Know the: Know the:

1.Urea Cycle: Reaction and Significance.1.Urea Cycle: Reaction and Significance.2.The Importance of the Cytosol and Mitochondria.2.The Importance of the Cytosol and Mitochondria.3.Enzyme Defect and Subsequent Clinical Diagnosis. 3.Enzyme Defect and Subsequent Clinical Diagnosis.

Lecture outline…Lecture outline…

2727thth Mar Mar 20122012

Learning Learning ObjectivesObjectives

II.Urea CycleII.Urea Cycle

A.Reactions of the urea cycle.A.Reactions of the urea cycle. 1.Synthesis of carbamoyl phosphate.1.Synthesis of carbamoyl phosphate. 2. Production of arginine by urea cycle.2. Production of arginine by urea cycle. 3. Cleavage of arginine to produce urea.3. Cleavage of arginine to produce urea.

B. Origin of ornithine.B. Origin of ornithine.

C. Regulation of the urea cycle. C. Regulation of the urea cycle.

D. Function of the urea cycle during fasting.D. Function of the urea cycle during fasting.

III. Disorders of the Urea Cycle. III. Disorders of the Urea Cycle. http://www.wiley.com/college/fob/quiz/quiz20/20-8.html

What is What is UreaUrea

Urea is a diamide, chiefUrea is a diamide, chief nitrogenous waste nitrogenous waste product. product.

Ammonia +COAmmonia +CO22 + Aspartate are the + Aspartate are the precursors of Urea.precursors of Urea.

Other nitrogenous waste products Other nitrogenous waste products which which includes includes uric acid & creatinineuric acid & creatinine..

What are the Precursors of What are the Precursors of UreaUrea

H2N C

O

NH2

Urea

HH22N-C-NHN-C-NH22

UreaUrea

OO

CarbonCarbon NitrogenNitrogen

Other Other N-N-Containing Containing CompoundsCompounds

Dietary ProteinsDietary ProteinsSummary of Amino Acid MetabolismSummary of Amino Acid Metabolism

Amino Acids in Amino Acids in BloodBlood

Amino AcidsAmino Acids

COCO22 + H + H22OOEnergy ATPEnergy ATP

ProteinProteinss

Digestion (Stomach Digestion (Stomach Intestine)Intestine)

MembranMembranee

-Aminobutyrate-AminobutyrateDopamineDopamineNorepinephrineNorepinephrineEpinephrineEpinephrineSerotoninSerotonin

-C--C-

UreaUrea

OO

CarboCarbonn

NitrogeNitrogenn

Fate of Amino Acid Nitrogen: Urea CycleFate of Amino Acid Nitrogen: Urea CycleAmino Amino AcidsAcids

AspartateAspartate

COCO22+H+H22OO

COO-COO-

H C-H C-

COO-COO-

StoragStoragee

EnergEnergyy

ATPATP

NHNH44++

+CO+CO22

----

--

NHNH22 Urea Urea

CycleCycleNHNH22

NHNH

22

What are the source of amino groups and carbonyl group for the biosynthesis of Urea?.

NH4+

Ammonium ion

Uric acidH2N-C-NH2

Urea

O

NH4+

O

O

HN

NH

NH

NH

O

Most terrestrial vertebrates Birds & reptiles

Fish & other aquatic

vertebrates

Some animals excrete NH4

+ or uric acid.

Amino Amino acidsacids

The carbon chains are broken down to molecules that feed into the TCA cycle.

Most mammals convert amino-acid nitrogen to urea for excretion

Excretion of Nitogenous Waste ProductsExcretion of Nitogenous Waste Products

An Overview of Urea An Overview of Urea CycleCycle

Urea biosynthesis occurs Urea biosynthesis occurs partly in the cytosol and partly in the cytosol and partly in the mitochondria of partly in the mitochondria of the liver.the liver.

What Are The Sites & The What Are The Sites & The Organ In Which Organ In Which

Biosynthesis Of Urea Biosynthesis Of Urea Occurs?Occurs?

Net reaction:

NH3 + CO2 + 2 ATP(HCO3)

Carbamoyl-Phosphate + 2 ADP + Pi

NH4+ + HCO3

- + 2 ATP

UreaArginine

Argininosuccinate

Citrulline

Carbamoyl phosphate

Mitochondrial Matrix

Cytoplasm

Arginase

Carbamoyl phosphate synthase I (CPS-I)

Ornithine Transcarbamoylase (OTC)

Argininosuccinate Synthetase

Argininosuccinate Lyase

Fumerate

The Urea Cycle in the LiverThe Urea Cycle in the Liver

Ornithine

Citrulline

Ornithine

Hep

ato

cyte

Aspartate

ATP

AMP+ Pi

1

2

3

45

What is Normal Value of UreaWhat is Normal Value of Urea Serum urea concentration is 15-40 Serum urea concentration is 15-40 mg/dL.mg/dL.

Accumulation of urea more than normal Accumulation of urea more than normal value in blood is referred asvalue in blood is referred as’””UremiaUremia”.”.

Urea Excretion: 20–30 gm / day.Urea Excretion: 20–30 gm / day.

Not yet identified, but probably may Not yet identified, but probably may involve ininvolve in ‘’’’’the maintenance of osmotic the maintenance of osmotic balance as abalance as a ‘’’’’crystalloid.crystalloid.

Is there any Specific functions of Is there any Specific functions of UreaUrea

Osmolality of Plasma: 285-295 milliosmoles / Kg.Osmolality of Plasma: 285-295 milliosmoles / Kg.

OsmolalityOsmolality is a measure of the solute particles is a measure of the solute particles present in the fluid medium.present in the fluid medium.

Plasma Osmolality Plasma Osmolality can be computed fromcan be computed from the the concentrations concentrations (mmol/l) of Na(mmol/l) of Na++, K, K++, urea, & glucose, urea, & glucoseas follows.as follows. 2(Na2(Na++) + 2(K) + 2(K++) + Urea + Glucose) + Urea + Glucose

The factor 2 is used for NaThe factor 2 is used for Na++ & K & K++ ions for the ions for theassociated anion concentration.associated anion concentration.

Plasma OsmolalityPlasma Osmolality will be altered in lipidaemias, ESRD, will be altered in lipidaemias, ESRD, hyperproteinaemia, ketoacidosis, diabetes insipidus,chronichyperproteinaemia, ketoacidosis, diabetes insipidus,chronicalcohol intoxication, diuretics & in chronic diarrhoea.alcohol intoxication, diuretics & in chronic diarrhoea.

Osmolality of PlasmaOsmolality of Plasma

Is any Clinical Conditions Are Is any Clinical Conditions Are related with Low Urea Levelrelated with Low Urea Level

Impairment of liver functions and Impairment of liver functions and congenital deficiencies of urea cycle congenital deficiencies of urea cycle enzymes will impair the biosynthesis of enzymes will impair the biosynthesis of urea, which ultimately leads to urea, which ultimately leads to accumulation of ammonia in blood and CSF accumulation of ammonia in blood and CSF causes “causes “azotemia”azotemia”..

What Is Meant By AzotemiaWhat Is Meant By Azotemia Accumulation of nitrogenous materials Accumulation of nitrogenous materials includingincludingammonia due to impairment in urea ammonia due to impairment in urea biosynthesis due to,biosynthesis due to, Urea cycle enzyme deficiencies,Urea cycle enzyme deficiencies,

Gastrointestinal tract (GI) bleeding, Gastrointestinal tract (GI) bleeding,

Valproic acid therapy andValproic acid therapy and

Renal impairments ect.,.Renal impairments ect.,.

What Are The Consequences of What Are The Consequences of UremiaUremia

Uremia will leads to ammonia toxicity, Uremia will leads to ammonia toxicity, hypercapnia & alkalosis which may leads to hypercapnia & alkalosis which may leads to neurotoxicity depending on the severity.neurotoxicity depending on the severity.

Impaired renal function leads to Impaired renal function leads to erythropoietinerythropoietin deficiency and hence deficiency and hence accumulation of COaccumulation of CO22..

Ureamia may occur as a result of Ureamia may occur as a result of impairment in impairment in ‘’’’’the the renal functionsrenal functions, , which occur as a result ofwhich occur as a result of nephrotic syndrome,nephrotic syndrome, heavy metal toxicities, heavy metal toxicities, ESRD and ESRD and

nitrogen load due to high protein diet. nitrogen load due to high protein diet.

What Are The Causes of What Are The Causes of UremiaUremia

Importance of Urea CycleImportance of Urea Cycle

Removal of CORemoval of CO22..

Removal of Ammonia.Removal of Ammonia.

Recycling of TCA cycle Recycling of TCA cycle intermediates.intermediates.

Recycling of amino acids & Recycling of amino acids & ketoacids.ketoacids.

Involvement of Peripheral Tissues And Liver in the Ammonia Metabolism

α-KG = α-Ketoglutaratee GDH = Glutamate Dehydrogenase.

α-Ketoglutarate

Glutamate Dehydrogenase

Glutamine Synthetase

Molecualr Interconversions in Handling of AmmoniaMolecualr Interconversions in Handling of Ammonia

NH4+

ATP

Alanine

Glutamate

Pyruvate

Glutamine

AST

ADP+Pi

Aspartate

Oxaloacetate

NADH+H+

NAD

NADPH+H+

NADPH+

ALT

NH4+

H2O

The major enzyme responsible for the interconversion of Glutamate into α-KG isGlutamate Dehydrogenase. ALT= Alanine aminotransferaseAST= Aspartate aminotransferase.

Reactions of the Urea CycleReactions of the Urea Cycle

1.1. Synthesis of Carbomoyl Phosphate.Synthesis of Carbomoyl Phosphate.2.2. Production of Arginine by the Urea Production of Arginine by the Urea

Cycle.Cycle.3.3. Cleavage of Arginine to Produce Urea.Cleavage of Arginine to Produce Urea.

Most terrestrial land animals convert excess nitrogen to urea, prior to excreting it.

Urea is less toxic than ammonia.

The Urea Cycle occurs mainly in liver.

The 2 nitrogen atoms of urea derived from NH3 and the amino nitrogen of aspartate.

H2N C

O

NH2

Urea

Carbamoyl Phosphate SynthaseCarbamoyl Phosphate Synthase is the is the committed committed stepstep of the Urea Cycle, and is subject to of the Urea Cycle, and is subject to regulationregulation. .

The NH3 and HCO3- that will be part of urea are

incorporated first into carbamoyl phosphate (CP).

H2N C OPO32

O

HCO3 + NH3 + 2 ATP

+ 2 ADP + Pi

Carbamoyl Phosphate Synthase-I

Carbamoyl phosphate

Carbamoyl Phosphate Synthase-ICarbamoyl Phosphate Synthase-I has an has an absolute requirement for an absolute requirement for an allostericallosteric activatoractivator NN-acetylglutamate-acetylglutamate..

This derivative of glutamate is synthesized from This derivative of glutamate is synthesized from acetyl-CoA & glutamate when cellular [glutamate] acetyl-CoA & glutamate when cellular [glutamate] is high, signaling an is high, signaling an excess of free amino acidsexcess of free amino acids due to protein breakdown or dietary intake. due to protein breakdown or dietary intake.

H3N+ C COO

CH2

CH2

COO

H

Glutamate (Glu)

NH

C COO

CH2

CH2

COO

H

CH3C

O

N-Acetylglutamate

Formation & Degradation of N-Formation & Degradation of N-Acetylglutamate (NAG)Acetylglutamate (NAG)

NAG NAG

NAG, is an allosteric activator of CPS-NAG, is an allosteric activator of CPS-I.I.

N-Acetyl glucoseamine (NAG)N-Acetyl glucoseamine (NAG) serves as an serves as an allosteric activatorallosteric activator for the enzyme CPS-I for the for the enzyme CPS-I for the biosynthesis of carbamoyl phosphate (CP) with the biosynthesis of carbamoyl phosphate (CP) with the help of HCOhelp of HCO33

- - and ATP.and ATP.

The The ReactionsReactions

of the Urea of the Urea CycleCycle

NAG:NAG:N-acetyl N-acetyl glutamate; (in the glutamate; (in the formation of urea, formation of urea, one amino group one amino group is derived from is derived from free NHfree NH44

++ ion, ion, while the other is while the other is from aspartate. from aspartate. Carbon is obtained Carbon is obtained from COfrom CO22. (*) . (*) mitochondrial mitochondrial enzymes, the restenzymes, the restof the enzymes of the enzymes are cytosolic).are cytosolic).

NAG*

*Cytosol

Mitochondria

Role of Glutamate in Urea Role of Glutamate in Urea ProductionProduction

TransaminationTransamination

GlutamateGlutamate

OxalaoacetateOxalaoacetate

AspartateAspartate

αα-Ketoacids-Ketoacids

UreaUrea

NHNH44++

Amino acidsAmino acids

TransaminationTransamination

αα-Ketoglutarate-Ketoglutarate

αα-Ketoglutarate-Ketoglutarate

Urea Urea CycleCycle

Other reactionsOther reactions

Role of Glutamate in Amino Acid Role of Glutamate in Amino Acid SynthesisSynthesis

PLPPLP

Glutamate Glutamate

Glutamate transfers Nitrogen by means of Glutamate transfers Nitrogen by means of transamination reactions to transamination reactions to αα–ketoacids to form AAs. –ketoacids to form AAs. The nitrogen is obtained by glutamate either from The nitrogen is obtained by glutamate either from transamination of other AAs or from NHtransamination of other AAs or from NH44

++ by means by means of the of the glutamate dehydrogenaseglutamate dehydrogenase (GDH) reaction (GDH) reaction

αα-Ketoglutarate-Ketoglutarate

Amino acidsAmino acids

TransaminationTransamination

αα-Ketoacid-Ketoacid

PLPPLP GDHGDH

αα-Ketoglutarate-Ketoglutarate

PLP= Pyridoxal phosphatePLP= Pyridoxal phosphate

OxaloacetateOxaloacetate is converted to is converted to aspartateaspartate via via transamination (e.g., from glutamate). transamination (e.g., from glutamate). Aspartate then reenters Urea Cycle, carrying an Aspartate then reenters Urea Cycle, carrying an amino group derived from another amino acid.amino group derived from another amino acid.

Aspartate -Ketoglutarate Oxaloacetate Glutamate

Aminotransferase (Transaminase)

COO

CH2

CH2

C

COO

O

COO

CH2

HC

COO

NH3+

COO

CH2

CH2

HC

COO

NH3+

COO

CH2

C

COO

O + +

Regulation of the Urea CycleRegulation of the Urea Cycle

Liver has a vast capacity to convert AA nitrogen Liver has a vast capacity to convert AA nitrogen to ureato urea

,thereby preventing toxic effects from ammonia.,thereby preventing toxic effects from ammonia. Urea cycle is regulated by substrate (S) Urea cycle is regulated by substrate (S)

availability; the higher the rate of NHavailability; the higher the rate of NH33 production, the higher the rate of urea production, the higher the rate of urea formation.formation.

Regulation by the S availability is a general Regulation by the S availability is a general characteristics of disposal pathways, such as the characteristics of disposal pathways, such as the urea cycle, which remove toxic compounds from urea cycle, which remove toxic compounds from the body.the body.

This is a type of “feed-forward” regulation, in This is a type of “feed-forward” regulation, in contrast to the “feed-back” regulation contrast to the “feed-back” regulation characteristic of pathways that produce characteristic of pathways that produce functional endproducts. functional endproducts.

Regulation of the Urea Cycle…Regulation of the Urea Cycle… The other type of regulation control the urea The other type of regulation control the urea

cycle: cycle: Allosteric activationAllosteric activation of CPS-I by N- of CPS-I by N-acetylglutamate (NAG) and acetylglutamate (NAG) and induction/repressioninduction/repression of the synthesis of urea cycle enzymes.of the synthesis of urea cycle enzymes.

NAG is formed specifically to activate CPS-I; it NAG is formed specifically to activate CPS-I; it has no other known function in mammals.has no other known function in mammals.

The synthesis of NAG from acetyl CoA and The synthesis of NAG from acetyl CoA and glutamate is stimulated by arginine.glutamate is stimulated by arginine.

Thus, the arginine level increase within the liver, Thus, the arginine level increase within the liver, 2 important reactions are stimulated.2 important reactions are stimulated.

The first is the synthesis of NAG, which will The first is the synthesis of NAG, which will increase the rate at which CP is produced.increase the rate at which CP is produced.

The 2The 2ndnd is to produce more ornithine (via is to produce more ornithine (via arginase rxn), such that the cycle can operate arginase rxn), such that the cycle can operate more rapidly. more rapidly.

D. Functions of Urea Cycle During D. Functions of Urea Cycle During FastingFasting During fasting, the liver maintains blood glucose During fasting, the liver maintains blood glucose

by by gluconeogenesisgluconeogenesis by utilizing muscle protein by utilizing muscle protein AAs as a carbon source.AAs as a carbon source.

As AA carbons are converted to As AA carbons are converted to glucoseglucose, the , the nitrogens are converted to urea, thus the nitrogens are converted to urea, thus the urinary excretion of urea is high during fasting.urinary excretion of urea is high during fasting.

As fasting progresses, the brain begins to use As fasting progresses, the brain begins to use ketone bodies, sparing blood glucose.ketone bodies, sparing blood glucose.

Less muscle protein is cleaved to provide AAs for Less muscle protein is cleaved to provide AAs for gluconeogenesis,and decreased production of gluconeogenesis,and decreased production of glucose from AAs for gluconeogenesis.glucose from AAs for gluconeogenesis.

Hence, Decreased production of glucose from Hence, Decreased production of glucose from AAs is accompanied by decreased production of AAs is accompanied by decreased production of urea. urea.

D.Functions of Urea Cycle During D.Functions of Urea Cycle During Fasting…Fasting… The major AA substrate for gluconeogenesis is The major AA substrate for gluconeogenesis is

AlanineAlanine, which is synthesized in peripheral , which is synthesized in peripheral tissues to act as a nitrogen carrier.tissues to act as a nitrogen carrier.

GlucagonGlucagon release, which is expected during release, which is expected during fasting, fasting, stimulates alanine transport into the stimulates alanine transport into the liverliver by activating the transcription of transport by activating the transcription of transport systems for alanine.systems for alanine.

Two molecules of alanine are required to Two molecules of alanine are required to generate one molecule of glucose.generate one molecule of glucose.

The nitrogen from 2 molecules of alanine is The nitrogen from 2 molecules of alanine is converted to one molecule of urea.converted to one molecule of urea.

Hereditary deficiencyHereditary deficiency of any of the Urea of any of the Urea Cycle enzymes leads to Cycle enzymes leads to hyperammonemiahyperammonemia - - elevated [ammonia] in blood. elevated [ammonia] in blood.

Total lack of any Urea Cycle enzyme is lethal. Total lack of any Urea Cycle enzyme is lethal.

Elevated ammonia is toxic, especially to the Elevated ammonia is toxic, especially to the brainbrain. .

If not treated immediately after birth, If not treated immediately after birth, severe severe mental retardationmental retardation results. results.

1.1.Ornithine Transcarbamylase Deficiency (OTC) Ornithine Transcarbamylase Deficiency (OTC) 2.2.Citrullinemia. Citrullinemia. 3.3.Arginase Deficiency Arginase Deficiency 4.4.Argininosuccinic Aciduria Argininosuccinic Aciduria 5.5.Carbamyl Phosphate Synthetase (CPS-I) Deficiency Carbamyl Phosphate Synthetase (CPS-I) Deficiency 6.6.N-Acetyl Glutamate Synthetase Deficiency (NAGS)N-Acetyl Glutamate Synthetase Deficiency (NAGS)

Genetic Deficiency of Urea SynthesisGenetic Deficiency of Urea Synthesis

May be caused by,

Genetic Deficiency of Urea SynthesisGenetic Deficiency of Urea Synthesis

Carbamoyl Phosphate Carbamoyl Phosphate Synthetase-ISynthetase-I

(CPS-I) Deficiency(CPS-I) Deficiency

Ornithine Ornithine Transcarbamylase Transcarbamylase

(OTC)(OTC)

↑↑[NH[NH44++]; hyperammonemia ]; hyperammonemia ↑↑[NH[NH44

++]; ];

hyperammonemiahyperammonemia

↑↑Blood GlutamineBlood Glutamine ↑↑Blood GlutamineBlood Glutamine

BUN is decreasedBUN is decreased BUN is decreasedBUN is decreased

No increase in uracil or orotic acidNo increase in uracil or orotic acid ↑↑Uracil & orotic acid in Uracil & orotic acid in blood & urineblood & urine

Cerbral edemaCerbral edema Cerebral edemaCerebral edema

Lethargy, convulsions, coma, Lethargy, convulsions, coma, deathdeath

Lethargy, convulsions, Lethargy, convulsions, coma, deathcoma, death

Disorders present in infants:Disorders present in infants:

Symptoms:Symptoms: Lethargy, swelling of the brain leads to Lethargy, swelling of the brain leads to mental retardation / brain damage.mental retardation / brain damage.

Diagnosis:Diagnosis: Low blood urea nitrogen (BUN) levels -high Low blood urea nitrogen (BUN) levels -high levels of ammonia in the blood elevated circulating levels of ammonia in the blood elevated circulating glutamine glutamine -other metabolites that accumulate depend on the -other metabolites that accumulate depend on the specific enzyme defect.specific enzyme defect.

Most common form:Most common form: Hyperammonemia Type II caused Hyperammonemia Type II caused by Ornithine Transcarbamylase (OTC) Deficiency by Ornithine Transcarbamylase (OTC) Deficiency elevated Carbamoyl-Phosphate (CP) levels in this elevated Carbamoyl-Phosphate (CP) levels in this deficiency causedeficiency causesecondary problems in pyrimidine metabolism.secondary problems in pyrimidine metabolism.

Metabolic Diseases of the Urea Cycle

Treatment:Treatment:

Long termLong term, dietary restriction., dietary restriction.Low protein diet. Supplemented with Arginine.Low protein diet. Supplemented with Arginine.

Short termShort termDialysis.Dialysis.Administration of Nitrogen “scavengers”.Administration of Nitrogen “scavengers”.

e.g. Phenylacetate.e.g. Phenylacetate.

Treatment of Hyperammonemia with Phenylacetate:Treatment of Hyperammonemia with Phenylacetate:taking advantage of metabolismtaking advantage of metabolism

PhenylbutyratePhenylbutyrate

Glycine + Benzoic Acid Hippuric Acid

Excreted in Urine

1.Which biomolecule acts as a positive modulator for the first step of the urea cycle which is catalyzed by carbamoyl phosphate synthetase-I (CPS-I).

A. Gultamate.B. Glutamine. C. N-Acetyl Glutamate (NAG).D. Aspartate.E. Ammonium ions (NH4+).

Ans: C.

7. Which of the following is a common compound shared by The TCA cycle and the urea cycle?

A. α-ketoglutarate.B. Succcinyl Coenzyme A (CoA).C. Oxaloacetate.D. Fumarate.*E. Arginine.

Ans: D.

8. Which of the following enzymes requires adenosine triphosphate (ATP) to mediate its reactions?

A. Argininosuccinate lyase.B. Argininosuccinate synthetase.C. Arginase.D. Glutaminase.E. Ornitine transcarbamoylase.

Ans: B.

10. Two days after a full-term normal delivery, a neonate begins to hyperventilate, develops hypothermia and cerebral edema, and becomes comatose. Urinalysis reveals high levels of glutamine and orotic acid. The BUN is below normal. Which enzyme is most likely to be deficient in this child?

A. Cytoplasmic glutaminase.B. Cytoplasmic carbamoyl phosphate synthetase.C. Cytoplasmic orotidylate decarboxylase.D. Mitochondrial carbamoyl phosphate synthetase.E. Mitochondrial ornitihine transcarbamoylase.

Ans: E.

11. Transamination reactions are essential for ammonia assimilation. What cofactor is required to catalyze transamination reactions?

A. pyridoxal phosphate (PLP).B. thiamin pyrophosphate.C. biotin.D. NAD+.E. NADPH.

Ans: A.

12. Which of the following enzymes function in the biologicalassimilation of ammonia?

A. Glutamate dehydrogenase.B. Glutamine synthetase.C. Glutamate synthase.D. All of the above.***E. None of the above.

Ans: D.

A 55-year-old man suffers from cirrhosis of the liver. Toxins suchas ammonia are not properly metabolized by the liver and can now damage structures such as the brain. Which of the followingamino acids covalently binds ammonia and transports and storesit in a nontoxic form? A. Aspartate.B. Glutamate.C. Serine.D. Cysteine.E. Histidine.

Ans: B.

14. Which amino acid participates as a pathway intermediate in the urea cycle?

A. Lysine.B. Arginine.***C. Glutamine.D. Histidine.E. Tyrosine.

Ans: B

Urea Cycle Must Know Key PointsUrea Cycle Must Know Key Points1.1. Urea cycle occurs partly in ……….. and partly in the ……….Urea cycle occurs partly in ……….. and partly in the ……….

Ans:Ans: Mitochondria, Cytosol / Cytoplasm. Mitochondria, Cytosol / Cytoplasm.

2. The rate-limiting step (committed step) in the urea cycle is 2. The rate-limiting step (committed step) in the urea cycle is catalyzed by the enzyme …………catalyzed by the enzyme …………

Ans:Ans: Carbamoyl phosphate synthetase-I (CPS-I). Carbamoyl phosphate synthetase-I (CPS-I).

3. The biomolecule acts as a positive modulator in the first step of 3. The biomolecule acts as a positive modulator in the first step of the urea cycle is……………the urea cycle is……………Ans:Ans: N-Acetyl glutamate (NAG). N-Acetyl glutamate (NAG).

4. ………….can be converted to arginine by a series of reactions,4. ………….can be converted to arginine by a series of reactions, some of which requires urea cycle enzymes.some of which requires urea cycle enzymes.Ans:Ans: Glutamate. Glutamate.

5. The enzymes are responsible for producing the direct donors of 5. The enzymes are responsible for producing the direct donors of nitrogen into the pathway producing urea, include ………. And nitrogen into the pathway producing urea, include ………. And ………..………..

Ans:Ans: Aspartate aminotransferase and carbmoyl phosphate Aspartate aminotransferase and carbmoyl phosphate sythetase.sythetase.

6. The accumulation of glutamine and orotic acid in the blood and6. The accumulation of glutamine and orotic acid in the blood andan ultimate ammonia toxicity in the newborn is due to the an ultimate ammonia toxicity in the newborn is due to the deficiency of the urea cycle enzyme……….deficiency of the urea cycle enzyme……….Ans: Ans: Mitochondrial ornitihine transcarbamoylase (OTC).Mitochondrial ornitihine transcarbamoylase (OTC).

7. The common nitrogen acceptor for all reactions involving 7. The common nitrogen acceptor for all reactions involving transaminase is ……………transaminase is ……………

Ans: α-ketoglutarate.Ans: α-ketoglutarate.

8. The common nitrogen donor for all reactions involving 8. The common nitrogen donor for all reactions involving transaminase is ……………transaminase is …………… Ans:Ans: Oxaloacetate (OAA). Oxaloacetate (OAA).

9. The common compound shared by the TCA cycle and the urea cycle is ……………… 9. The common compound shared by the TCA cycle and the urea cycle is ……………… Ans:Ans: Fumarate. Fumarate.

10. Hyperammonemia caused by a congenital defect of the urea cycle 10. Hyperammonemia caused by a congenital defect of the urea cycle enzyme………..which is characterized by accumulation of excess amount of arginine enzyme………..which is characterized by accumulation of excess amount of arginine in the blood.in the blood.

Ans: Ans: Arginase.Arginase.

11. The urea cycle enzyme which requires adenosine triphosphate (ATP) to mediate its 11. The urea cycle enzyme which requires adenosine triphosphate (ATP) to mediate its reactions is ?. reactions is ?.

Ans: Ans: Argininosuccinate synthetase.Argininosuccinate synthetase.

12. The reaction catalyzed by an urea cycle enzyme 12. The reaction catalyzed by an urea cycle enzyme ornithine aminotransferaseornithine aminotransferase which which requires --------- as a coenzyme, which converts ornithine and requires --------- as a coenzyme, which converts ornithine and αα-ketoglutarate into -ketoglutarate into glutamate. glutamate.

Ans:Ans: Pyridoxal phosphate (PLP). Pyridoxal phosphate (PLP).

NoNo DisorderDisorder Answer Answer Enzyme DefectEnzyme Defect

11 Hyperammonemia type IHyperammonemia type I A. Argininosuccinate synthaseA. Argininosuccinate synthase

22 Hyperammonemia type IIHyperammonemia type II B. Ornithine transcarbomylase (OTC)B. Ornithine transcarbomylase (OTC)

33 CitrullinemiaCitrullinemia C. Argininosuccinate lyaseC. Argininosuccinate lyase

44 Argininosuccinic AciduriaArgininosuccinic Aciduria D. ArginaseD. Arginase

55 HyperargininemiaHyperargininemia E. Carbamoyl-P synthase-I (CPS)E. Carbamoyl-P synthase-I (CPS)

Answer:Answer: 1E; 2-B; 3-A; 4-C; 5-D1E; 2-B; 3-A; 4-C; 5-D

Match the Urea Cycle Disorders With Match the Urea Cycle Disorders With the Enzyme Defectthe Enzyme Defect

Genetic Deficiencies in Some of the Urea Cycle Enzymes can be Treated Genetic Deficiencies in Some of the Urea Cycle Enzymes can be Treated Pharmacologically by eliminating the amino acids such as glycine and glutamine by Pharmacologically by eliminating the amino acids such as glycine and glutamine by administering, administering,

A. Aspartic acid.A. Aspartic acid.B. B. Benzoic acid and phenylacetate.Benzoic acid and phenylacetate.C. GABA.C. GABA.D. Pyridoxal phosphate.D. Pyridoxal phosphate.E. Methionine.E. Methionine.

Ans: B. Ans: B. The amide products of these reactions (hippurate and The amide products of these reactions (hippurate and phenylacetylglutamine) are excreted in the urine. Synthesizing the Gly or Gln phenylacetylglutamine) are excreted in the urine. Synthesizing the Gly or Gln removes ammonia.removes ammonia.

The Reaction Catalyzed by Glutamate Dehydrogenase whichThe Reaction Catalyzed by Glutamate Dehydrogenase whichreversibly converts glutamate to reversibly converts glutamate to αα-ketoglutarate require the -ketoglutarate require the cofactor,cofactor,

A.A. ATPATPB.B. NADNADC.C. NAD(P)NAD(P)++ /NAD(P)H. /NAD(P)H. D.D. BiotinBiotinE.E. Pyridoxal phosphate.Pyridoxal phosphate.

Ans: C. Ans: C. The cofactors require by the glutamate dehydrogenase The cofactors require by the glutamate dehydrogenase is NAD(P)+ /NAD(P)H. is NAD(P)+ /NAD(P)H.