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2. Transamination (Biosynthesis of Amino Acids)

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2. Transamination (Biosynthesis of Amino Acids) 2. Transamination (Biosynthesis of Amino Acids) Amino acids in the pool (from diet or tissue proteins) are degraded releasing amino groups and carbon skeletons. Amino groups: help to synthesize new amino acids (transamination) or other N- containing molecules and if in excess released as urea (urea cycle). Carbon skeletons: synthesize glucose, fatty acid or ATP. Biochemistry (FT 5103) 1 Amino Acid Metabolism Tissue proteins Amino alcohols Nucleotides Dietary protein Amino acids Purines Pyrimidines Glucose 1 Carbon Urea Pyruvate NH + Urea skeleton 4 Cycle 2 TCA Acetyl-CoA Cycle ATP 1 Glucogenic AA Fatty Ketone acids bodies 2 Ketogenic AA Biochemistry (FT 5103) 2 Metabolic Classification of AA (1) Essential and Non-essential AA (2) Glucogenic and Ketogenic AA Non-essential AA Not required in diet Can be formed from α-keto acids by transamination and subsequent reactions • Alanine • Glycine • Asparagine • Proline • Aspartate • Serine • Glutamate • Cysteine • Glutamine • Tyrosine Biochemistry (FT 5103) 3 Essential AA Required in diet Humans incapable of forming requisite carbon skeleton • Arginine* • Lysine • Histidine* • Methionine • Isoleucine • Threonine • Leucine • Phenylalanine • Valine • Tryptophan * Essential in children, not in adults Biochemistry (FT 5103) 4 Glucogenic AA Metabolized to pyruvate, α-ketoglutarate, succinyl CoA, fumarate or oxaloacetate Phosphoenolpyruvate Glucose • Aspartate • Methionine • Alanine • Asparagine • Valine • Serine • Arginine • Glutamine • Cysteine • Phenylalanine • Glutamate • Glycine • Tyrosine • Proline • Threonine • Isoleucine • Histidine • Tryptophan Biochemistry (FT 5103) 5 Ketogenic AA Metabolized to acetyl CoA or acetoacetyl CoA Animals cannot convert acetyl CoA or acetoacetyl CoA to pyruvate (therefore not produce glucose) Produce ketone bodies or ATP through TCA cycle • Leucine • Lysine Biochemistry (FT 5103) 6 Oxidative deamination: Removes the amino group as an ammonium ion from glutamate. Provides -ketoglutarate for transamination. + NH3 | - - + OOC-CHCH2CH2COO + NAD + H2O Glutamate Glutamate dehydrogenase O || - - + OOC-CCH2CH2COO + NH4 + NADH α-Ketoglutarate + During digestion of AA in the liver, NH4 ions are formed. Biochemistry (FT 5103) 7 + Incorporation of NH4 (from AA) into Organic Compounds 1) Carbamoyl Phosphate + - -2 NH4 + HCO3 + 2 ATP Synthase I NH2CO2PO3 + 2 ADP + (Carbamoyl (CPS-I) + Phosphate) Pi + 2 H 2) TCA Cycle mitochondria Glutamate + O dehydrogenase NH3 + - - - - NH4 + O2 CCH 2 CH2 CCO 2 O2CCH 2CH2CHCO 2 (α-Ketoglutarate) (Glutamate) NADPH + NADP+ H+ 3) + Glutamine O NH3 Synthase NH + + H NCCH CH CHCO - 4 Mg++ 2 2 2 2 2 ATP (Glutamate) (Glutamine) N of glutamine donated to other compounds in synthesis of purines, pyrimidines, and other amino acids Biochemistry (FT 5103) 8 In transamination: An amino group is transferred from an amino acid to an -keto acid, usually -ketoglutarate. The reaction is catalyzed by a transaminase or aminotransferase. (mitochondria, cytosol) A new amino acid, usually glutamate, and a new -keto acid are formed. Amino Acid1 +-Keto Acid2 Amino Acid2 +-Keto Acid+ 1 O NH3 - - - - O2 CCH 2 CH2 CCO 2 O2CCH 2CH2CHCO 2 O - + R-CCO 2 (Glutamate) Glutamate Transaminases/ dehydrogenase Aminotransferase (Coenzyme is pyridoxal PO4) NH 2 + - (-Ketoglutarate) R-CHCO 2 Biochemistry (FT 5103) 9 Eg: Glutamate-Pyruvate Glutamate Aminotransferase -Ketoglutarate + + Pyruvate (Alanine Transferase / Alanine ALT) Glutamate-Oxaloacetate Glutamate Aminotransferase -Ketoglutarate + + Oxaloacetate (Aspartate Transferase / Aspartate AST) All AA except Thr & Lys can undergo transamination with α-ketoglutarate. Equilibrium of reaction is close to 1 therefore reaction direction depends on the [reactants] which are directed by other cellular processes. Therefore, remove or add AA to the pool. Urea synthesis directs reactions by withdrawing amino groups from the AA pool (which increase deamination and AA catabolism). Biochemistry (FT 5103) 10 Role of transamination (i) Redistridution of amino groups to balance AA pool Dietary proteins provide a mixture of AA whose proportions differ from AA pool required by body Creates a correct imbalance (ii) AA synthesis / degradation performed in conjunction with glutamate dehydrogenase (GDH) GDH can remove or add amino groups to the AA pool Most -amino groups glutamate due to the action of transaminases When there is a surplus of AAs in the pool, the amino groups can be funneled through + glutamate and released as NH4 Biochemistry (FT 5103) 11 By coupling GDH reaction with many different aminotransferases, produces large variety of AA Many amino Corresponding acids α-Keto acids Transaminases α-Ketoglutarate Glutamate GDH + NADH + NH4 NAD+ The release of amino groups as NH4+ is catalysed by glutamate dehydrogenase through oxidative deamination. Since the reaction is reversible it can also synthesize amino groups..
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