Lecture 2 - 2024 Amino Acid Degradation PDF

Dr. Sarray Sameh Amino acid degradation Most of the absorbed dietary amino acids are catabolized by 2 subsequent steps: I. Removal of -amino group: the -amino group is removed in the form of ammonia (NH3). This occurs in most amino acids by transamination followed by oxidative deamination II. Brea...

Dr. Sarray Sameh Amino acid degradation Most of the absorbed dietary amino acids are catabolized by 2 subsequent steps: I. Removal of -amino group: the -amino group is removed in the form of ammonia (NH3). This occurs in most amino acids by transamination followed by oxidative deamination II. Breakdown of carbon skeleton Removal of α-amino group  The removal of α-amino group in the form of ammonia (NH3) occurs in 2 steps: -Transamination (that produce glutamate) followed by - Oxidative deamination to give ammonia.  Transamination: - Occurs primarily in the liver (the principal site of amino acid metabolism), but also in other tissues, such as the kidney, the small intestine, muscles, and adipose tissue. - Is the transfer of α-amino group from α-amino acid to α- ketoacid (mostly is α- ketoglutarate: amino group acceptor)) to produce a new amino acid (Glutamate) and α-keto acid (derived from the original amino acid). - The enzymes that catalyze transamination are called transaminases or aminotransferases - All amino acids participate in the transamination EXCEPT Lysine and Threonine which lose their amino groups by deamination. lysine and threonine directly go through deamination transamination. , not Substrate specificity of aminotransferases  Each aminotransferase is specific for one or a few amino group donors  Aminotransferases are named after the amino group donor  The most common examples on transaminases that are clinically important are: alanine aminotransferase (ALT) & aspartate aminotransferase (AST) Alanine aminotransferase (ALT)  The enzyme catalyzes the transfer of the amino group of alanine to α-ketoglutarate, resulting in the formation of pyruvate and glutamate  Bi-directional reaction; During amino acid catabolism, functions in the direction of glutamate synthesis transfer amino alanine - - to grp from ketoglutarate alanine will become Retoglutarate will pyruvate become glutamate Aspartate aminotransferase (AST)  AST catalyzed reversible reactions.  AST transfer the amino group from glutamate to oxaloacetate so the glutamate is converted into alpha-ketoglutarate and the oxaloacetate is converted into aspartate.  Aspartate is used as a source of nitrogen in the urea cycle Mechanism of action of aminotransferases active form of vitamin Do AST and ALT are PLP dependent. pyridoxal phosphate, a derivative of vitamin B6).     Pyridoxal phosphate covalently attached to the specific lysine aa at the active site of the enzyme. Aminotransferases transfer the amino group of an amino acid to the pyridoxal part of the coenzyme to generate pyridoxamine phosphate. The pyridoxamine then reacts with oxaloacetate to form Asp, at the same time regenerating the original form of the coenzyme. becomes & Diagnostic value of ALT and AST they are only ↑ low levels, It present so in when gets high something , s wrong  Aminotransferases are intracellular enzymes, found only in low levels in the plasma meaningwhenblood testshow high e a  The presence of elevated levels of aminotransferases indicate damage of cells rich in these enzymes; e.g. ALT and AST are present in the liver, so their elevation indicate liver damage such as hepatitis, toxic injury, and prolonged circulatory collapse Oxidative deamination of glutamate * removal of from amino grp amino acia  This reaction occurs in the liver and kidney  Deamination is a removal of amino group from amino acid, resulting in the formation of corresponding keto acids and ammonia.  The reaction produces α-ketoglutarate and ammonia (source of nitrogen in urea synthesis) for that , it needs NAD + as co-enzyme M  Reaction catalyzed by glutamate dehydrogenase (GDH) using NAD+ opposite of oxidative deamination  GDH catalyzed the reverse reaction called reductive amination using NADPH amount  Direction of reaction depends on the relative concentrations of glutamate, α-ketoglutarate, and ammonia. - Oxidative deamination : After ingestion of a meal containing protein, glutamate levels in the liver are nigh elevated, and the reaction proceeds in the direction of amino acid degradation and the formation of ammonia. high amonia : ammonia is toxic to Cells , so reductive amination will take Place to reduce amount , of ammonia in blood High ammonia levels are required to drive the reaction to glutamate synthesis  The glutamate dehydrogenase is regulated by allosteric effectors: GTP and ATP are inhibitors however GDP and ADP are activators Removal of ammonia & disposal routes The produced ammonia is toxic and it must be removed. The disposal routes: I. Excreted with urine 3 ways to clear body from ammonia 1 pee 2.. glutamate + ammonia-glutamine II. React with glutamate yielding glutamine by glutamine synthetase 3. form urea in live III. Used in formation of urea in the liver: this is the most important disposal route of ammonia  TWO mechanisms of transport of ammonia from the peripheral tissues to the liver for its ultimate conversion to urea 1st Mechanism: - Used by most tissues. - Combine ammonia with Glutamate to form glutamine (non-toxic form of ammonia) by glutamine synthetase ammonia +  glutamate = glutamine Glutamine is transported into blood then to the liver where it is cleaved by glutaminase to produce Glutamate and free ammonia /giutamate glutamine ammonia  Ammonia is then converted to urea. one way to have for the muscles , by amino acid into glucose converting sugar 2nd mechanism: - Used primarily by the muscle - Formation of Alanine by transamination of pyruvate using ALT. - Alanine is transported by blood and then to the liver where is converted to pyruvate by transamination. - Pyruvate is used to synthesize glucose by gluconeogenesis, which can enter blood and be used by muscle. This pathway is called glucose-alanine cycle 2 formation of alanine alanine converted to pyruvate. 3 Pyruvate enter blood and be 1. by muscles used (gircose-alanine cycle) Liver catabolizes toxic ammonia into urea. Urea is synthesized in the liver in 5 steps by 5 enzymes: The first 2 reactions occur in Mitochondria while steps 3,4 and 5 occur in cytoplasm  i ons + 102 ammonium = Carbomoyl Phosphate enzyme : CPS-1 consume 2 ATP Mitochondria Step 1: Formation of Carbamoylphosphate: - Irreversible step & rate limiting step - Ammonium ions reacts with CO2 to form carbamoyl phosphate by the Carbamoyl phosphate synthetase-I (CPS-I) - This step consumes two molecules of ATP - x Carbomovl Phosphate + ornithine - = Citrulline enzyme: ot( - Citrulline then transported to cytosol enzyme : oruitune translocase Mitochondria Step 2- Formation of citrulline: ◦ Carbamoyl phosphate combines with ornithine to form citrulline by ornithine transcarbamylase (OTC) ◦ Then, citrulline is transported to the cytosol by an ornithine translocase Cytosol Step 3- Synthesis of argininosuccinate: o Citrulline condenses with Aspartate (the source of the second amino group on urea) to form argininosuccinate. Reaction is catalyzed by argininosuccinate synthetase and consumes 1 ATP ((the total of ATP consumed in the formation of urea to 3 molecules). - Citrulline + aspartate enzyme : consume = arginosuccinate organinosuccinate synthetase 1 ATP total so far : 3ATP Transport to the cytosol organinosuccinate organine fumarate Step 4- Cleavage of argininosuccinate  Argininosuccinate is cleaved by argininosuccinate lyase to yield arginine and fumarate cleared by  argininosuccinate lase The fumarate (intermediate of TCA cycle) is hydrated to malate, oxidized to oxaloacetate , and transaminated to Aspartate Step 5- formation of urea organine cred ornithine enzyme used :  arginase Arginine is cleaved by arginase to yield urea and regenerate ornithine which enters mitochondria for its reuse in the urea cycle.  Arginase is mostly found in the liver, while the rest of the enzymes (4) of urea cycle are also present in other tissues  Arginine synthesis may occur in many tissues BUT ONLY the LIVER CAN ULTIMATELY PRODUCE UREA. Urea Cycle Carbamoyl phosphate synthetase (CPS-I) is rate limiting enzyme in urea cycle.  CPS-I is activated by N-Acetylglutamate (NAG)  glutamate + acetyl  COA = N-acetylglutamate NAG is synthesized from glutamate and acetyl Co A by N-acetylglutamate synthase. - The rate of urea synthesis in liver is correlated with the concentration of NAG (NAG urea synthesis ) higher The consumption of a protein rich meal increases the level of NAG in the liver leading to enhanced urea synthesis. NAG - a theres activation of CPS-1 < urea cycle occur I. Diffuse from liver, transported in blood to kidney and excreted in the urine. Urea is used as kidney function test: increased blood urea indicate renal disease. II. A portion diffuse from blood to intestine and cleaved by bacterial urease into ammonia and CO2. ammonia passes with stool Portion of cleared by crease urea is producing. ammonia and CO2 ammonia then released with Strol END!