Urea cycle

UREA CYCLE
PRESENTED BY:
MAULIK PATEL
MSc. Biochemistry
Roll no: 11
P-401

Presentation Overview
1) Introduction
2) History
3) Urea Cycle and It’s Reactions
4) Link between Urea Cycle and TCA Cycle
5) Regulation
6) Energetics
7) Disorder
8) Reference

Introduction
 Urea is the major disposal form of amino groups derived from amino
acids pool and 90% of nitrogen containing component of urine
 Urea cycle is a cyclic process
 Urea formation takes place in the liver
 Some reaction occur in mitochondria(1,2) and some In cytosol(3,4,5)
 Synthesis of 1 molecule of urea need
• 3 Molecule of ATP
• 1 Molecule of ammonium ion
• 1 molecule of α-amino nitrogen of aspartate
 5 enzyme catalyzed the numbered reaction

History
 The urea cycle is the first metabolic
pathway to be elucidated.
 Thecycleisknownas Krebs–Henseleit
Urea Cycle.
 It was discovered five years before the
discovery of the TCA cycle.
 It takes place primarily in the liver
and, to a lesser extent, in the kidneys.

Properties Of Urea
 Non toxic
 Water soluble
 Combines two waste products into
one molecule:
• CO2
• NH3

Pros Of Urea Against Ammonia
Ammonia is an extremely toxic base and its
accumulation in our body would be quickly
fatal.
Liver contains a system of carrier molecules &
enzymes which coverts ammonia to urea.

Blood Urea Nitrogen
 Normal range: 7-18 mg/dL
 Elevated in amino acid catabolism
 Elevated in renal insufficiency
 Decreased in hepatic failure

2 ATP 2 ADP
NH3 Pi
Step-1-Formation of Carbamoyl Phosphate
 Reactionof bicarbonate with ATP forms carbonyl phosphate and ADP.
 Ammonia then displaces ADP, forming carbamate and orthophosphate.
 Phosphorylation of carbamate by the second ATP then forms carbamoyl phosphate.

 This enzyme has no regulatory significance. The remainder of the urea cycle
steps take place in the cytosol. This requires the continuous export of
citrulline and the uptake of ornithine across the inner mitochondrial
membrane.
Step-2- Formation of Citrulline
 The Carbamoyl group of Carbamoyl
phosphate is transferred to ornithine,forming
Citrulline and Ortho Phosphate
 The reaction is catalyzed
by Ornithine
transcarbamoylase
 Subsequent metabolism of Citrulline take
place in the cytosol.
 Entry of ornithine into mitochondria and exit
of citrulline from mitochondria involves
mitochondrial inner membrane transport
systems

 Production of arginino-succinate is an energetically expensive process, since the ATP is split
to AMP and pyrophosphate.
 The pyrophosphate is then cleaved to inorganic phosphate using pyrophosphatase , so
the overall reaction costs two equivalents of high energy phosphate per mole.
 The reaction requires ATP and involves intermediate formation of citruIlyl-AMP.
Subsequent displacement of AMP by aspartate then forms Argininosuccinate.
Step-3- Formation Of Arginosuccinate

Step-4- Cleavage Of Arginosuccinate
 Cleavage of argininosuccinate catalyzed by argininosuccinate lyase
(ASL), proceeds with retention of nitrogen in arginine and release of
the aspartate skeleton as fumarate.
 Addition of water to fumarate forms L-malate, and subsequent NAD+-
dependent oxidation of malate forms oxaloacetate.
 Transamination of oxaloacetate by glutamate aminotransferase then
reforms aspartate. carbon skeleton of aspartate-fumarate thus acts as a
carrier of the nitrogen of glutamate into a precursor of urea

 In each case fumarate is formed as a by-product. Fumarate is not
transported by mitochondria, so this requires the presence of
cytosolic fumarase to form malate.
Fumarate Malate
Fumarase MDH
Oxaloacetate Aspartate
Aminotransferase
NAD+ NADH+H+

Step-5- Cleavage of Arginine
 Hydrolytic cleavage of the guanidino group of
arginine,catalyzed by liver arginase (ARGl)
releases urea, the other product,Ornithine,
reenters liver mitochondria for addition a l
rounds of urea synthesis.
 Ornithine and lysine a repotent inhibitors of
arginase,competitive with arginine.
 Arginase is activated by Co2+ & Mn2+
 Ornithine & lysine compete with arginine
(competitive inhibition).

Link Between Citric Acid
Cycle And Urea Cycle
 The fumarate produced in the urea cycle is an intermediate in
citric acid cycle
 Aspartate formed in mitochondria by transamination between
oxaloacetate and glutamate which is transported to the cytosol.
Where it serves as nitrogen donor in the urea cycle.
 These reactions , making up the aspartate-arginosuccinate shunt,
provide metabolic link between these two pathways.

Regulations Of Urea Cycle
 Coarse regulation
• Enzyme level changes with protein content of diet
• Starvation, urea cycle elevated to meet increase rate of
protein catabolism
 Fine regulation (allosterically)
• Majorly via CPS-1, through positive effector is N-acetyl
glutamate (NAG)
• Arginine activate NAG synthase

Energetics Of Urea Cycle
 The overall reaction may be summarized as:
NH3 + CO2 + aspartate → urea + fumarate
 2 ATP are used in 1st reaction
 Another ATP is converted to AMP + ppi in the 3rd step which is equivalent to
two ATPs
 The urea cycle consumes 4 high energy PHOSPHATE BONDS.
 Fumarate formed in the 4th step may be converted to malate
 Malate when oxidised to oxaloacetate produces 1 NADH equivalent to 2.5 ATP.
• So net energy expenditure is only 1.5 high energy phosphates.
• The urea cycle & TCA cycle are interlinked & it is called as "urea bicycle".

 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, i.e.,
Urea cycle leads to Hyperammonaemia or NH3 intoxication.
Disorders of Urea Cycle

 Inherited disorders of urea
cycleenzymes- familial
hyperammonaemia.
 Acquired disorders- Liver
Disease, severe Renal
disease -Acquired
hyperammonaemia.
Hyperammonaemia

 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.
Glutamate
NADPH + H+ NADP+
GDH
α- Ketoglutarate + NH3
Ammonia Toxicity

 In diseases of the liver, hepatic failure can finally lead to hepatic coma &
death.
 Hyperammonemia is the characteristic feature of liver failure.
 The condition is also known as portal systemic encephalopathy.
 Normally the ammonia & 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 & their concentration in systemic circulation rises.
Hepatic Coma (Acquired
Hyperammonemia)

Disorders Defective Enzyme
Products
accumulated
Hyperammonaemia-1 Carbamoyl Phosphate Syntethase -1 Ammonia
Hyperammonaemia-2
Ornithine transcarbomylase
(orotic aciduria-most Common)
Ammonia
Citrullinemia Argininino succinate Syntheatase Citrulline
Arginosuccinic aciduria Argininosuccinate lyase Arginosuccinate
Argininemia Arginase Arginine
Inherited Disorders Of Urea Cycle

References
 Textbook of Biochemistry-U Satyanarayana
 TextbookofBiochemistry-DM Vasudevan

Urea cycle

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