3. INTRODUCTION
• Amino acids make a significant contribution to
the generation of metabolic energy.
• Carnivores can obtain up to 90% of their energy
requirements from amino acid oxidation,
whereas herbivores may fill only a small fraction
of their energy needs by this route.
• In plants, the purpose of amino acid catabolism
is to produce metabolites for other biosynthetic
pathways, rarely for energy.
4. Organisms witnessing amino acid
catabolism
I. Ammonotelic organisms – It includes most aquatic
vertebrates, such as bony fishes & the larvae of
amphibians who excrete it in the form of
ammonia (as ammonium ion).
II. Ureotelic organisms – It includes many terrestrial
vertebrates & also sharks who excrete it in form of
urea.
III. Uricotelic organisms – It includes birds, reptiles
who excrete it in form of uric acid.
5. Associated Pathway
1.
a) L-amino acids reach the liver.
b) The enzyme “aminotransferases or transaminases”
remove their α-amino group by transamination reactions
and transfer it to the α-carbon atom of α-ketoglutarate
leaving behind α-keto acid.
• ENZYME- Aminotransferases
• Prosthetic group- Pyridoxal phosphate
• E.C Number – 2.6.1.4
• The enzyme belongs to the class transferases.
• It transfers nitrogenous groups.
• It belongs to sub-sub class transaminases.
6. In hepatocytes, glutamate is transported from the
cytosol to mitochondria where it goes oxidative
deamination under the action of “L-glutamate
dehydrogenase” to yield α-ketoglutarate.
• ENZYME- L-Glutamate dehydrogenase
• E.C Number – 1.4.1.2
• Systematic name – L-glutamate:NAD+ oxidoreductase
• It belongs to class oxidoreductase.
• It acts on the CH-NH₂ group of donors.
• It has NAD or NADP as acceptor.
• It has been given a name glutamate dehydrogenase.
7. 2.
a) The free ammonia produced in tissues is combined with
glutamate under the action of another enzyme “glutamine
synthetase”.
b) This is because the toxic ammonia has to be converted into
a non-toxic compound before export into the blood.
c) Hence, ATP & glutamate react to form ADP and ү-glutamyl
phosphate(intermediate) which then reacts with ammonia
to produce glutamine & inorganic phosphate.
d) This glutamine is a non-toxic transport form of ammonia.
• ENZYME- Glutamine synthetase
• E.C Number- 6.3.1.2
• Systematic name – L-glutamate:ammonia ligase
• It belongs to ligase class.
• It forms carbon-nitrogen bonds.
• It belongs to sub-sub class acid-ammonia ligase.
8. 3.
a) Glutamine is now converted into Glutamate & NH₄⁺ under
the action of the enzyme Glutaminase.
b) The NH₄⁺ from intestine & kidney is transported in the
blood to the liver.
c) In the liver, the ammonia from all sources is disposed off by
urea synthesis.
• ENZYME – Glutaminase
• E.C Number – 3.5.1.2
• Systematic Name – L-Glutamine amidohydrolase.
• It is a hydrolase.
• It acts on carbon-nitrogen bonds other than peptide bonds.
• It acts on linear amides.
• It is named as Glutaminase.
• It always acts in the presence of an acceptor water molecule.
9. UREA CYCLE
1.
• Ammonia deposited in the mitochondria of
hepatocytes is required to be converted into urea.
• It spans two cellular compartments; mitochondria
of liver & the cytosol.
• The NH₄⁺ generated in the liver & CO₂ produced by
mitochondrial respiration forms ‘carbamoyl
phosphate’ in the matrix,catalayzed by ‘Carbamoyl
phosphate synthetase І’.
10. • Enzyme – Carbamoyl phosphate synthetase І.
• E.C Number – 6.3.4.16
• Systematic name- Hydrogen carbonate:ammonia ligase.
• It is a ligase.
• It forms carbon-nitrogen bonds.
• It is other carbon-nitrogen ligase.
• It is carbamoyl phosphate synthetase І.
11. 2.
a) The carbamoyl phosphate donates its carbamoyl
group to ornithine to form citrulline with the
release of Pi.
b) The enzyme catalyzing it is Ornithine
transcarbamoylase.
• ENZYME – Ornithine transcarbamoylase
• E.C Number – 2.1.3.3
• Systematic Name - Carbamoyl-phosphate:L-ornithine
carbamoyltransferase
• It is a transferase.
• It tranfers one-carbon groups.
• These are carboxy and carbamoyltransferses.
• It is named ornithine transcarbamoylase.
• The citrulline passes from the mitochondrion to the cytosol.
12.
13. 3.
a) A condensation reaction takes place between the
amino group of aspartate(generated in
mitochondria & transported into cytosol) and the
ureido (carbonyl) group of citrulline.
b) It leads to the formation of argininosuccinate.
c) It is catalyzed by argininosuccinate synthetase.
• ENZYME – Argininosuccinate synthetase
• E.C Number – 6.3.4.5
• Sytematic name – L-citrulline:L-aspartate ligase
• It is a ligase.
• It forms carbon-nitrogen bonds.
• It is other carbon-nitrogen ligase.
• Its name is argininosuccinate synthetase.
14. 4.
a) The argininosuccinate is then cleaved by the
enzyme argininosuccinase.
b) It forms free arginine & fumarate.
c) This fumarate enters mitochondria to participate
Kreb’s cycle.
• ENZYME – Argininosuccinase
• E.C Number – 4.3.2.1
• It is a lyase.
• It acts on carbon-nitrogen bonds.
• It acts on amides,amidines etc.
• It is arginosuccinate lyase.
15. 5.
a) Arginine is cleaved by the cytosolic enzyme
‘arginase’.
b) It yields urea & ornithine.
c) This ornithine is transported into the mitochondria
to start another round of urea cycle.
• ENZYME – Arginase
• E.C Number – 3.5.3.1
• Systematic name – L-arginine:amidino hydrolase
• It is a hydrolase.
• It acts on carbon-nitrogen bonds.
• It acts on other than peptide bonds in linear
amidines.
• It is arginase.
16. CONCLUSION
• The liver is the major site of degradation for most
amino acids.All amino acids contain atleast one
nitrogen atom,which forms their α-amino group.
• Nitrogen is removed from the carbon skeleton and
transferred to α-ketoglutarate , which yields glutamate.
• The carbon skeletons are converted to intermediates of
the mainstream carbon oxidation pathways.
• Surplus nitrogen is removed from
glutamate,incorporated into urea,and excreted.
