma-globulinobtained from the electrophoresis of human serum. Because the rabbit has previously been injected with human serum, its blood contains antibodies (substances formed in response to a foreign substance introduced into the body) against each of the human serum proteins; each antibody combines with the serum protein (antigen) that caused its formation in the rabbit. The result is the formation of about 20 regions of insoluble antigen-antibody precipitate, which appear as white arcs in the transparent gel of the electrophoresis medium. Each region corresponds to a different human serum protein. Serum albumin is much less heterogeneous (i.e., contains fewer distinct proteins) than are the globulins; in fact, it is one of the few serum proteins that can be obtained in a crystalline form. Serum albumin combines easily with many acidic dyes (e.g., Congo red and methyl orange); with bilirubin, the yellow bile pigment; and with fatty acids. It seems to act, in living organisms, as a carrier for certain biological substances. Present in blood serum in relatively high concentration, serum albumin also acts as a protective colloid, a protein that stabilizes other proteins. Albumin (molecular weight of 68,000) has a single free sulfhydryl (-SH) group, which on oxidation forms a disulfide bond with the sulfhydryl group of another serum albumin molecule, thus forming a dimer. The isoelectric point of serum albumin is pH 4.7. The alpha-globulin fraction of blood serum is a mixture of several conjugated proteins. The best known are an a-lipoprotein (combination of lipid and protein) and two mucoproteins (combinations of carbohydrate and protein). One mucoprotein is called orosomucoid, or a1-acid glycoprotein; the other is called haptoglobin because it combines specifically with globin, the protein component of hemoglobin. Haptoglobin contains about 20 percent carbohydrate. The beta-globulin fraction of serum contains, in addition to lipoproteins and mucoproteins, two metal-binding proteins, transferrin and ceruloplasmin, which bind iron and copper, respectively. They are the principal iron and copper carriers of the blood. The gamma-globulins are the most heterogeneous globulins. Although most have a molecular weight of approximately 150,000, that of some, called macroglobulins, is as high as 800,000. Because typical antibodies are of the same size and exhibit the same electrophoretic behaviour as g-globulins, they are called immunoglobulins. The designation IgM or gamma M (gM) is used for the macroglobulins; the designation IgG or gamma G (gG) is used for g-globulins of molecular weight 150,000. Special structure and function of proteins Immunoglobulins and antibodies Antibodies, proteins that combat foreign substances in the body, are associated with the globulin fraction of the immune serum. As stated previously, when the serum globulins are separated into a-, b-, and g- fractions, antibodies are associated with the g-globulins. Antibodies can be purified by precipitation with the antigen (i.e., the foreign substance) that caused their formation, followed by separation of the antigen-antibody complex. Antibodies prepared in this way consist of a mixture of many similar antibody molecules, which differ in molecular weight, amino acid composition, and other properties. The same differences are found in the g-globulins of normal blood serums. It is believed that the g-globulin of normal blood serum is a mixture of thousands of different g-globulins, each of which occurs in amounts too small for isolation. Because the physical and chemical properties of normal g-globulins are the same as those of antibodies, the g-globulins are frequently called immunoglobulins. They may be considered to be antibodies against unknown antigens. If solutions of g-globulin are resolved by gel filtration through dextran, the first fraction has a molecular weight of 800,000. This fraction is called IgM or gM; Ig is an abbreviation for immunoglobulin and M for macroglobulin. The next two fractions are IgA (gA) and IgG (gG), with molecular weights of about 300,000 and 150,000 respectively. Two other immunoglobulins, known as IgD and IgE, have also been detected in much smaller amounts in some immune sera. The bulk of the immunoglobulins is found in the IgG fraction, which also contains most of the antibodies. The IgM molecules are apparently pentamersaggregates of five of the IgG molecules. Electron microscopy shows their five subunits to be linked to each other by disulfide bonds in the form of a pentagon. The IgA molecules are found principally in milk and in secretions of the intestinal mucosa. Some of them contain, in addition to a dimer of IgG, a secretory piece that enables the passage of IgA molecules between tissue and fluid; the structure of the secretory piece is not yet known. The IgM and IgA immunoglobulins and antibodies contain 10 to 15 percent carbohydrate; the carbohydrate content of the IgG molecules is 2 to 3 percent. Figure 6: Diagram of an IgG immunoglobulin. IgG molecules treated with the enzyme papain split into three fragments of almost identical molecular weight of 50,000. Two of these, called Fab fragments, are identical; the third is abbreviated Fc. Reduction to sulfhydryl groups of some of the disulfide bonds of IgG results in the formation of two heavy, or H, chains (molecular weight 55,000) and two light, or L, chains (molecular weight 22,000). They are linked by disulfide bonds in the order L-H-H-L. Each H chain contains four intrachain disulfide bonds, each L chain contains two. The structure of antibodies and normal immunoglobulins of the IgG type is shown in Figure 6. Antibody preparations of the IgG type, even after removal of IgM and IgA antibodies, are heterogeneous. The H and L chains consist of a large number of different L chains and a variety of H chains. Pure IgG, IgM, and IgA immunoglobulins, however, occur in the blood serum of patients suffering from myelomas, which are malignant tumours of the bone marrow. The tumours produce either an IgG, an IgM, or an IgA protein, but rarely more than one class. A protein called the Bence-Jones protein, which is found in the urine of patients suffering from myeloma tumours, is identical with the L chains of the myeloma protein. Each patient has a different Bence-Jones protein; no two of the more than 100 Bence-Jones proteins that have been analyzed thus far are identical. It is thought that one lymphoid cell among hundreds of thousands becomes malignant and multiplies rapidly, forming the mass of a myeloma tumour that produces one g-globulin. Figure 6: Diagram of an IgG immunoglobulin. Analyses of the Bence-Jones proteins have revealed that the L chains of man and other mammals are of two quite different types, kappa (k) and lambda (l). Both consist of approximately 220 amino acids. The Nterminal halves of k- and l-chains are variable, differing in each Bence-Jones protein. The Cterminal halves of these same L chains have a constant amino acid sequence of either the k- or the l-type. The fact that one half of a peptide chain is variable and the other half invariant is contradictory to the view that the amino acid sequence of each peptide chain is determined by one gene (see Genetics and Heredity: The gene). Evidently, two genes, one of them variable, the other invariant, fuse to form the gene for the single peptide chain of the L chains. Whereas the normal human L chains are always mixtures of the k- and l-types, the H chains of IgG, IgM, and IgA are different. They have been designated as gamma (g), mu (m), and alpha (a) chains, respectively. The N-terminal quarter of the H chains has a variable amino acid sequence; the C-terminal three-quarters of the H chains have a constant amino acid sequence, as indicated in Figure 6. Some of the amino acid sequences in the L and H chains are transmitted from generation to generation. As a result, the constant portion of the human L chains of the k-type has in position 191 either valine or leucine. They correspond to two alleles (character-determining portions) of a gene; the two types are called allotypes. The valine-containing genetic type has been designated as InV(a+), the leucine-containing type as InV(b+). Many more allotypes, called Gm allotypes, have been found in the gamma chains of the human IgG immunoglobulins; more than 20 Gm allotypes are now known. Certain combinations of Gm types occur; the combination of Gm types 5, 6, and 11 has been found in Caucasians and Negroes but not in Chinese; the combination of 1, 2, and 17 has not been found in Negroes; and the combination of 1, 4, and 17 has not been found in Caucasians. Allotypes have also been discovered to occur in a number of other animals, including rabbits and mice. It is understandable from the occurrence of a large number of allotypes that antibodies, even if produced in response to a single antigen, are mixtures of different allotypes. The existence of several classes of antibodies, of different allotypes, and of adaptation of the variable portions of antibodies to different regions of an antigen molecule results in a multiplicity of antibody molecules even if only a single antigen is administered. For this reason it has not yet been possible to unravel the amino acid sequence in the variable portion of antibody molecules. Much of the amino acid sequence in the constant regions of the L and H chains of man and rabbit immunoglobulins, however, has been resolved. Felix Haurowitz The Editors of the Encyclopdia Britannica Special structure and function of proteins Protein hormones Some hormones that are products of endocrine glands are proteins or peptides; others are steroids. (The origin of hormones, their physiological role, and their mode of action are dealt with in the article hormone.) None of the hormones has any enzymatic activity. Each has a target organ in which it elicits some biological action; e.g., secretion of gastric or pancreatic juice, production of milk, production of steroid hormones. The mechanism by which the hormones exert their effects is not fully understood. Cyclic adenosine monophosphate is involved in the transmittance of the hormonal stimulus to the cells whose activity is specifically increased by the hormone. Hormones of the thyroid gland Thyroglobulin, the active groups of which are two molecules of the iodine-containing compound thyroxine (see Figure 1), has a molecular weight of 670,000. Thyroglobulin also contains thyroxine with two and three iodine atoms instead of the four shown in Figure 1, and trosine, with one and two iodine atoms. Injection of the hormone causes an increase in metabolism; lack of it results in a slowdown. Another hormone, calcitonin, which lowers the calcium level of the blood, occurs in the thyroid gland. The structure of human calcitonin is given in Formula 7 (see Figure 1 for structures of amino acids corresponding to the one-letter codes). The amino acid sequences of calcitonin from pig, beef, and salmon differ from human calcitonin in some amino acids. All of them, however, have the half-cystines and the prolinamide in the same position. Porcine calcitonin has been synthesized in the laboratory. The parathyroid hormone (parathormone), produced in small glands that are embedded in or lie behind the thyroid gland, is essential for maintaining the calcium level of the blood. Its lack results in the disease hypocalcemia. Bovine parathormone has a molecular weight of 8,500; it contains no cystine or cysteine and is rich in aspartic acid, glutamic acid, or their amides. Special structure and function of proteins Despite its weaknesses, a functional classification is used here in order to demonstrate, whenever possible, the correlation between the structure and function of a protein. The structural, fibrous proteins are presented first, because their structure is simpler than that of the globular proteins and more clearly related to their function, which is the maintenance of either a rigid or a flexible structure. Structural proteins Scleroproteins Collagen Collagen is the structural protein of bones, tendons, ligaments, and skin. For many years collagen was considered to be insoluble in water. Part of the collagen of calf skin, however, can be extracted with citrate buffer at pH 3.7. A precursor of collagen called procollagen is converted in the body into collagen. Procollagen has a molecular weight of 120,000. Cleavage of one or a few peptide bonds of procollagen yields collagen, which has three subunits, each with a molecular weight of 95,000; therefore, the molecular weight of collagen is 285,000 (3 95,000). The three subunits are wound as spirals around an elongated straight axis. The length of each subunit is 2,900 angstroms, and its diameter is approximately 15 angstroms. The three chains are staggered, so that the trimer has no definite terminal limits. The amino acid composition of collagen is shown in Table 1. It differs from all other proteins in its high content of proline and hydroxyproline. Hydroxyproline does not occur in significant amounts in any other protein except elastin. Most of the proline in collagen is present in the sequence glycineproline-X, in which X is frequently alanine or hydroxyproline. Collagen does not contain cystine or tryptophan and therefore cannot substitute for other proteins in the diet. The presence of proline causes kinks in the peptide chain and thus reduces the length of the amino acid unit from 3.7 angstroms in the extended chain of the b-structure to 2.86 angstroms in the collagen chain. In the intertwined triple helix, the glycines are inside, close to the axis; the prolines are outside. Native collagen resists the action of trypsin but is hydrolyzed by the bacterial enzyme collagenase. When collagen is boiled with water, the triple helix is destroyed, and the subunits are partially hydrolyzed; the product is gelatin. The unfolded peptide chains of gelatin trap large amounts of water, resulting in a hydrated molecule. When collagen is treated with tannic acid or with chromium salts, cross links form between the collagen fibres, and it becomes insoluble; the conversion of hide into leather is based on this tanning process. The tanned material is insoluble in hot water and cannot be converted to gelatin. On exposure to water at 62 to 63 C (144 to 145 F), however, the cross links formed by the tanning agents collapse, and the leather contracts irreversibly to about one-third its original volume. Collagen seems to undergo an aging process in living organisms that may be caused by the formation of cross links between collagen fibres. They are formed by the conversion of some lysine side chains to aldehydes (compounds with the general structure RCHO), and the combination of the aldehydes with the e-amino groups of intact lysine side chains. The protein elastin, which occurs in the elastic fibres of connective tissue, contains similar cross links and may result from the combination of collagen fibres with other proteins. When cross-linked collagen or elastin is degraded, products of the cross-linked lysine fragments, called desmosins and isodesmosins, are formed.