Lecture 13: Classification and Catabolism of Proteins PDF

Lippincott’s illustrated reviews Chapter 19, Page 245 Lecture 13 Classification and Catabolism of Proteins 1  Specific Objectives By the end of this lecture students can be able to: Classify proteins acco...

Lippincott’s illustrated reviews Chapter 19, Page 245 Lecture 13 Classification and Catabolism of Proteins 1  Specific Objectives By the end of this lecture students can be able to: Classify proteins according to shape, function, solubility and nutritional requirements. Differentiate between high and low quality proteins. Know role of digestive enzymes in protein digestion. 2  Classification of proteins I- Based on shapes: 1- Globular proteins: e.g. plasma albumins and globulins and many enzymes. They have spheroidal shape. 2- Fibrous proteins: e.g. keratin, myosin, fibrin and collagen. 3  II.Classification based on functions Catalytic proteins, e.g. enzyme. Structural proteins, e.g. collagen, elastin, keratin. Contractile proteins, e.g. myosin, actin, flagellar proteins. Transport proteins, e.g. hemoglobin, myoglobin, albumin, transferrin. Regulatory proteins or hormones, e.g. ACTH, insulin, growth hormone. Genetic proteins, e.g. histones. Protective proteins, e.g. immunoglobulins, clotting factors. 4 water hemostasy e 5 6  III. Classification based on solubility Proteins may be divided into three major groups; simple, conjugated and derived. job A. Simple proteins: Posible Short not According to definition, they contain only amino acids. But they also contain very small quantity of carbohydrates. Types of simple protein: Albumins Globulins Protamine Gasp Scleroprotein 7 8 B- Conjugated proteins: They are combinations of protein with a non-protein part, called prosthetic group. Conjugated proteins may be classified as follows: 1. Glycoproteins: These are proteins combined with carbohydrates. Blood group antigens and many serum proteins are glycoproteins. 2. Lipoproteins: These are proteins loosely combined with lipid components. They occur in blood and cell membranes 9  3. Nucleoproteins: These are proteins attached to nucleic acids, e.g. Histones. 4. Chromoproteins: These are proteins with colored prosthetic groups. Hemoglobin (Heme, red); Flavoproteins (Riboflavin, yellow), Visual purple (Vitamin A, purple) are some examples of chromoproteins. 5. Phosphoproteins: Cillia These contain phosphorus. Ex. Casein of milk and vitellin of egg yolk. 10  Defention p 6. Mettaloproteins: an et They contain metal ions. Ex. Hemoglobin (iron), cytochrome (iron), tyrosinase (copper) and carbonic anhydrase (zinc). C. Derived proteins: They are degradation products of native proteins. Progressive hydrolysis of protein results in smaller and smaller chains: Protein → Peptones → Peptides → Amino acids. 11  IV. Classification based on nutritional value A. Nutritionally rich proteins: They are also called as complete proteins or first class proteins. They contain all the essential amino acids in the required proportion. On supplying these proteins in the diet, the young individuals will grow satisfactorily. A good example is casein of milk. Any animal sourff 12 B. Incomplete proteins: They lack one essential amino acid. They cannot promote body growth in young individuals; but may be able to sustain the body weight in adults. Proteins from pulses are deficient in methionine, while proteins of cereals lack in lysine. If both of them are combined in the diet, good growth could be obtained. 13 C. Poor proteins: They lack in many essential amino acids and a diet based on these proteins will not even sustain the original body weight. Zein from corn lacks tryptophan and lysine. 14  Catabolism of Proteins There are two types of proteins: Dietary Proteins Catabolism Anabolism catabolism Body Proteins 15  Digestion of Dietary proteins The dietary proteins are denatured on cooking and therefore more easily to digest by digestive enzymes. digestion starts at stomche Proteolytic enzymes hydrolyze proteins by breaking down the peptide bonds of the protein. They are secreted as inactive zymogens which are converted to their active form in the intestinal lumen. This would prevent autodigestion of the secretory acini. 16 The proteolytic enzymes include: Endopeptidases: They act on peptide bonds inside the protein molecule, so that the protein becomes successively smaller and smaller units. This group includes pepsin, trypsin, chymotrypsin Exopeptidases: This group acts at the peptide bond only at the end region of the chain. This includes carboxypeptidase acting on the peptide only at the carboxyl terminal end on the chain and aminopeptidase, which acts on the peptide bond only at the amino terminal end of the chain. 17 18 Digestion of protein takes place by three types of secretions 1. Gastric secretion: Hydrochloric acid: It kills some bacteria, denatures the proteins, it makes the pH optimum for the action of pepsin and also activates pepsin by conversion of inactive pepsinogen to active pepsin. break ALL weak bonds It will from hydrochloric acid Ph 2 2 will get 19 20 Rennin: Pepsin Rennin otherwise called chymosin, is active in infants and is involved in the curdling of milk. It is absent in adults. Milk protein, casein is converted to paracasein by the action of rennin. So pepsin can break due to it being weaken in children The denatured protein is easily digested further by pepsin. 21 Pepsin: It is secreted by the chief cells of stomach as inactive pepsinogen. The conversion of pepsinogen to pepsin is brought about by the hydrochloric acid. The optimum pH for activity of pepsin is around 2. Pepsin is an endopeptidase. By the action of pepsin, proteins are broken into 22 proteoses. 2. Pancreatic secretion: The optimum pH for the activity of pancreatic enzyme (pH 8) is provided by the alkaline bile and pancreatic juice. Iff The secretion of pancreatic juice is stimulated by the peptide hormones, cholecystokinin and D pancreozymin. Dabney Pancreatic juice contains the important endopeptidases, namely trypsin, Elastase, 5 chymotrypsin, and carboxypeptidase 23 1) Trypsin: Trypsinogen is secreated from pancrease and activated by enterokinase present on the intestinal microvillus membranes. TITI Once activated, the trypsin activates other enzyme molecules. Break down Trypsin catalyzes hydrolysis of the bonds formed by carboxyl groups of Arginine and Lysine. Acute pancreatitis: Premature activation of trypsinogen inside the pancreas itself will result in the autodigestion of pancreatic cells. The result is acute pancreatitis. It is a life-threatening condition Destruction of pencritic cells 24 2) Elastase: The only one stable does not break Elastase catalyzes hydrolysis of the bonds formed by carboxyl groups of Alanine, glycine and serine. It is not degraded during intestinal transit and is generally not affected by increases or decreases in intestinal transit time. Doctors can test a person’s pancreatic elastase levels by measuring the presence of the enzyme in a stool sample. Low levels of pancreatic elastase could indicate exocrine pancreatic oh insufficiency. _t 25 BUEY Est or 26 3) Chymotrypsin: Trypsin activates chymotrypsinogen, to active chemotrypsin. Trypsin, elastase and chymotrypsin degrade the proteoses into small peptides. 4) Carboxypeptidases: EXO Small peptides are further hydrolyzed into dipeptides and tripeptides by carboxypeptidases present in the pancreatic juice. They are metallo-enzymes requiring zinc. 27 3. Intestinal secretion: who does 17 Complete digestion1 of the small peptides to the level of amino acids is brought about by enzymes present in intestinal juice (succus entericus). without it awill indig tions The luminal surface of intestinal epithelial cells contains Amino- peptidases, which release the N-terminal amino acids successively. 28 29 Reference Book: Vasudevan, D. M., Sreekumari, S., and Kannan, V.., 2011. Textbook of biochemistry for medical students, 6th Edition. 30 Reference Book: Champe, P. C., Harvey, R. A. and Ferrier, D. R., 2005. Biochemistry “Lippincott’s Illustrated Reviews”, 5th or 6th Edition 31