Conversion of aa to Specialized products
Describe roles of arginine and ornithine in metabolic pathways other than
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urea cycle (in NO and synthesis, respectively)Introduction
In addition to serving as building blocks for proteins, aa are
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precursors of many nitrogen-containing compounds that haveimportant physiologic functions
These molecules include porphyrins (involved in heme biosynthesis),
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hormones, purines, and pyrimidines, neurotransmitters.
Conversion of arginine, ornithine & proline to specialized products
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Fig 30.1. Harper's Illustrated Biochemistry 30th EditionCreatine & Creatinine
? Creatinine is formed in muscle from
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creatine phosphate by irreversible, non-enzymatic dehydration, and loss of
phosphate
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? Glycine, arginine, and methionine all
participate in creatine biosynthesis
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Fig 30.13. Harper's Illustrated Biochemistry 30th EditionCont--
Creatine converted into creatine phosphate by creatine kinase using
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ATP as a phosphate donor
Presence of creatine kinase in plasma is indicative of heart damage
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and is used in diagnosis of myocardial infarctionConversion of cysteine to Taurine
? Three enzyme catalyzed reactions convert
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cysteine to taurine? Taurine displace coenzyme A moiety of
cholyl-CoA to form bile acid taurocholic
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acid
Fig 30.4. Harper's Illustrated Biochemistry 30th Edition
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Biosynthesis of hippurate from glycine? Many metabolites and pharmaceuticals are
excreted as water soluble glycine conjugates
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? Ex. include glycocholic acid and hippuric acid
formed from food additive benzoate
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? Many drugs, drug metabolites, and othercompounds with carboxyl groups are conjugated
with glycine, which makes them more water-
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soluble and thereby facilitates their excretion in
urine
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Fig 30.5. Harper's Illustrated Biochemistry 30th EditionDerivatives of Histidine
? Decarboxylation of histidine to histamine is
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catalyzed by pyridoxal 5-phosphate-dependent enzyme histidine decarboxylase
? Histamine functions in allergic reactions
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and gastric secretion
Fig 30.6. Harper's Illustrated Biochemistry 30th Edition
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Derivatives of MethionineMethionine adenosyl transferase
? These polyamines function in cell
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proliferation and growth, are growth
factors for cultured mammalian cells,
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and stabilize intact cells, subcellularorganelles, and membranes
? They bear multiple positive charges,
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polyamines readily associate with DNA
and RNA
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Fig 30.9. Harper's Illustrated Biochemistry 30th EditionDerivatives of Tryptophan
Hydroxylation of tryptophan to 5-hydroxytryptophan by liver tryptophan
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hydroxylase subsequent decarboxylation forms serotonin a potentvasoconstrictor and stimulator of smooth muscle contraction.
Catabolism of serotonin is initiated by deamination to 5-hydroxyindole-3-
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acetate, a reaction catalyzed by monoamine oxidase
Cont--
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N-Acetylation of serotonin, followed by its O-methylation in pinealbody, forms melatonin
Kidney tissue, liver tissue, and fecal bacteria all convert tryptophan to
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tryptamine, then to indole 3-acetate
Normal urinary catabolites of tryptophan are 5-hydroxyindoleacetate
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and indole 3-acetate.Fig 30.11. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Tyrosine
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Melanin: Tyrosine form DOPA by Tyrosinase in melanocytes, L-Dopa can beconverted, via tyrosinase, into dopaquinone followed melanin synthesis
Tyrosine forms DOPA by Tyrosine Hydroxylase in adrenal chromaffin
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cellsNor-Epinephrine and Epinephrine:
Neural cells convert tyrosine to epinephrine and norepinephrine
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Dopa decarboxylase, a PLP-dependent enzyme, forms dopamineCont--
Subsequent hydroxylation, catalyzed by dopamine -oxidase forms
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norepinephrineIn adrenal medulla, phenylethanolamine-N-methyltransferases
utilizes S-adenosylmethionine to methylate the primary amine of
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norepinephrine, forming epinephrine
T3 and T4: Tyrosine is also a precursor of triiodothyronine and
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thyroxineDerivatives of Tyrosine: Melanin
DM Vasudevan' s Textbook of Biochemistry for Medical Students-6th Ed
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Derivatives of Tyrosine: EpinephrineFig 30.12. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Tyrosine: T3 and T4
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DM Vasudevan' s Textbook of Biochemistry for Medical Students-6th Ed
Disorder related to Tyrosine derivative
Albinism: A deficiency of tyrosinase in melanocytes causes one form of
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albinism; it is inherited as an autosomal recessive disorder
Pigmentation of skin, hair and iris is reduced and eyes may appear pink
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Reduced pigmentation of iris causes photosensitivity and decreased skinpigmentation associated with increased incidence of certain skin cancers
Tyrosinase involved in catecholamine synthesis is a different isoenzyme,
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controlled by different gene; consequently, adrenaline (epinephrine)
metabolism is normal
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Metabolism of -AminoButyrate (GABA)Active form of Vit B6
Fig 30.14. Harper's Illustrated Biochemistry 30th Edition
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Disorder related to GABA4-hydroxybutyric aciduria
Defects in succinic semialdehyde dehydrogenase, are responsible for
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4-hydroxybutyric aciduria a rare metabolic disorder of -aminobutyratecatabolism
Characterized by presence of 4-hydroxybutyrate in urine, plasma and
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cerebrospinal fluid
No present treatment is available for accompanying mild to severe
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neurologic symptoms.Clinical-cases discussed
Reference Books
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231) Lehninger Principles of Biochemistry, 6th Ed.
2) Harper's Il ustrated Biochemistry-30th edition
3) Biochemistry, Lippincott's Il ustrated Reviews, 6th Ed
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4) Gregory S. Ducker and Joshua D Rabinowitz. Cell Metab.2017 Jan 10;25(1):27-42
Thank you
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