Urea Cycle

Dr. Zahid AzeemAssistant Professor of Biochemistry

AJ&K Medical College Muzaffarabad

For MBBS-batch 2017

• Turnover mean both degradation and synthesis

• Proteins are constantly degraded and

resynthesized

• Why degradation?

To remove

a) Damaged Proteins

b) Un-needed Proteins

c) Defective Proteins

d) Old Proteins (Actually the half life of many proteins is

determined at the time of their synthesis)

Protein Turnover

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Half life of aminoacids

Highly stabilizing residues (t1/2>20 hours)

Ala Cys Gly Met

Pro Ser Thr Val

Intrinsically destabilizing residues

(t1/2= 2 to 30 minutes)

Arg His Ile Leu

Lys Phe Trp Tyr

Destabilizing residues after chemical

modification (t1/2= 3 to 30 minutes)

Asn Asp Gln Glu

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Amino acid oxidation and the production of urea

Waste or reuse

Oxidation

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Ammonia has to be eliminated

• Ammonia originates in the catabolism of amino acidsthat are primarily produced by the degradation of proteins – dietary as well as existing within the cell:

digestive enzymes

proteins released by digestion of cells sloughed-off the walls of the GIT

muscle proteins

hemoglobin

intracellular proteins (damaged, unnecessary)

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Ammonia has to be eliminated

• Ammonia is toxic, especially for the CNS, because it reacts with -ketoglutarate, thus making it limiting for the TCA cycle decrease in the ATP level

• Liver damage or metabolic disorders associated with elevated ammonia can lead to tremor, slurred speech, blurred vision, coma, and death

• Normal conc. of ammonia in blood: 30-60 µM

6Dr zahid

Overview of amino acid catabolism in mammals

2 CHOICES1. Reuse

2. Urea cycle

Fumarate

Oxaloacetate

7Dr. Inayat Abbasi

Nitrogen removal from amino acids

Transamination

Oxidativedeamination

Urea cycle

AminotransferasePLP

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Nitrogen removal from amino acids

Step 1: Remove amino group

Step 2: Take amino group to liver for nitrogen excretion

Step 3: Entry into mitochondria

Step 4: Prepare nitrogen to enter urea cycle

Step 5: Urea cycle

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Step 1. Remove amino group ; Transamination

• Transfer of the amino group of an amino acid to an -ketoacid the original AA is converted to the corresponding -keto acid and vice versa: Transamination is catalyzed by transaminases (aminotransferases) that require participation of pyridoxalphosphate

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Step 2: Take amino group to liver for nitrogen excretion; Oxidation

deaminationn

Glutamatedehydrogenase

The glutamate dehydrogenase of mammalian liverhas the unusual capacity to use either NAD+ orNADP+ as cofactor

Glutamate releases its amino group asammonia in the liver.

The amino groups from many of the-amino acids are collected in theliver in the form of the amino groupof L-glutamate molecules.

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Oxidationdeamination

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1. Glutamate

transferres one amino group WITHIN cells:

Aminotransferase → makes glutamate from -ketogluta-rate

Glutamate dehydrogenase → opposite

2. Glutamine

transferres two amino group BETWEEN cells → releases its amino group in the liver

3. Alanine

transferres amino group from tissue (muscle) into the liver

Nitrogen carriers

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Glucose-alanine cycle

Ala is the carrier of ammonia and of the carbonskeleton of pyruvate from muscle to liver.The ammonia is excreted and the pyruvate is usedto produce glucose, which is returned to themuscle.

Alanine plays a special role in transporting amino groups to liver.

According to D. L. Nelson, M. M. Cox :LEHNINGER. PRINCIPLES

OF BIOCHEMISTRY Fifth edition

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Step 3: entry of nitrogen to mitochondria

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Step 4: prepare nitrogen to enter urea cycle

Regulation

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Step 5: Urea cycleaspartate

Ornithine transcarbamoylase

Argininosuccinate synthase

Argininosuccinate lyase

Arginase 1

19Dr. Inayat Abbasi

Dr. Inayat Abbasi 20

OOA

Oxaloacetate → aspartate

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The overall chemical balance of the biosynthesis of urea

NH3 + CO2 + 2ATP → carbamoyl phosphate + 2ADP + Pi

Carbamoyl phosphate + ornithine → citrulline + Pi

Citrulline + ATP + aspartate → argininosuccinate + AMP + PPi

Argininosuccinate → arginine + fumarate

Arginine → urea + ornithine

Sum: 2NH3 + CO2 + 3ATP urea + 2ADP + AMP + PPi + 2Pi

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Deficiencies of urea cycle enzymes

Ammonia toxicity

Ammonia encephalopathy

• Increased concentration of ammonia in the blood and other biological fluids → ammonia difuses into cells, across blood/brain barrier → increased synthesis of glutamate from -ketoglutarate, increased synthesis of glutamine

-ketoglutarate is depleted from CNS → inhibition of TCA cycle and production of ATP

• Neurotransmitters – glutamate (excitatory neurotr.) and GABA (inhibitory neurotr.), may contribute to the CNS effects – bizarre behaviour

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Deficiencies of urea cycle enzymes

• Infant born with total deficiency of one or more enzymessurvive at least several days.

• Many enzymes deficiencies are partial → enzymes havealtered Km values.

• Case are known of deficiencies of each enzymes.

• Interruption of the cycle at each point affected nitrogenmetabolism differently - some of the intermediates candiffuse from hepatocytes → accumulate in the blood → passinto the urine.

• If symptoms are not detected early enough → severe mentalretardation → brain damage is irreversible.

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• Urea Level in Blood:

• In clinical practice, blood urea level is taken asan indicator of renal function. The normalurea level in plasma is from 20 t0 40 mg/dl.Blood urea level is increased where renalfunction is inadequate.

• Urinary excretion of urea is 15 to 30g/day (6-15g nitrogen/day). Urea constitutes 80% ofurinary organic solids.

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• Clinical importance of urea:• An active man consuming 300grams

carbohydrates, 100grams of fats and 100 gramsof proteins excretes 16.5 grams of N daily. 95%Nis eliminated through kidneys and remaining 5%through feces.

• Normal Urea level:• Healthy adult in fasting condition has urea 20-

40mg/100ml of plasma. Pakistani individualtaking less protein may have normal level of urea15-40mg/100ml of plasma.

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Causes of Increased Urea Levels (Uremia):

• Pre-renal Causes:

• Here the plasma volume is mostly reduced such as salt and water depletion.

• Sever and prolonged diarrhea/dehydration.

• Hemorrhage and shock; shock due to sever burns.

• Ulcerative colitis with severe chloride loss

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Thank you for your attention