Conversion of Amino acids into Non-protein Nitrogenous Compounds

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    Conversion ofAmino acids into

    Non-protein

    NitrogenousCompounds

    Dr. Contessa S. Tac-an

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    Glycine

    Heme synthesis

    Synthesis ofpurines forms positions 4,

    5, 7 of the purine ring Constituent ofglutathione

    Conjugates with cholic acid to formglycocholic acid

    Conjugates with benzoic acid to formhippuric acid

    Synthesis ofcreatine

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    Alanine

    Carnosine - is a -alanyl dipeptide

    that occurs in human

    skeletal muscle. Anserine - also a -alanyl dipeptide

    - derived from the diet

    - occurs in skeletal muscle

    characterized by rapid

    contractile activity.

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    Structure of Carnosine

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    Structure ofAnserine

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    Carnosine & Anserine

    Functions:

    1. Activate myosin ATPaseactivity.

    2. Chelate copper and enhance

    copper uptake.

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    Cont.

    Synthesis: Carnosinesynthetase

    -alanine + L-histidine + ATPp

    Carnosine + AMP + PPi

    Carnosine N-methyltransferaseCarnosine + SAM pSAH

    + Anserine

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    Cont.

    Degradation:

    Carnosinase

    Carnosinep -Alanine + L-Histidine

    Carnosinase deficiency - heritabled/o characterized by persistentcarnosinuria.

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    Homocarnosine

    CNS dipeptide

    Related structurally &metabolically

    to carnosine but is present in humanbrain at levels 100x higher thancarnosine levels

    Synthesis in brain tissue is catalyzed

    by carnosine synthetase

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    Structure ofHomocarnosine

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    Serine, threonine & tyrosine

    Phosphorylation (by proteinkinases) & dephosphorylation

    (by protein phosphatases) ofser, thr & tyr serve importantregulatory functions.

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    Serine

    Biosynthesis ofsphingosine

    Purines & pyrimidinespp

    -carbon of ser provides methylgroups of thymine (& of choline)

    and of carbons 2 & 8 of purines.

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    Cysteine

    Urinary sulfate arises from theoxidation of L-cysteine. (Sulfur ofmethionine contributes to urinarySO4 indirectly, via cysteine.

    Precursor of thioethanolamineportion of CoA

    Converted to Taurine, thatconjugates with bile acids such as

    taurocholic acid

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    Histidine

    Decarboxylation of histidine formshistamine, catalyzed by aromatic L-amino acid decarboxylase orhistidine decarboxylase.

    Other histidine compounds include:ergothioneine, carnosine, and

    dietary anserine.

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    Structure of ergothioneine

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    Arginine

    Arginine - formamidine donor forcreatine synthesis

    Conversion, via ornithine, toputrescine, spermine & spermidine.

    Precursor ofnitric oxide (NO), w/c isa neurotransmitter, smooth muscle

    relaxant & vasodilator.

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    Arginine

    NO synthase

    L-Arginine + O2 + NADPHp

    L-Citrulline + NO

    Precursor of polyamines, spermine

    And spermidine, w/c function in cellproliferation and growth.

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    Spermine & Spermidine

    Growth factors for cultured cells.

    Stabilization of intact cells,subcellular organelles &membranes.

    Pharmacologic doses of polyaminesare hypothermic and hypotensive.

    Associate w/ polyanions such asDNA & RNA

    Stimulate DNA & RNA synthesis

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    Cont.

    DNA stabilization

    Packaging of DNA in bacteriophage

    Exert diverse effects in proteinsynthesis

    Inhibit protein kinases

    Polyamine biosynthesis catalyzedby ornithine decarboxylase.

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    Polyamine biosynthesis

    L-ornithine - precursor ofputrescine portion of spermine &spermidine

    SAM - precursor ofdiaminopropane portion

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    Cont.

    Enzymes:

    1. Ornithine decarboxylase & SAMdecarboxylase - inducible enzymesw/ short half-lives

    2. Spermine & Spermidine synthases

    - are neither inducible nor

    unusually labile enzymes.

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    Ornithine decarboxylase

    Half life 10 mins.

    Ten- to 200-fold increase in itsactivity after administration tocultured cells ofGH, corticosteroids,testosterone orEGF

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    SAM decarboxylase

    Cofactor: pyruvate

    Half life 1-2 hours

    Responds to promoters of cellgrowth

    Inhibited by decarboxylated SAM

    Activated by putrescine

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    Polyamine degradation

    Polyamine oxidase, of liver peroxisomes,oxidizes spermine to spermidine;subsequently oxidizes

    spermidine to putrescine. Putrescine oxidizes to NH4 + CO2.

    Both aminopropane is converted to -aminopropionaldehyde.

    Major fraction of putrescine &spermidine are excreted in urine asconjugates (acetyl derivatives).

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    Tryptophan

    Forms serotonin

    Synthesis/Reactions:

    1.

    Hydro

    xylation of trp to 5-O

    Htrp,catalyzed by livertyrosine

    hydroxylase

    2. Decarboxylation of 5-OHtrp forms

    serotonin(5-hydroxytryptamine)

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    Synthesis of Serotonin

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    Serotonin

    Potent vasoconstrictor

    Stimulator of smooth musclecontraction

    Catabolism initiated by monoamineoxidase-catalyzed oxidativedeamination to 5-

    hydroxyindoleacetate, excreted inthe urine (2-8mg/d)

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    Cont.

    Malignant carcinoid increasesserotonin production.

    Catabolites detected in the urineinclude N-acetylserotoninglucuronide and 5-hydroxyindoleaceturate.

    Reduced synthesis of niacin (Vit.B3), causing pellagra.

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    Serotonin forms melatonin

    Reactions:

    N-acetylation

    O-methylation

    Melatonin - taken up by all tissues(incl. brain); rapidly metabolized by

    hydroxylation at position 6, thenconjugated w/ SO4 or glucuronicacid.

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    Trp metabolites

    Excreted in urine and feces

    Principal normal urinary catabolitesare 5-hydroxyindoleacetate andindole-3-acetate.

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    Tyrosine

    Biosynthesis of melanins, catalyzed bytyrosine hydroxylase.

    Neuronal cells convert tyrosine toepinephrine and norepinephrine

    Dopamine synthesis, via dopadecarboxylase (B6PO4 as cofactor)

    Norepinephrine synthesis catalyzed bydopamine -oxidase(Vit. C and copper

    dependent enzyme) Methylation of NE (adrenal medulla)catalyzed by phenylethanolamine-N-methyltransferase, forms epinephrine

    Precursor of T3 and T4.

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    Synthesis of cathecolamines

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    ALBINISM

    Defective melanin biosynthesis

    Hypomelanosis due to heritable defectsin eye and skin melanocytes

    Oculocutaneous albinism Forms:

    1. Tyrosine hydoxylase negative albinos lack all visual pigment

    -- hair bulbs fail to converttyrosine to pigment; melanocytescontain unpigmented melanosomes

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    Creatine biosynthesis

    Derived from glycine, arginine &methionine

    Transfer of guanidino group fromarginine to glycine, formingguanidoacetate(glycocyamine);occurs in kidney

    Methylation of guanidoacetate bySAM in the liver

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    Cont.

    Creatine and phosphocreatine arepresent in muscle, brain & blood.

    Forms creatinine (creatineanhydride) in muscle, from creatinePO4 by dehydration & loss ofphosphate

    24 hr creatinine excretion in urine isconstant from day to day &proportionate to muscle mass

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    L-glutamate

    Biosynthesis ofK-Aminobutyrate(GABA), via decarboxylation of L-glutamate catalyzed by glutamate

    decarboxylase(B6PO4 cofactor)present in CNS

    Addtition:

    Putrescine can be converted to

    GABA, either by deamination (bydiamine oxidase) or via N-acetylatedintermediates.

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    Cont.

    GABA degradation

    GABA transaminase

    GABAp succinate semi-

    aldehyde

    Succinate semialdehyde mayundergo either:

    1.

    Reduction to K-hydro

    xybutyrate2. Oxidation to succinate to CO2 &

    H2O

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