Amino acid metabolism I Jana Novotná Department of the Medical Chemistry and Clinical Biochemistry The 2nd Faculty of Medicine, Charles Univ.
Metabolic relationship of amino acids DIETARY PROTEINS GLYCOLYSIS KREBS CYCLE Proteosynthesis Digestion Transamination BODY PROTEINS 250 300 g/day AMINO ACIDS Conversion (Carbon skeleton) UREA NH 3 Degradation NONPROTEIN DERIVATIVES Porphyrins Purines Pyrimidines Neurotransmitters Hormones Komplex lipids Aminosugars GLUCOSE ACETYL CoA CO 2 KETONBODIES
General reactions of amino acid catabolism NH 2 R CH COO - deamination transamination O + NH 4 + R C COO - O R C COO - NH 2 R CH COO - oxidative decarboxylation NH 3 + + R CH 2 CO 2
Transamination. Exchange of NH 2 group with C=O NH 3 + R CH COO + - - - O OOC CH 2 CH 2 C COO Amino acid α-ketoglutarate O R C COO + NH 3 + - - - OOC CH 2 CH 2 CH COO α-ketoacid Glutamate
The fate of the amino group during amino acid catabolism
Transamination reaction The first step in the catabolism of most amino acids is removal of a-amino groups by enzymes transaminases or aminotransferases All aminotransferases have the same prostethic group and the same reaction mechanism. The prostethic group is pyridoxal phosphate (PPL), the coenzyme form of pyridoxine (vitamin B 6 )
Active metabolic form of vitamin B6
Mechanism of transamination reaction: PLP complex with enzyme accept an amino group to form pyridoxamine phosphate, which can donate its amino group to an α-keto acid.
All amino acids except threonine, lysine, and proline can be transaminated Transaminases are differ in their specificity for L- amino acids. The enzymes are named for the amino group donor.
Clinicaly important transaminases Alanine transaminase ALT (previously called serum glutamate-pyruvate transaminase SGPT) Aspartate transaminase AST (previously called serum glutamate-oxaloacetate transaminase SGOT) Important in the diagnosis of liver (viral hepatitis drug toxicity), heart and skeletal muscle injury (heart attack). ALT
Relationship between glutamate, glutamine and α-ketoglutarate NH 3 NH 3 α-ketoglutarate glutamate glutamine NH 3 NH 3 A. Glutamate dehydrogenase glutamate + NAD + + H 2O α-ketoglutarate + NH 3 + NADH From transamination reactions To urea cycle B. Glutamine synthetase (liver) ATP ADP glutamate + NH glutamine 3 C. Glutaminase (kidney) glutamine + H 2 O glutamate + NH 3
A. Oxidative deamination Amino acids + FMN + H 2 O α keto acids + FMNH 2 + NH 3 L-amino acid oxidase O 2 Oxidative deamination catalse L-amino acid oxidase produces ammonia and α-keto acid directly, using FMN as cofactor. The reduced form of flavin must be regenerated by O 2 molecule. This reaction produces H 2 O 2 molecule which is decompensated by catalase. H 2 O + O 2 FMN H 2 O 2 B. Nonoxidative deamination serine Is possible only for hydroxy amino acids threonine Serin-threonin dehydratase pyruvate + NH 3 α-ketobutyrate + NH 3
Synthesis and degradation of amino acids
Overview of the synthesis of nonessential amino acids The carbon of 10 AA may be produced from glucose through intermediates of glycolysis or the TCA cycle. Tyrosine from phenylalanine. The sulphur of cysteine from methionine.
Degradation of AA 20 amino acids are converted to 7 products: pyruvate acetyl-coa acetoacetate α-ketoglutarate succinyl-coa oxalacetate fumarate
Glucogenic amino acids formed: α-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-coa Aspartate Asparagine Arginine Phenylalanine Tyrosine Isoleucine Methionine Valine Glutamine Glutamate Proline Histidine Alanine Serine Cysteine Glycine Threonine Tryptophan
Ketogenic amino acids formed acetyl CoA or acetoacetate Lysine Leucine
Both glucogenic and ketogenic amino acids formed: α-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-coa in addition to acetyl CoA or acetoacetate Isoleucine Threonine Tryptophan Phenylalanine Tyrosine
Amino acids derived from intermediates of glycolysis
The major pathways for serine synthesis from glucose and serine degradation
Glycine biosynthesis from serine Reaction involves the transfer of the hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N 5,N 10 -methylene-thf. Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
Glycine oxidation to CO 2 Glycine produced from serine or from the diet can also be oxidized by glycine decarboxylase (also referred to as the glycine cleavage complex, GCC) to yield a second equivalent of N 5,N 10 -methylene-tetrahydrofolate as well as ammonia and CO 2. Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
Tetrahydrofolate acts as a carrier of reactive single C units Copy from: http://www.chembio.uoguelph.ca/educmat/chm452/lectur25.htm
Metabolism of glycine
Cysteine synthesis Conversion of homocysteine back to Met. N 5 - methyl-thf is donor of methyl group. * *folate + vit B 12 1. Conversion of SAM to homocysteine. 2. Condensation of homocysteine with serine to cystathione. 3. Cystathione is cleavaged to cysteine. Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
Homocystinuria Genetic defects for both the synthase and the lyase. Missing or impaired cystathionine synthase leads to homocystinuria. High concentration of homocysteine and methionine in the urine. Homocysteine is highly reactive molecule. Disease is often associated with mental retardation, multisystemic disorder of connective tissue, muscle, CNS, and cardiovascular system.
Amino acids related through glutamate
Synthesis and degradation of proline
Amino acids related to oxalacetate Aspartate and asparagine
Amino acids that form succinyl-coa
Cysteine and methionine are metabolically related The sulfur for cysteine synthesis comes from the essential amino acid methionine. SAM Condensation of ATP and methionine yield S-adenosylmethionine (SAM) SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the conversion of norepinephrine to epinenephrine).
Catabolism of branched amino acids valine isoleucine leucine α-ketoglutarate glutamate (transamination) α-ketoisovalerate α-keto-β-methylbutyrate α-ketoisokaproate NAD + CO oxidative decarboxylation 2 Dehydrogenase of α-keto acids* NADH + H + isobutyryl CoA α-methylbutyryl CoA isovaleryl CoA Dehydrogenation etc., similar to fatty acid β-oxidation propionyl CoA acetyl CoA acetyl CoA + + propionyl CoA acetoacetate
Branched-chain aminoaciduria Disease also called Maple Syrup Urine Disease (MSUD) (because of the characteristic odor of the urine in affected individuals). Deficiency in an enzyme, branched-chain α-keto acid dehydrogenase leads to an accumulation of three branchedchain amino acids and their corresponding branched-chain α-keto acids which are excreted in the urine. There is only one dehydrogenase enzyme for all three amino acids. Mental retardation in these cases is extensive.
Amino acids that form acetyl-coa and acetoacetate
Phenylalanine and tyrosine
Biosynthesis of tyrosine from phenylalanine Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase
Tetrahydrobiopterin as a cofactor of hydroxylases Dihydrobiopterin
Phenylketonuria Hyperphenylalaninemia - complete deficiency of phenylalanine hydroxylase (plasma level of Phe raises from normal 0.5 to 2 mg/dl to more than 20 mg/dl). The mental retardation is caused by the accumulation of phenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of α-ketoglutarate. Absence of α-ketoglutarate in the brain shuts down the TCA cycle and the associated production of aerobic energy, which is essential to normal brain development. Newborns are routinelly tested for blood concentration of Phe. The diet with low-phenylalanine diet.
Tryptophan catabolism Tryptophan has complex catabolic pathway: 1. the indol ring is ketogenic 2. the side chain alanin gluconeogenesis Xanthurenic acid is excrete in the urine. Nicotinamide NAD and NADP.
Enzymes which metabolised amino acides containe vitamines as cofactors THIAMINE B 1 (thiamine diphosphate) oxidative decarboxylation of α-ketoacids RIBOFLAVIN B 2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD) oxidses of α-amino acids NIACIN B 3 nicotinic acid (nikotinamide adenine dinucleotide NAD + nikotinamide adenine dinukleotide phosphate NADP + ) dehydrogenases, reductase PYRIDOXIN B 6 (pyridoxalphosphate) transamination reaction and decarboxylation FOLIC ACID (tetrahydropholate) Meny enzymes of amino acid metabolism
Pictures were taken from textbooks: Marks Basic Medical Biochemistry A Clinical Approach. Four edition M. Lieberman, A.D. Marks ed., 2013. Essentials of Medical Biochemistry With Clinical Cases. First edition. N.V. Bhagavan, Chung-Eun Ha ed., 2011.