Amino Acid Metabolism 2024 PDF

Amino acid metabolism Learning objectives 1. Explain the significance of amino acid oxidation 2. Recognize the reactions steps for NH3 removal and the urea cycle (enzymes, metabolites, chemical changes, flow of C and N) 3. Briefly understand the oxidation of the carbon skeleton in diff...

Amino acid metabolism Learning objectives 1. Explain the significance of amino acid oxidation 2. Recognize the reactions steps for NH3 removal and the urea cycle (enzymes, metabolites, chemical changes, flow of C and N) 3. Briefly understand the oxidation of the carbon skeleton in different amino acids – note the flow of C, N, and S 4. Explain the significance of amino acid biosynthesis 5. Recognize the reaction steps for NH3 assimilation 6. Briefly understand biosynthesis of non-essential amino acids – note the flow of C, N, and S  Amino acid catabolism - During times of starvation, amino acids are used to replenish TCA cycle intermediates and used as precursors for gluconeogenesis - Organisms with a diet rich in proteins can oxidize excess amino acids as fuels - Excess amino acids are not stored – they are catabolized (i.e. oxidized) - In animals, amino acids released from proteins are the major sources of nitrogen. - What is the key difference between amino acids and the other 2 types of oxidizable biomolecules (i.e. carbohydrates and fatty acids)? 2  Removal of amino group from amino acids Removal of ammonia 3 Transamination in liver (cytosol): - The first step in the catabolism of most amino acids Universal amino group acceptor PLP : pyridoxal phosphate (prosthetic group) 4 Oxidative deamination of glutamate in liver (mitochondrial matrix): Dehydrogenation (oxidation) Glutamate Dehydrogenase deamination urea 5 Non-liver (non-hepatic) tissues: Glutamate Degradation of amino acids, nucleotides, etc. Amino group Glutamine 6 Amide group (transport in bloodstream) Transport of glutamine to liver for deamination: Oxidative deamination α – Ketoglutarate + NH4+ 7 Urea Removal of ammonia as urea in liver Formation of carbamoyl phosphate - Ammonia generated in liver is condensed with CO2 (as HCO3-) - Enzyme: carbamoyl phosphate synthetase I - Mitochondrial matrix ATP 8 The Urea Cycle - 1: in matrix - 2-4: in cytosol - Urea is circulated through bloodstream to kidneys for excretion in urine. Ornithine transcarbamoylase Argininosuccinate Arginase synthetase Argininosuccinate lyase 9  Sources of nitrogen in urea Liver: Glutamine (from non-liver tissues) Glutamate glutaminase (α-KG) Amino NH2 α-KG acid Urea cycle Glutamate α-Keto acid 10 (See p. 11) Linkage between the urea cycle and the TCA cycle Cytosolic fumarase transamination OAA 2ATP HCO3- + NH3 fumarase 11 Metabolism of carbon skeleton 12 Oxidation of carbon skeletons in amino acids Glucogenic amino acids - Degraded to pyruvate, oxaloacetate, fumarate, succinyl-CoA, or α- ketoglutarate, and may be further degraded to CO2 and H2O to yield ATP (complete oxidation) - Precursors for glucose (gluconeogenesis) Ketogenic amino acids - Degraded to acetyl-CoA or acetoacetate, and may be further degraded to CO2 and H2O to yield ATP (complete oxidation) - Precursors for fatty acids or ketone bodies Some amino acids are both glucogenic and ketogenic 13 Glutamate, Glutamine, Histidine, Proline, and Arginine - Degradation to -ketoglutarate - Deamination or transamination - Ring cleavage Deamination - 1C group transfer Transamination (tetrahydrofolate) THF - A cofactor involved in 1-C group transfer Deamination Oxidative deamination 14 Asparagine and Aspartate - Degradation to oxaloacetate (OAA) Asparaginase aspartate Aminotransferase 15 Alanine, Cysteine, Glycine, Serine, and Threonine - Degradation to pyruvate Threonine - Carbon skeleton is degraded to pyruvate (through glycine) and acetyl-CoA - There are two pathways for degradation to glycine and acetyl-CoA Deamination 16 Methionine - Degradation to succinyl-CoA (through propionyl-CoA) Methionine cycle Methionine - Involving an adenine cycle nucleotide - Recycling of a methyl group - Homocysteine leaves the cycle for carbon skeleton degradation carboxylation [recall: propionyl-CoA is also formed from 17 beta-oxidation of “odd” fatty acids] Leucine, Valine, Isoleucine (branched-chain amino acids) - Degradation to acetyl-CoA and/or succinyl-CoA (through propionyl-CoA) Same reaction as the first step in beta- oxidation of fatty acids 18 Lysine (6 Cs, 2 Ns) - Degradation to acetoacetate (4 Cs) - Transaminations - Decarboxylations 19 Tryptophan - Degradation to acetoacetate and pyruvate pyruvate - including decarboxylation and deamination 20 Phenylalanine and Tyrosine - Degradation to acetoacetate and fumarate 21 Phenylketonuria - Metabolic disease (1 in 10,000 newborns) due to defective phenylalanine hydroxylase X Phenylalanine hydroxylase COO - CH 2 COO - H2O Phenylacetate* CO2 O C COO - CH 2 *Accumulates in HO CH phenylketonuria patients CH 2 and may cause mental retardation Phenylpyruvate* Phenyllactate* 22  “phenylketonurics: contains phenylalanine” Aspartame -Artificial sweetener -Dipeptide of phenylalanine methyl ester and aspartate 23  Amino Acid Biosynthesis The Nitrogen Cycle [N2 as electron acceptor] - N2 fixing bacteria (free living in soil or symbiotic in legume plants) Denitrification Nitrifying bacteria Plants, bacteria, fungi 24 Nitrogen fixation by the nitrogenase complex: N2 + 8 H+ + 8 e- + 16 ATP 2NH3 + H2 + 16 ADP + 16 Pi Nitrogen-fixing bacteria in root nodules of legume plants 25 Ammonia Assimilation 26 Essential and Non-essential Amino Acids (in mammals) Essential Amino Acids: Histidine, Isoleucine, Leucine, Lysine, Methionine Phenylalanine, Tryptophan, Threonine, Valine Non-essential amino acids (precursor): Glutamate, glutamine, proline, arginine (-ketoglutarate) Alanine (pyruvate) Aspartate, asparagine (oxaloacetate) Serine, glycine (3-phophoglycerate) *Cysteine (methionine) *Tyrosine (phenylalanine) *Conditional essential 27  Biosynthesis of Amino Acids 28 Non-essential amino acids Alanine, aspartate, glutamate, asparagine, and glutamine biosynthesis glutamate glutamate -ketoglutarate -ketoglutarate 29 Proline and arginine biosynthesis α-Ketoglutarate → “N-acetyl” - Acetylation of the amino group Non-enzymatic Urea cycle cyclization enzymes (first three) 30 Serine and glycine biosynthesis (A glycolysis intermediate) Serine hydroxymethyltransferase - An 1-C group transfer cofactor 31 Tyrosine biosynthesis from phenylalanine (an essential amino acid) NH3+ CH2 CH COO- Phenylalanine O2 (defective in phenylketonurics) NADH + H+ Phenylalanine hydroxylase NAD+ H2O NH3+ HO CH2 CH COO- Tyrosine In human and animals, tyrosine is a conditional essential amino acid 32 Cysteine biosynthesis from methionine (an essential amino acid) and serine - In human and animals, cysteine is a conditional essential amino acid - Methionine is the major dietary S source for the biosynthesis of S- containing organic molecules such Methionine cycle as cysteine. - Serine provides the carbon skeleton for cysteine. Serine obtains its C skeleton from a glycolysis metabolite. Cystathionine β-synthase (CBS) - Plants and bacteria can assimilate sulfate into cysteine and other organic molecules. Homocystinuria - Genetic defect in CBS - Homocysteine level in blood stream increases - Increased risks of heart diseases 33