Protein Metabolism (Nursing)

٢ Any excess protein intake is not stored in the body but undergoes catabolism (degradation). About 300 g of body proteins, principally muscle proteins, undergo breakdown and resynthesis each day. Of the liberated amino acids about 75%- 80% are reutilized for protein synthesis. The remaining 20%- 2...

٢ Any excess protein intake is not stored in the body but undergoes catabolism (degradation). About 300 g of body proteins, principally muscle proteins, undergo breakdown and resynthesis each day. Of the liberated amino acids about 75%- 80% are reutilized for protein synthesis. The remaining 20%- 25% undergo deamination and final excretion, mostly in the form of urea. ٣  Body proteins Resynthesis Breakdown 75-80% of 1-2% (∼ 300g) Amino acids pool Deamination 20-25% ٤ We need to eat proteins to compensate for the catabolized amino acids. The recommended dietary allowance (RDA) of protein for adults is 0.8 g/kg body weight/day, i.e., about 56 g/day for a 70-kg subject. The turnover rate varies widely for individual proteins. ٥  Overall protein metabolism Urea Fat Glycogen Body proteins NH3 Glucose Dietary Carbon CO2 + Amino acid pool skeleton proteins H 2O + ∼ 100 g energy Ketone bodies Synthesis of non- essential amino acids ▪ Heme Fat, sterol ▪ Creatine ▪ Purines ▪ Pyrimidines ▪ Neurotransmitters ▪ Hormones ▪ Melanin ▪ Niacin ▪ Other nitrogenous compounds ٦ Amino acid pool: Amino acids released by hydrolysis of dietary or tissue protein, or synthesized de novo, mix with other free amino acids distributed throughout the body. The amino acid pool, contains about 100 g of amino acids. ٧  Digestion of proteins In the stomach 1. Gastric HCL It produces denaturation of proteins which causes unfolding of the polypeptide chains. 2. Pepsin – It is secreted by the chief cells in the form of inactive zymogen, pepsinogen. Pepsinogen is activated by HCl. HCl – Pepsinogen ⎯⎯⎯⎯⎯→ Pepsin + Polypeptide. ٨ 3. Rennin – It is present in infants. It converts casein of milk to paracasein which in presence of calcium it forms ca-paracaseinate or milk clot. – Milk clot prevents the rapid passage of milk through stomach and intestine which allows good time for digestion and absorption of milk constituents. – Paracasein is further digested by pepsin and other proteases. ٩  In the small intestine Digestion in small intestine is due to the action of proteases present in both pancreatic and intestinal secretions. Pancreatic enzymes 1. Trypsin 2. Chymotrypsin 3. Elastase 4. Carboxypeptidase Intestinal enzymes They complete digestion of proteins as follows: 1. Aminopeptidase 2. Tripeptidases and dipeptidases Absorption of Amino acids L-amino acids are rapidly absorbed from the small intestine by a process of active transport. ١٠ Dietary protein Pepsin Polypeptides and amino acids Stomach Trypsin Chymotrypsin Carboxypeptidase Elastase To liver Pancreas Oligopeptides and amino acids Aminopeptidases Small intestine Tripeptidase Dipeptidase Amino acids Digestion of dietary proteins by the proteolytic enzymes of the gastrointestinal tract. ١١  Fate of absorbed amino acids These mix with the amino acids produced from hydrolysis of body protein and those synthesized in the body to form a common amino acid pool (about 100g). This pool is drawn upon for anabolism and for catabolism of amino acids. Catabolic fates Anabolic fates ١٢ Anabolic fates These include the synthesis of proteins, e.g., tissue, milk, and plasma proteins, enzymes, and some hormones. They also include the synthesis of other nitrogenous substances, e.g., glutathione, adrenaline, thyroxine, melanin, niacin, purines, pyrimidines, aminosugars, and the nitrogenous bases of phospholipids. ١٣  Catabolic fates Most catabolic reactions are preceded by cleavage of the amino acids into ammonia and the carbon skeleton (usually in the form of an α-keto acid). The ammonia is mostly converted to urea, which is excreted in urine. Little ammonia is also excreted in urine. The carbon skeleton may be converted to glucose or ketone bodies, or may be oxidized to carbon dioxide and water. ١٤ Nitrogen balance The nitrogen balance is the quantitative difference between the nitrogen intake and output. The balance between the two represents the balance between protein anabolism and catabolism. Three States may exist: 1- Nitrogen equilibrium. 2- Positive nitrogen balance. 3- Negative nitrogen balance. ١٥ 1- Nitrogen equilibrium It exists when output equals intake. It occurs in the normal healthy adult on an adequate diet. 2- Positive nitrogen balance It exists when intake exceeds output. It occurs whenever new tissues are built, e.g., during growth, pregnancy, muscular training, and convalescence from states of negative nitrogen balance. ١٦ 3- Negative nitrogen balance Inadequate protein intake – This occurs in cases of starvation, malnutrition, and gastrointestinal diseases. Loss of protein – This occurs in cases of chronic hemorrhage, albuminuria, and during lactation on an inadequate diet. Increased protein catabolism – This occurs in cases of diabetes mellitus, Cushing's syndrome, hyperthyroidism, and infectious fevers. ١٧  ١٨ Contents Transamination…………………………………………… Oxidative deamination………………………………….. Transdeamination………………………………………… Fate of products of deamination……………………… 1. Ammonia……………………………………………….. 2. Carbon skeleton………………………………………. ١٩ General method applies to most amino acids. It primarily includes two mechanisms Transamination Oxidative deamination However, neither of these mechanisms, alone, can meet the physiologic requirements of the body, so, a combination of the two mechanisms has been suggested as a possible mechanism, i.e. Transdeamination ٢٠ Transamination ٢١ Transamination is the transfer of an amino group usually from an α- amino acid to an α-keto acid forming a new α-amino acid and a new α-keto acid. The enzymes that catalyze this type of reaction are called aminotransferases and also called transaminases. They are present in the CYTOSOL and mitochondria of all tissues especially the liver. R.CH.COOH HOOC.CH2.CH2.C.COOH I II ☞ NH2 O transaminase α-Amino acid α-Ketoglutaric acid R.C.COOH HOOC.CH2.CH2.CH.COOH II I O NH2 ☞ α-Keto acid Glutamic acid ٢٢ The reaction is reversible, so it is important in deamination and in the biosynthesis of the nonessential amino acids. It requires pyridoxal phosphate (PLP) as intermediate carrier of the amino group. Transamination involves neither the uptake nor the release of free ammonia. The two most important and active aminotransferase reactions are catalyzed by 1.alanine aminotransferase and 2.aspartate aminotransferase ٢٣ Diagnostic value of plasma aminotransferases Aminotransferases are normally intracellular enzymes, with the low levels found in the plasma representing the release of cellular contents during normal cell turnover. The presence of elevated plasma levels of aminotransferases indicates damage to cells rich in these enzymes. Two aminotransferases AST and ALT are of particular diagnostic value when they are found in the plasma: 1. Liver disease: Plasma AST and ALT are elevated in nearly all liver diseases, but are particularly high in conditions that cause extensive cell necrosis, such as severe viral hepatitis, toxic injury, and prolonged circulatory collapse. 2. Nonhepatic disease: Aminotransferases may be elevated in nonhepatic disease, such as myocardial infarction and muscle disorders. ٢٤ Oxidative deamination Enzymes concerned with oxidative deamination of amino acids in the body, are: L-glutamate dehydrogenase ٢٥  Fate of products of deamination Ammonia Carbon skeleton ٢٦ Fate of the ammonia removed ٢٧ The major source of ammonia in the body is : The deamination of amino acids. Other minor sources include the deamination of pruines and pyrimidines as well as the catabolism of the latter. In addition, bacterial putrefaction in the intestines produces small amounts of ammonia, which become of great importance in cases of liver cirrhosis. The fate of ammonia will be discussed under 2 headings: (1) anabolic pathways. (2) catabolic and excretory pathways. ٢٨ (1) Anabolic pathways The ammonia removed by deamination may be used for the synthesis of some compounds. Actually, virtually all the nitrogenous compounds in the body are derived from amino acids. The two reactions for fixation of ammonia in organic compound are catalyzed by: 1. Glutamate dehydrogenase. 2. Glutamine synthetase. ٢٩  (2) Catabolic and excretory pathways Being highly toxic to tissues, the ammonia produced in excess of the requirements for anabolic purposes is rapidly disposed of. The method of disposal depends upon the tissue in which deamination occurs (liver, extrarenal , renal and muscles). 1-In the liver The liver is the main site of deamination of amino acids. Most of the ammonia released and that come from other tissues in the form of glutamine or alanine are converted to urea. Urea is the major disposal form of ammonia. The urea formed goes via the blood to the kidneys to be excreted in urine. ٣٠ 2. In extrarenal tissues The ammonia resulting from the deamination of amino acids in extrarenal tissues, particularly the brain, is converted to glutamine as follows: Glutamine synthetase HOOC.CH2.CH2.CH.COOH HOOC.CH.CH2.CH2.CO-NH2 I Mg2+ I NH2 NH3 ATP ADP + Pi NH2 L-Glutamic acid Glutamine glutamine goes, via the blood, to the kidneys where it becomes hydrolyzed by glutaminase into glutamic acid and ammonia. The ammonia is excreted in the urine, accounting for about 60% of urinary ammonia, or to the liver ,transporting ammonia for urea synthesis. Glutaminase HOOC.CH.CH2.CH2.CO-NH2 HOOC.CH2.CH2.CH.COOH I I NH2 H 2O NH3 NH2 Glutamine L-Glutamic acid ٣١  3. In the kidneys The ammonia resulting from the deamination of amino acids in the kidneys is directly excreted in the urine. This accounts for about 40% of the urinary ammonia. “Urinary ammonia comes from deamination of amino acids in the kidney and from the action of glutaminase on glutamine coming from extrarenal tissues”. 4. In muscles A good part of the ammonia is converted to alanine, which goes to the liver. ٣٢ Transport of ammonia to the liver Two mechanisms are available in humans for the transport of ammonia from the peripheral tissues to the liver for its ultimate conversion to urea: 1. The first, found in most tissues, uses glutamine synthetase to combine ammonia with glutamate to form glutamine,a nontoxic transport form of ammonia. 2. The second transport mechanism, used primarily by muscle, involves transamination of pyruvate (the end-product of aerobic glyclosysis) to form alanine. ٣٣  Brain Liver Amino acids NH3 Urea Glutamine Alanine Kidney Muscle NH3 ٣٤ ٣٥  Importance and site of urea synthesis ٣٦ This is the principal pathway of disposal of ammonia resulting from the deamination of amino acids. It allows the body to get rid of about 80-90% of the amino groups of amino acids in a neutral non-toxic form. Urea has no role in animal metabolism other than an end product that is excreted. Urea formation occurs only in the liver. ٣٧  Regulation of urea cycle Fine regulation Gross regulation ٣٨ I- Fine regulation Through the rate-limiting step of the urea cycle & allosteric activator II- Gross regulation An increase in the supply of amino acids to the liver, such as occurs on a high protein diet, during the administration of the protein catabolic hormones, and during prolonged starvation, increases the synthesis of the urea cycle enzymes 10-20-fold. This increases urea formation. ٣٩  Fate of urea ٤٠ The urea formed by the liver goes, via the blood (plasma level 10-50 mg/dL), to the kidneys to be excreted in the urine. In renal failure the plasma level of urea increases. In hepatic failure ammonia remains in the blood, leading to hyperammonemia and ammonia intoxication. ٤١  Hyperammonemia ٤٢ When the liver function is affected, due either to genetic defects of the urea cycle, or liver disease, blood levels of amonia can rise above 1000 mmol/L. Such hyperammonemia is a medical emergency, because ammonia has a direct neurotoxic effect on the CNS. Cause the symptoms of ammonia intoxication, which include tremors, slurring of speech, somnolence, vomiting, cerebral edema, and blurring of vision. At high concentrations, ammonia can cause coma and death. ٤٣  ٤٤ Fates of carbon skeleton of amino acids ٤٥ The catabolism of the amino acids found in proteins involves the removal of α-amino groups, followed by the breakdown of the resulting carbon skeletons. These pathways converge to form seven intermediate products: pyruvate, α-ketoglutarate, succinyl CoA, fumarate, oxaloacetate, acetyl CoA, and acetoacetyl CoA. These products directly enter the pathways of intermediary metabolism, resulting either in the synthesis of glucose or lipids, or in the production of energy through their oxidation to CO2 and water by the citric acid cycle. ٤٦ A. Glucogenic amino acids Amino acids whose catabolism yields pyruvate or one of the intermediates of the citric acid cycle are termed glucogenic or glycogenic.(e.g. Glycine Cysteine Serine Alanine Threonine Tryptophan) These are substrates for gluconeogenesis and, therefore, can give rise to the net formation of glucose or glycogen in the liver and glycogen in the muscle. ٤٧ B. Ketogenic amino acids Amino acids whose catabolism yields either acetoacetate or one of its precursor, (acetyl CoA or acetoacetyl CoA) are termed ketogenic. Acetoacetate is one of the ketone bodies, which also include 3- hydroxybutyrate and acetone. Leucine and lysine are the only exclusively ketogenic amino acids found in proteins. Their carbon skeletons are not substrates for gluconeogenesis. ٤٨ C. Both glucogenic and ketogenic Isolucine, phenylalanine, tyrosine and tryptophan are glucogenic and ketogenic amino acids as part of its carbon skeleton can give glucose, the other can give ketone bodies ٤٩  Phenylalanine Tyrosine ٥٠ Hydroxylation of phenylalanine leads to the formation of tyrosine. Thus, the metabolism of phenylalanine and tyrosine merge, leading ultimately to the formation of fumarate and acetoacetate. Phenylalanine and tyrosine are, therefore, both glucogenic and ketogenic. L-Phenylalanine Tetrahydrobiopterin NADP+ + O2 Phenylalanine hydroxylase Dihydrobiopterrin NADPH + H 2O L-Tyropsine Fumarate Acetoacetate ٥١  Phenylketonuria (PKU) Classical PKU is a deficiency of phenylalanine hydroxylase caused by an autosomal recessive gene. The name comes from the excretion of phenylpyruvic acid (a phenylketone) in the urine. An oxidation product of phenylpyruvate, phenylacetate, is also excreted and gives the urine a “mousey” odor. Normal Phenylketonuria Phenylpyruvate Phenyllactate Phenylpyruvate Phenyllactate Phenylacetate Phenylacetate Phenylalanine Tissue proteins Phenylalanine Tissue proteins Melanin Melanin Thyroid hormones Thyroid hormones Tyrosine Tyrosine Catecholamines Catecholamines Fumarate Fumarate Acetoacetate Acetoacetate ٥٢ Phenylketonuria blood test ٥٣