Metabolism of Amino acids including transamination, deamination and urea cycle

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In the name of Allah, the
most beneficent, The most
Gracious and the most
merciful!

2. Prepared by:
M.Shayan Adnan
shayanadnan00@yahoo.com
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3. • Proteins are the most abundant organic compounds &
constitute a major part of the body dry weight (10-12kgin
adults).
• Perform a wide variety of structural and dynamic (enzymes,
hormones, clotting factors, receptors) functions.
• Proteins are nitrogen containing macromolecules consistingof L-α
- amino acids as the repeating units.
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4. • 20 amino acids found in proteins, half can be synthesized by
the body andhalf are supplied through diet.
• Theproteins on degradation release individual amino acids.
• Eachamino acid undergoes itsown metabolism performs
specificfunctions.
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5. • Some amino acidsserve as precursors for the synthesis of many
biologically important compounds.
• Certain amino acids may directly act as neurotransmitters
(e.g glycine, aspartate, glutamate)
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6. • About 100g of free amino acids which represent the amino acid
pool of the body.
• Glutamate and glutamine together constitute about 5
0 an
d essential
amino acids about 10%of the body pool (100g).
• Theconcentration of intracellular amino acids is always higher
than the extracellular amino acids.
• Enter the cells against a concentration gradient.
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7. • Turnover of body intake of dietary protein and the synthesis of
non-essential amino acids contribute to the body amino acid
pool.
• Protein turnover:
• Theproteins in the body isin a dynamic state.
• About 300-400g of protein per day isconstantly degraded and
synthesized which represents body protein turnover.
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8. • Theturnover of the protein isinfluenced by many
factors.
• A small polypeptide called ubiquitin (8,500) tags with the
protein andfacilitatesdegradation.
• Certain proteins with amino acid sequence proline,
glutamine, serine andthreonineare
rapidly degraded.
Control of protein turnover:
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9. • There isa regular lossof nitrogen from the body due to
degradation of amino acids.
• About 30-50gof protein islost every day.
• Thisamount of protein issupplied through diet to maintain
nitrogen balance.
• There isno storage form of amino acids in the
body.
• Excessintake of amino acids isoxidized to provide energy.
Dietary protein
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10. • Proteins function as enzymes, hormones, immunoproteins,
contractile proteins etc.
• Many important nitrogenous compounds (porphyrins,
purines, pyrimidines, etc) are produced from the amino
acids.
• About 10-15%of body energy requirements are met from the
amino acids.
• Theamino acids are converted into carbohydrates andfats.
Utilization of amino acids from body pool
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11. ➢ Transamination
➢ Oxidative Deamination
➢ Ammonia Transport
➢ Urea Cycle
Catabolism of amino acids occur in 4 stages
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12. • Thetransfer of an amino (-NH2) group from an amino acid to a
ketoacid, with the formation of a new amino acid anda new
ketoacid.
• Catalysed by a group of enzymescalled transaminases
(aminotransferases)
• Pyridoxalphosphate (PLP)– Co-factor.
• liver, kidney, heart, brain - adequate amount of these enzymes.
Transamination
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13. 13

14. • All transaminases require PLP
.
• No free NH3 liberated, only the transfer of amino group.
• Transamination is reversible.
• There are multiple transaminase enzymes which vary in
substrate specificity.
• AST andAL
Tmake a significant contribution for
transamination.
Silent features of Transamination
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15. • Transamination isimportant for redistribution of amino
groups andproduction of non-essential amino acids.
• It diverts excess amino acids towards the energy
generation.
• Amino acids undergo transamination to finally
concentrate nitrogen in glutamate.
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16. • Glutamate undergoes oxidative deamination to liberate
free NH3 for urea synthesis.
• All amino acids except, lysine, threonine, proline and
hydroxyproline participate in transamination.
• It involves both anabolism andcatabolism, since–
reversible.
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17. AA1+ α- KG
KETOACID1 +
GLUTAMATE
Alanine + α- KG
Aspartate + α- KG
Pyruvate + Glutamate
Oxaloacetetae +Glutamate
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20. • Step:1
➢ Transfer of amino group from AA1to the coenzyme PLPto form
pyridoxamine phosphate.
➢ Amino acid1isconverted to Keto acid2.
• Step:2
➢ Amino group of pyridoxamine phosphate is then transferred to a keto
acid1to produce a new AA 2&enzyme with PLPis regenerated.
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21. • The amino group of most of the amino acids is released by a
coupled reaction, trans- deamination.
• Transamination followed by oxidative deamination.
• Transamination takes place in the cytoplasm.
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Trans-deamination

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23. • Theremoval of amino group from the amino acids as NH3 is
deamination.
• Deamination results in the liberation of ammonia for
urea synthesis.
• Thecarbon skeleton of amino acidsis converted to
keto acids.
• Deamination may be either oxidative or non-
oxidative
Deamination
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24. • Only liver mitochondria contain glutamate dehydrogenase
(GDH) which deaminates glutamate to α-ketoglutarate
and ammonia.
• It needs NAD+ asco-enzyme.
• It isan allosteric enzyme.
• It isactivated by ADP&inhibited by GTP.
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25. • Oxidative deamination isthe liberation offree ammonia from
the amino group of amino acids coupled with oxidation.
• Site: Mostly in liver andkidney.
• Oxidative deamination isto provide NH3 for urea synthesis
andα-keto acids for a variety of reactions, including energy
generation.
Oxidative deamination
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26. • Glutamate isa 'collection centre' for amino groups.
• Glutamate rapidly undergoes oxidative deamination.
• Catalysed by GDH to liberate ammonia.
• It can utilize either NAD+ orNADP+.
• Thisconversion occursthrough the formation of
an α-iminoglutarate
Role of glutamate dehydrogenase
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27. COO-
I
CH2
I
CH2
I
3
H -C-NH +
I
COO-
COO-
I
CH2
I
CH2
I
C=NH
I
COO-
COO-
I
CH2
I
CH2
I
I
C=O
I
COO-
NAD(P)+
GDH H2O
GDH
4
CH2 + NH +
L-Glutamate α- Iminoglutarate α- ketoglutarate
NAD(P)H+H+
Oxidation of glutamate by GDH
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28. ➢ Reversible Reaction
➢ BothAnabolic & Catabolic.
➢ Regulation of GDH activity:
➢ Zinccontaining mitochondrial, allosteric enzyme.
➢ Consistsof 6identical subunits.
➢ Molecular weight is56,000
Metabolic significance
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29. • L-amino acid oxidase andD-Amino acid oxidase.
• Flavoproteins and Cofactors are FMN &FAD.
• Acton corresponding amino acids to produce α-keto acids and
NH3
• Site: Liver, kidney, Peroxisomes.
• Activity of L-Amino acid oxidase is low.
• Plays a minor role inAmino acid catabolism.
Amino acid oxidases
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30. L-amino acid
L-AMINOACID OXIDASE
Α- KETO ACID +
NH3
FMN FMNH2
H2O2 ½ O2
Catalase
H2O
Oxidative deamination of amino acids
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31. • L-Amino acid Oxidase actson all Amino acids,except
glycine and dicarboxylic acids.
• Activity of D-Amino oxidase ishigh than that of L-Amino acid
oxidase
• D-Amino oxidase degrades D-Amino acids in bacterial cell
wall.
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32. • D-amino acids are found in plants and
microorganisms.
• They are not present in mammalian proteins.
• D-amino acids are taken in the diet/bacterial cell wall,
absorbed from gut - D-Amino acid oxidase converts them to
respective α-keto acids.
Fate of D-amino acids
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33. • The α-ketoacids undergo transamination to be converted to
L-amino acids which participate in various metabolic
pathways.
• Keto acids may be oxidized to generate energy or serve as
precursors for glucose and fat synthesis.
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34. • Direct deamination, without oxidation.
• Amino acid Dehydratases:
• Serine, threonine &homoserine are the hydroxy
amino acids.
• They undergo non-oxidative deamination catalyzed by
PLP-dependent dehydratases
Non oxidative deamination
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35. Serine
Threonine
Homoserine
Respective Ketoacid
Dehydratase
NH3
PLP
Non oxidative deamination
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36. CYSTEINE &HOMOCYSTEINE UNDERGO
DEAMINATION COUPLED WITH DESULFHYDRATION
TO GIVE KETO ACIDS.
Cysteine
Desulfhydrases
Pyruvate
NH3 +H2S
Uroconate
• Deamination of histidine:
Histidase
Histidine
NH3
Amino acids Desulfhydrases
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37. UREA CYCLE
• the UREA CYCLE IS THE FIRST METABOLIC PATHWAY TO BE
ELUCIDATED.
• THE CYCLE IS KNOWN AS KREBS–HENSEL UREA CYCLE.
• ORNITHINE IS THE FIRST MEMBER OF THE REACTION, IT IS
ALSO CALLED AS ORNITHINE CYCLE.
• UREA IS SYNTHESIZED IN LIVER & TRANSPORTED TO KIDNEYS
FOR EXCRETION IN URINE.
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38. • the two nitrogen atoms of urea are derived from two
different sources, one from ammonia and the other directly
from the a- amino group of aspartic acid.
• carbon atom is supplied by co2
• urea is the end product of protein metabolism (amino acid
metabolism).
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39. • Urea accountsfor 80-90%of the nitrogen containing
substancesexcreted in urine.
• Urea synthesis isa five-step cyclicprocess, with five distinct
enzymes.
• Thefirst two enzymes are present in mitochondria while
the rest are localized in cytosol.
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Step:1 Formation of carbonyl phosphate
• Carbamoyl phosphate synthase I (CPS I) of
mitochondria catalyzes the condensation of NH4+
ions with CO2 to form carbamoyl phosphate.
• This step consumes two ATP and is irreversible.
• It is a rate-limiting.

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• CPS I requires N-acetyl glutamate for its activity.
• Carbamoyl phosphate synthase II (CPS II) - involved
in pyrimidine synthesis & it is present in cytosol.
• It accepts amino group from glutamine and does not
require N-acetyl glutamate for its activity.

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Step2: Formulation of citrulline
• The second reaction is also mitochondrial.
• Citrulline is synthesized from carbamoyl phosphate
and ornithine by ornithine trans carbamoyl.
• Ornithine is regenerated and used in urea cycle.

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• Ornithine and citrulline are basic amino acids.
(Never found in protein structure due to lack of
codons).
• Citrulline is transported to cytosol by a transporter
system.
• Citrulline is neither present in tissue proteins nor in
blood; but it is present in milk.

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Step 3: Formation of arginosuccinate
• Citrulline condenses with aspartate to form
arginosuccinate by the enzyme Arginosuccinate
synthetase.
• Second amino group of urea is incorporated.
• It requires ATP, it is cleaved to AMP & PPi
• 2 High energy bonds are required.
• Immediately broken down to inorganic phosphate
(Pi).

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Step:4 Formation of Arginine or cleavage of
Arginosuccinate
• The enzyme Arginosuccinate or arginosuccinate lyase cleaves
arginosuccinate to arginine and fumarate (an intermediate in TCA
cycle)
• Fumarate provides connecting link with TCA cycle or
gluconeogenesis.

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• The fumarate is converted to oxaloacetate via
fumarase and MDH and transaminase to aspartate.
• Aspartate is regenerated in this reaction

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Step:5 Formation of Urea
• Arginase is the 5th and final enzyme that cleaves
arginine to yield urea and ornithine.
• Ornithine is regenerated, enters mitochondria for its
reuse in the urea cycle.
• Arginase is activated by Co2+ & Mn2+
• Ornithine and lysine compete with arginine
(competitive inhibition).

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• Arginase is mostly found in the liver, while the rest
of the enzymes (four) of urea cycle are also present
in other tissues.
• Arginine synthesis may occur to varying degrees in
many tissues.
• But only the liver can ultimately produce urea.

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Significance of Urea cycle
• Toxic ammonia is converted into non-toxic urea.
• Synthesis of semi-essential amino acid-arginine.
• Ornithine is precursor of Proline, Polyamines.
• Polyamines include putrescine, spermidine,
spermine.
• Polyamines have diverse roles in cell growth &
proliferation.

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Disorders or Urea cycle
• The main function of Urea cycle is to remove toxic
ammonia from blood as urea.
• Defects in the metabolism of conversion of ammonia
to urea, Urea cycle leads to Hyperammonemia or
NH3 intoxication.

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Hyperammonemia
• Inherited disorders of urea cycle enzymes- familial
hyperammonemia.
• Acquired disorders- Liver Disease, severe Renal
disease - Acquired hyperammonemia.

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Ammonia toxicity
• Increased levels of ammonia crosses BBB, formation of
glutamate.
• More utilization of α-ketoglutarate.
• Decreased levels of α- Ketoglutarate in Brain.
• α-KG is a key intermediate in TCA cycle.
• Decreased levels impairs TCA cycle.
• Decreased ATP production

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Hepatic coma (acquired hyperammonemia)
• In diseases of the liver, hepatic failure can finally
lead to hepatic coma and death.
• Hyperammonemia is the characteristic feature of
liver failure.
• The condition is also known as portal systemic
encephalopathy.

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• Normally the ammonia and other toxic compounds
produced by intestinal bacterial metabolism are
transported to liver by portal circulation & detoxified
by the liver.
• But when there is portal systemic shunting of blood,
the toxins bypass the liver and their concentration in
systemic circulation rises.

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Signs/Symptoms and treatment
• CNS dysfunction or manifestations of failure of liver
function (ascites, jaundice, hepatomegaly, edema,
hemorrhage).
• The management of the condition is difficult.
• A low protein diet & intestinal disinfection (bowel
clearing and antibiotics), withholding hepatotoxic
drugs and maintenance of electrolyte & acid-base
balance.

58. • Textbook of biochemistry-u Satyanarayana
• Textbook of Biochemistry-DM Vasudevan
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