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Amino acid metabolism III. Brake down of amino acids, glucoplastic and ketoplastic amino acids

Amino acid metabolism III. Brake down of amino acids, glucoplastic and ketoplastic amino acids. Figures : Lehninger-4ed; chapter: 18 Stryer-5ed; chapter: 23. Overview of amino acid catabolism in mammals. Summary of amino acid catabolism: fates of the carbon chain.

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Amino acid metabolism III. Brake down of amino acids, glucoplastic and ketoplastic amino acids

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  1. Amino acid metabolism III.Brake down of amino acids, glucoplastic and ketoplastic amino acids Figures: Lehninger-4ed; chapter: 18 Stryer-5ed; chapter: 23

  2. Overview of amino acid catabolism in mammals

  3. Summary of amino acid catabolism: fates of the carbon chain

  4. Purely ketogenic amino acids: can yield ketone bodies in the liver •leucine (Leu)  very common in proteins •lysine (Lys) Glucogenic amino acids: can be converted to glucose and glycogen •alanine (Ala) •cysteine (Cys) •glycine (Gly) •serine (Ser) •asparagine (Asn) •aspartate (Asp) •methionine (Met) •valine (Val) •arginine (Arg) •glutamine (Gln) •glutamate (Glu) •histidine (His) •proline (Pro)

  5. Mixed amino acids (both ketogenic and glucogenic): •tryptophan (Trp) •phenylalanine (Phe) •tyrosine (Tyr) •threonine (Thr) •isoleucine (Ile)

  6. Enzyme cofactors in amino acid catabolism These cofactors transfer one-carbon groups in different oxidation states: •Biotin(most oxidized: COO–) •Tetrahydrofolate (intermediate ox. state: methylene, methenyl, formyl, formimino groups, and sometimes methyl) •S-Adenosylmethionine (most reduced: methyl)

  7. Conversion of one-carbon units in tetrahydrofolate

  8. The preferred cofactor for biological methyl group transfer: adoMet This Me group is about 1,000 times more reactive than the Me group from N5-Me-tetrahydrofolate! The only other known reaction in whichtriphosphate is displaced from ATP occurs in the synthesis of coenzyme B12!

  9. Coenzyme B12-dependent reactions in mammals: • methionine synthase reaction • rearrangament of L-methylmalonyl-CoA to succinyl-CoA Vitamin B12 deficiency disease: metabolic folates become trapped in the N5-methyl form!

  10. Minor pathway in humans: 10-30% of Thr catabolism

  11. Oxidative cleavage pathway (the 2 C-atoms from Gly do not enter the citric acid cycle!!!)

  12. Another pathway of Gly degradation: D-amino acid oxidase: • is present at high levels in the kidney • has as primary function the detoxification of ingested D-amino acids Calcium oxalate: 75% of kidney stones!

  13. Try and Phe are precursors for biologically active molecules!

  14. The first step in Phe degradation requires the cofactor tetrahydrobiopterin: (mixed function oxidase) Mixed function oxidases: catalyze simultaneous hydroxylation of a substrate by an oxygen atom of O2 and reduction of the other oxygen atom to H2O

  15. (Urea cycle) Allosteric activator: ADP Allosteric inhibitor: GTP

  16. The primary pathway for Thr degradation in humans!

  17. • much of the catabolism of amino acids takes place in the liver •branched-chain amino acids are oxidized as fuels primarily in the muscles, adipose, kidney, and brain tissue (absent in the liver!)

  18. • branched-chain -keto acid dehydrogenase complex • pyruvate dehydrogenase complex •-ketoglutarate dehydrogenase complex similar structure, same reaction mechanism catalyze homologous reactions five cofactors: thiamine pyrophosphate FAD NAD lipoate coenzyme A inactive enzyme complex = phosphorylated form! (when the dietary intake of branched-chain amino acids is low)

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