Nitrogen Metabolism Ch 18 PDF

Date: /  Nitrogen Metabolism Disposal of nitrogen. Urea cycle. Catabolism of the carbon skeleton. N-containing substances. 1  Amino Acids Metabolism: Disposal of Nitrogen. Amino acids can’t be stored, excess a.a (more than the needs o...

Date: /  Nitrogen Metabolism Disposal of nitrogen. Urea cycle. Catabolism of the carbon skeleton. N-containing substances. 1  Amino Acids Metabolism: Disposal of Nitrogen. Amino acids can’t be stored, excess a.a (more than the needs of cells in the protein synthesis or other compound) are catabolized and degraded immediately. Catabolism of a.a has two phases: 1. Phase I: removal of α-amino group and forming NH4+ and α-keto acid. By process called transamination and oxidative deamination. The amino group: 1. Recycled in biosynthesis of a. a. 2. Excreted in urine. 3. Formation of urea  excreted in urine The most important rout for disposal of N is urea. 2. Phase II: is carbon skeleton of the α-ketoacids are converted into common intermediates of energy producing metabolic pathway. So can be metabolized into CO2, H2O, glucose, fatty acids, … 2 The fate of N2 in different organisms 3 Digestion of dietary proteins 4 After the complete digestion, free a.a and dipeptides are absorbed by the epithelial cells in which dipeptides are hydrolyzed to a.a in the cytosol before entering the portal circulation. Transport of a.a into cells by active transporters 5 Overall nitrogen metabolism Amino acids are precursors of nitrogen-containing compounds Amino acid catabolism is part of nitrogen metabolism in the body. N2 enters the body (by food) in different forms  converted to a. a and then exits from the body in form of urea and little amounts of NH4+. 6  Metabolic fates of amino groups: Transamination and Oxidative deamination The first step of a.a catabolism is the transfer of the α-amino group to α- ketoglutarate. The product is α-keto acid and glutamate. This process is called transamination and is mediated by aminotransferase -- Transamination: the funneling of amino groups to glutamate. Occurs in the cytosol of the hepatocytes Donor of NH2 gp in the biosynthesis of non-essential a.a The product Glutamate Oxidative deamination  release of NH4+ 7  Substrate specificity of aminotransferase: Each aminotransferase is specific for one or few a.a, they can be named by a.a donor. Because almost the acceptor is α-ketoglutarate. Equilibrium of transamination reactions Most of the transamination reactions have an equilibrium constant near 1, allowing the reaction to proceed in both a.a degradation and biosynthesis depending on the relative concentrations. 8  Mechanism of action of aminotransferase - All aminotransferases need pyridoxal phosphate derivatives of vit B6. - All a.a except threonine and serine participate in transamination. In Lippincot’s (lysine and threonine) 9  Threonine and serine Dehydratase enzyme (dehydration and then hydration with deamination. 10 Two important transferases: Alanine aminotransferas (ALT) called also Glutamate–Pyruvate transferase (GPT), found in many tissues catalyzes the transfer of amino gp of alanine to produce pyruvate and glutamate. Aspartate aminotransferase (AST) called also Glutamate–Oxaloacetate transferase (GOT). During the catabolism of a. a AST takes amino group from glutamate to oxaloacetate forming aspartate. Which used as source of NH4 gp in Urea synthesis. Aspartate  source of NH4+ in the urea cycle. Diagnostic value of plasma aminotransferases Plasma AST = SGOT Plasma ALT = SGPT 11  Oxidative deamination Amino groups of many a.a are collected in the liver in the form of the amino group of L-glutamate. Glutamate can be used as a donor of amino group in the biosynthesis of non-essential a.a. In hepatocytes, glutamate is transported from the cytosol into mitochondria, where it undergoes Oxidative deamination catalyzed by L-glutamate dehydrogenase. The combined action of aminotransferase and glutamate dehydrogenase is referred as transdeamination. Undergoes rapid oxidative 12 deamination  Transdeamination The combined action of aminotransferase and glutamate dehydrogenase is referred as transdeamination. The over all reaction of a.a catabolism Transamination Reaction Oxidative Deamination Transdeamination 13  Oxidative deamination Regulation of oxidative deamination Direction of reaction: 1. Depends on the relative concentration of glutamate, α- ketoglutarate and ammonia, the ratio of oxidized to reduced coenzymes. After high protein meal  increase glutamate  increase ammonia production. 2. Allosteric regulation of Glutamate-dehydrogenase ATP, GTP = inhibitors ADP, GDP = activators Low level of energy increase catabolism of a.a α-ketoglutarate as substrate for TCA cycle. 14  Oxidative deamination The enzyme glutamate dehydrogenase presents in mitochondrial matrix and can use either NAD+ or NADP+ as oxidants. The oxidative deamination results in: Liberation of the amino group as free ammonia. Occur primarily in the mitochondria of liver and kidney and provide α-ketoacid. 15  Glutamine transports ammonia Many extrahepatic tissues (brain) produce NH4+ from metabolic processes as nucleotide degradation. This toxic ammonia is converted into amino group of glutamine that transported to liver or kidneys. Glutamine: non-toxic transport form of NH4+ and also source of amino group in many biosynthesis reactions. The amide nitrogen of glutamine is released as ammonia only in liver and kidney’s mitochondria by the enzyme “Glutaminase” which convert glutamine into glutamate + NH4+ Glutamine glutaminase Glutamate + NH4+ glutamate urea dehydrogenase 16 α-ketoglutarate + NH4+  “Glucose-alanine cycle” Alanine transports ammonia from muscles to liver. In muscle, a.a are degraded, the amino groups are collected in form of glutamate by transamination. α-amino group can transferred to pyruvate (resulted from glycolysis) by enzyme Alanine Amino Transferase (ALT). Alanine is reconverted into pyruvate in the cytosol of hepatocytes and enters the gluconeogenic pathway to produce glucose. In the liver the formed glutamate enters the mitochondria where glutamate dehydrogenase releases NH4+ 17 18 Synthesis of nitrogen- containing compounds Synthesis of non-essential a.a 19  Urea Cycle Urea is the major disposal form of amino group derived from a.a. One nitrogen is supplied by free NH4+ and the other from Aspartate. Glutamate is the immediate precursor of both ammonia through oxidative deamination and by aspartate aminotransferase. Carbon and Oxygen are derived from CO2 Urea is produced in the liver then transported in the blood to the kidneys for excretion in the urine. The first two reactions lead to the synthesis of urea occur in mitochondria where the remaining cycle enzymes are located in cytosol. 20  Formation of carbamoyl phosphate The enzyme has an absolute Carbamoyl phosphate requirement for N- synthetase I acetylglutamate which act as an allosteric activator. Resulted from oxidatative Carbamoyl phosphate deamination of glutamate synthetase II, participates in biosynthesis of pyrimidines, does not require N- acetylglutamate and located in the cytosol. 21 22 Formation of N-acetylglutamate N-acetylglutamate is essential activator of carbamoyl phosphate synthetase I, which catalyzes the rate limiting step in urea cycle. The intrahepatic concentration of N-acetylglutamate increases after ingestion of a protein-rich meal  increase of urea synthesis. 23 Urea Cycle 24 Urea Cycle 25  Overall stoichiometry of the urea cycle Aspartate + NH3 + CO2 + 3 ATP + H2O → urea + fumarate + 2 ADP + AMP + 2 P i + PPi 26 Urea Cycle 27 The citric acid and urea cycles are linked 28 The citric acid and urea cycles are linked 29  Fate of urea Urea diffuses from liver and transported to kidneys  excreted to urine. Portion of the urea diffuses from blood to intestine and is cleaved to CO2 + NH3 by bacterial urease. Ammonia is lost by feces and little is reabsorbed by blood and excreted by kidney in urine. Kidney failure: NH4+ in blood is elevated.  Ammonia toxicity. 30  Metabolism of Ammonia If not used in the synthesis of new a.a or other nitrogenous compound it should exit from the body because it is very toxic to the CNS. Sources of ammonia 1. Liver produces ammonia from a.a by aminotransferases and glutamate dehydrogenase. 2. Renal glutaminase produces from glutamine  NH4+ is released. 3. Bacteria action in the intestinal. 4. From amines: amines obtained from diet and ammonia can be produced by amine oxidase. 5. The catabolism of purines and pyrimidines: in of purines and pyrimidines. 31 Metabolism of Ammonia 32  Metabolism of Ammonia Transport of ammonia in the circulation Ammonia is continuously produced by tissues, but it is rapidly removed from the body in form of urea which is the most important disposal route for ammonia travels from liver to kidneys Glutamine: provides non-toxic storage and transport form of ammonia. Glutamine occurs in skeletal muscle, liver and brain and hydrolyzed to give NH4+ in the kidney by “glutaminase”. 33  The catabolism of the branched-chain amino acids Isolecine, Leucine, Valine are essential a.a Can be metabolized by peripheral tissues mainly skeletal muscles rather than by the liver. Catabolism of these a.a 1. Transamination: catabolized by branched-chain α-amino acid transferase to produce α-keto acids. 2. Oxidative decarboxylation: removing the carboxyl group (COO-) derived from the a.a. Catalyzed by branched-chain α-ketoacid dehydrogenase. Deficiency of this enzyme  accumulation of α-acids  maple-syrup urine disease. 3. Dehydrogenation. 4. End product. Isoleucine  acetyl CoA + succinyl CoA Leucine  acetyl CoA + acetoacetate Valine  succinyl CoA 34  The catabolism Carbon Skeleton Amino Acids According to the nature of metabolic end product amino acids are classified into Glucogenic and ketogenic amino acids. Ketogenic: acetoacetate or acetyl CoA. Leucine and lysine are the only exclusively ketogenic amino acids. Glucogenic: pyruvate or one of the intermediates of citric acid cycle, and these intermediates are also substrate for gluconeogenesis 35  The catabolism Carbon Skeleton Amino Acids The catabolism of carbon skeleton of amino acids. Normally 10 – 15% of energy is from proteins. The catabolism of carbon skeletons of a.a can forms seven products: 1. Oxaloacetate. 2. α-ketoglutarate. 3. Pyruvate. 4. Fumarate. 5. Acetyl CoA. 6. Acetoacetyl CoA. 7. Succinyl CoA These products end with production of glucose or fat or energy by 36 entering citric acid cycle. 37 38 39 The End 40