Metabolism of amino acids, purine and pyrimidine bases Pavla Balínová

Amino acids (AAs) Sources of AAs: diet synthesis de novo protein degradation dietary proteins proteosynthesis body proteins AAs pool N-compound synthes. de novo biosynthesis degradation (E,glc,fat)

Biosynthesis of amino acids (AA) Humans can synthesize only 10 of the 20 AA. Essential AA = AA that cannot be synthesized „de novo“. They must be obtained from diet. Nonessential AA: Ala is synthesized from pyruvate. Cys is synthesized from Met and Ser. Tyr is formed by hydroxylation from Phe. Tyr Val Ser Trp Pro Thr Gly Phe Glu Met Gln Lys Cys Leu Asp Ile Asn His Ala Arg Nonessential AA Essential AA

Synthesis of AAs in a human body - 5 substrates - oxaloacetate → Asp, Asn -ketoglutarate → Glu, Gln, Pro, (Arg) pyruvate → Ala 3-phosphoglycerate → Ser, Cys, Gly Phe → Tyr

Synthesis of thyrosine from phenylalanine Figure is found at http://themedicalbiochemistrypage.org/amino-acid-metabolism.html#tyrosine

Formation of activated methionine = S-adenosylmethionine (SAM) SAM is used as –CH3 group donor in metabolic methylations Figure is found at http://themedicalbiochemistrypage.org/amino-acid-metabolism.html#cysteine

Synthesis of Cys from Met and Ser Figure is found at http://themedicalbiochemistrypage.org/amino-acid-metabolism.html#cysteine

Degradation of AA 20 different multienzyme sequences exist for catabolism of AAs. All common 20 AAs are converted to only 7 compounds: pyruvate acetyl-CoA acetoacetyl-CoA α-ketoglutarate succinyl-CoA fumarate oxaloacetate

Three types of reactions are typical for degradation of AAs: Transamination 2. Deamination 3. Decarboxylation

Transamination = an exchange of –NH2 between amino acid and α-ketoacid These reactions are catalyzed by transaminases (aminotransferases). Most of them require α-ketoglutarate as an acceptor of –NH2. Coenzyme of transaminases: pyridoxal phosphate (vit. B6 derivative) Figure is found at http://web.indstate.edu/thcme/mwking/nitrogen-metabolism.html

Aminotransferases (transaminases) important in medicine Alanine aminotransferase (ALT) Aspartate aminotransferase (AST) Figure was adopted from Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley‑Liss, Inc., New York, 1997. ISBN 0‑471‑15451‑2

Deamination e. g. oxidative deamination of Glu Glu → α-ketoglutarate by glutamate dehydrogenase Figure is found at http://www.sbuniv.edu/~ggray/CHE3364/b1c25out.html

Decarboxylation → primary amines a) decarboxylation of His → histamine b) decarboxylation of Trp → serotonin c) decarboxylation of Tyr → epinephrine and norepinephrine d) decarboxylation of Glu → GABA (γ-aminobutyrate) Figure is found at http://www.sbuniv.edu/~ggray/CHE3364/b1c25out.html

Ammonia transport and detoxification Glutamine (Gln) is the major transport form of ammonia. Figure is found at http://www.sbuniv.edu/~ggray/CHE3364/b1c25out.html

Glucose-Alanine cycle Figure is found at http://www.sbuniv.edu/~ggray/CHE3364/b1c25out.html

Urea cycle (ornithine cycle) substrates: NH4+, HCO3-, Asp, ATP product: urea function: synthesis of non-toxic urea subcellular location: mitochondria and cytosol organ location: liver regulatory enzyme: carbamoyl phosphate synthetase I

Urea cycle (ornithine cycle) Figure is found at http://web.indstate.edu/thcme/mwking/nitrogen-metabolism.html

The fate of carbon skeletons of AA during catabolism The strategy of the cell is to convert carbon skeletons to compounds useful in gluconeogenesis or CAC. Glucogenic AAs = AA that can form any of intermediates of carbohydrate metabolism Gly, Ala, Ser, Cys, Thr → pyruvate Glu, Pro, Arg, His → Glu → α-ketoglutarate Met, Ile, Val → succinyl-CoA Ketogenic AAs are converted to acetyl-CoA and acetoacetyl-CoA. They yield ketone bodies. Leu, Lys ● Glucogenic-ketogenic AAs = Thr, Phe, Tyr, Ile

Figure is found at http://www. biocarta

De novo synthesis of purine nucleotides Figure is found at http://web.indstate.edu/thcme/mwking/nucleotide-metabolism.html

Important notes about biosynthesis of purine nucleotides Subcellular location: cytoplasm PRPP = phosphoribosyl pyrophosphate is derived from ribose-5-P IMP = inosine monophosphate serves as the common precursor of AMP and GMP synthesis Gln, Gly, Asp are donors of C and N atoms CO2 is a source of C C1 units are transferred via tetrahydrofolate „Salvage pathway“: purines from normal turnover of cellular NA can be converted to nucleoside triphosphates substrates: purine bases, PRPP, ATP

Degradation of purine nucleotides → uric acid is formed by enzyme xanthine oxidase

De novo synthesis of pyrimidine nucleotides Figure is found at http://web.indstate.edu/thcme/mwking/nucleotide-metabolism.html

Important notes about synthesis of pyrimidine nucleotides Carbamoyl phosphate is formed from Gln and CO2 (2 ATP are consumed). This reaction is catalyzed by carbamoyl-P synthetase II (cytosolic enzyme) = regulatory step pathway occurs in cytoplasm but formation of orotate occurs in mitochondrion → orotate is linked by PRPP → OMP → UMP UMP → UTP → CTP TTP ● UTP inhibits regulatory enzyme, activator is PRPP „Salvage pathway“: ● pyrimidine nucleosides are phosphorylated (ATP) to nucleotides TTP

Degradation of pyrimidine bases → β-amino acids are formed