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Conversion of amino acids to Specialized products
Department of Biochemistry
Specific Learning Objectives
Conversion of aa to Specialized products
Describe roles of arginine and ornithine in metabolic pathways other than
urea cycle (in NO and synthesis, respectively)
Introduction
In addition to serving as building blocks for proteins, aa are
precursors of many nitrogen-containing compounds that have
important physiologic functions
These molecules include porphyrins (involved in heme biosynthesis),
hormones, purines, and pyrimidines, neurotransmitters.
Conversion of arginine, ornithine & proline to specialized products
Fig 30.1. Harper's Illustrated Biochemistry 30th Edition
Creatine & Creatinine
? Creatinine is formed in muscle from
creatine phosphate by irreversible, non-
enzymatic dehydration, and loss of
phosphate
? Glycine, arginine, and methionine all
participate in creatine biosynthesis
Fig 30.13. Harper's Illustrated Biochemistry 30th Edition
Cont--
Creatine converted into creatine phosphate by creatine kinase using
ATP as a phosphate donor
Presence of creatine kinase in plasma is indicative of heart damage
and is used in diagnosis of myocardial infarction
Conversion of cysteine to Taurine
? Three enzyme catalyzed reactions convert
cysteine to taurine
? Taurine displace coenzyme A moiety of
cholyl-CoA to form bile acid taurocholic
acid
Fig 30.4. Harper's Illustrated Biochemistry 30th Edition
Biosynthesis of hippurate from glycine
? Many metabolites and pharmaceuticals are
excreted as water soluble glycine conjugates
? Ex. include glycocholic acid and hippuric acid
formed from food additive benzoate
? Many drugs, drug metabolites, and other
compounds with carboxyl groups are conjugated
with glycine, which makes them more water-
soluble and thereby facilitates their excretion in
urine
Fig 30.5. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Histidine
? Decarboxylation of histidine to histamine is
catalyzed by pyridoxal 5-phosphate-
dependent enzyme histidine decarboxylase
? Histamine functions in allergic reactions
and gastric secretion
Fig 30.6. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Methionine
Methionine adenosyl transferase
? These polyamines function in cell
proliferation and growth, are growth
factors for cultured mammalian cells,
and stabilize intact cells, subcellular
organelles, and membranes
? They bear multiple positive charges,
polyamines readily associate with DNA
and RNA
Fig 30.9. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Tryptophan
Hydroxylation of tryptophan to 5-hydroxytryptophan by liver tryptophan
hydroxylase subsequent decarboxylation forms serotonin a potent
vasoconstrictor and stimulator of smooth muscle contraction.
Catabolism of serotonin is initiated by deamination to 5-hydroxyindole-3-
acetate, a reaction catalyzed by monoamine oxidase
Cont--
N-Acetylation of serotonin, followed by its O-methylation in pineal
body, forms melatonin
Kidney tissue, liver tissue, and fecal bacteria all convert tryptophan to
tryptamine, then to indole 3-acetate
Normal urinary catabolites of tryptophan are 5-hydroxyindoleacetate
and indole 3-acetate.
Fig 30.11. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Tyrosine
Melanin: Tyrosine form DOPA by Tyrosinase in melanocytes, L-Dopa can be
converted, via tyrosinase, into dopaquinone followed melanin synthesis
Tyrosine forms DOPA by Tyrosine Hydroxylase in adrenal chromaffin
cells
Nor-Epinephrine and Epinephrine:
Neural cells convert tyrosine to epinephrine and norepinephrine
Dopa decarboxylase, a PLP-dependent enzyme, forms dopamine
Cont--
Subsequent hydroxylation, catalyzed by dopamine -oxidase forms
norepinephrine
In adrenal medulla, phenylethanolamine-N-methyltransferases
utilizes S-adenosylmethionine to methylate the primary amine of
norepinephrine, forming epinephrine
T3 and T4: Tyrosine is also a precursor of triiodothyronine and
thyroxine
Derivatives of Tyrosine: Melanin
DM Vasudevan' s Textbook of Biochemistry for Medical Students-6th Ed
Derivatives of Tyrosine: Epinephrine
Fig 30.12. Harper's Illustrated Biochemistry 30th Edition
Derivatives of Tyrosine: T3 and T4
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
albinism; it is inherited as an autosomal recessive disorder
Pigmentation of skin, hair and iris is reduced and eyes may appear pink
Reduced pigmentation of iris causes photosensitivity and decreased skin
pigmentation associated with increased incidence of certain skin cancers
Tyrosinase involved in catecholamine synthesis is a different isoenzyme,
controlled by different gene; consequently, adrenaline (epinephrine)
metabolism is normal
Metabolism of -AminoButyrate (GABA)
Active form of Vit B6
Fig 30.14. Harper's Illustrated Biochemistry 30th Edition
Disorder related to GABA
4-hydroxybutyric aciduria
Defects in succinic semialdehyde dehydrogenase, are responsible for
4-hydroxybutyric aciduria a rare metabolic disorder of -aminobutyrate
catabolism
Characterized by presence of 4-hydroxybutyrate in urine, plasma and
cerebrospinal fluid
No present treatment is available for accompanying mild to severe
neurologic symptoms.
Clinical-cases discussed
Reference Books
23
1) Lehninger Principles of Biochemistry, 6th Ed.
2) Harper's Il ustrated Biochemistry-30th edition
3) Biochemistry, Lippincott's Il ustrated Reviews, 6th Ed
4) Gregory S. Ducker and Joshua D Rabinowitz. Cell Metab.
2017 Jan 10;25(1):27-42
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This post was last modified on 05 April 2022