Ammonia Metabolism Urea Cycle

AMMONIA METABOLISM UREA CYCLE

Compiled by:PRATEEK CHOPRA BT/BIO/05/310022 AMITY INSTITUTE OF BIOTECHNOLOGY NOIDA

OBJECTIVES 1. Define protein balance, nitrogen balance and essential amino acid. 2. Describe the transaminase, and glutamate dehydrogenase reactions and discuss their roles in the removal of nitrogen waste in the body. 3. Identify the direct sources of nitrogen for the urea cycle. 4. Define hyperammonemia and discuss why a defect in either carbamoyl phosphate synthetase I or ornithine transcarbamoylase leads to hyperammonemia 5. Distinguish between ketogenic and gluconeogenic (glycogenic) amino acids. 6. Describe the phenylalanine hydroxylase reaction and explain its relationship to phenylketonuria;

PHYSIOLOGICAL PREMISE Have you ever carefully read a packet of EqualTM? If so, you may have noticed a warning to phenylketonurics. The chemical sweetener in equal is a dipeptide containing phenylalanine and aspartate. Some individuals are born with one of the more common amino acid disorders, phenylketonuria. They are unable to metabolize phenylalanine to tyrosine. Consequently vast amounts of phenylalanine will accumulate in the blood if too much of this amino acid is consumed in the diet. Constant excess of phenylalanine in the blood can cause severe mental retardation. Hence this is one of several diseases tested for in newborns in all states.

Table 1- The essential and non-essential amino acids Essential Argininea Histidine

Nonessential

Methionineb

Alanine

Glutamine

Aspartate

Glycine

Isoleucine

Phenylalaninec Threonine

Asparagine

Proline

Leucine

Tryptophan

Cysteine

Serine

Lysine

Valine

Glutamate

Tyrosine

a

Arg is synthesized in the urea cycle, but the rate is too slow to meet the needs of growth in children

b

Met is required to produce cysteine if the latter is not supplied adequately by the diet.

c

Phe is needed in larger amounts to form tyr if the latter is not by the diet.

supplied

BODY PROTEIN Proteolysis Carbon compounds + nitrogen Dietary amino acids

Protein synthesis

De novo synthesis Amino Acid Pool

Catabolism

Urea + CO2

Biosynthesis of nitrogen compounds Porphyrins, creatine, carnitine, hormones, nucleotides

Amino acid sources Fates of amino acids

Figure 1. Sources and fates of amino acids

PROTEIN BALANCE • positive: synthesis > degradation (e.g., growth, body building) • negative: synthesis < degradation (e.g., starvation, trauma, cancer cachexia)

BODY PROTEIN Proteolysis

Protein synthesis

Amino Acid Pool

NH2 HOOC-CH-R α-Amino acid

O HOOC-C-CH2CH2COOH α-Ketoglutarate Cofactor = pyridoxal phosphate

α-Keto acid

Glutamate

O

NH2

HOOC-C-R

HOOC-CH-CH2CH2COOH

Figure 2. Depiction of a general transamination (aminotransferase) reaction. The α-amino acid other than glutamate can be a wide variety

Aspartate aminotransferase (glutamate-oxaloacetate transaminase) NH2

Aspartate

HOOC-CH-CH2COOH + α-ketoglutarate

O Oxaloacetate HOOC-C-CH2COOH + glutamate

Alanine aminotransferase (glutamate-pyruvate transaminase) NH2

Alanine

HOOC-CH-CH3 + α-ketoglutarate

O Pyruvate HOOC-C-CH3 + glutamate

Figure 3. The reactions catalyzed by aspartate aminotransferase and alanine aminotransferase.

Glutamate dehydrogenase NADH α-Ketoglutarate + NH4+

NAD+ Glutamate NH3 + ATP Glutamine synthetase ADP + Pi Glutamine

Figure 3. In non-hepatic tissues the linked reactions of glutamate dehydrogenase and glutamine synthetase remove two ammonia molecules from the tissues as a way of ridding the tissues of nitrogen waste. The glutamine deposits the ammonia in the kidney for excretion.

Glutamine Glutaminase NAD+

NADH

NH4+ Glutamate Glutamate dehydrogenase

α-Ketoglutarate + NH4+

Figure 5. Kidney production of ammonia for excretion following successive removal of amino groups from glutamine via glutaminase and glutamate dehydrogenase

α-Amino acid

α-Ketoglutarate

Aminotransferase

α-Keto acid

NADH + NH NH44++

Glu dehydrogenase

Glutamate

Urea cycle UREA

NAD+ + H2O

Figure 6. In liver, nitrogen waste from amino acids ends up in urea. Amino acids are derived either from the breakdown of protein in various tissues or from what is synthesized in those tissues

O

Fumarate (returns to TCA cycle)

UREA

H2N-C- NH2 Arginine 

Argininosuccinate AMP+PPi



 Ornithine

Ornithine

 Aspartate OOC-CH-NH3+ -

ATP

Carbamoyl phosphate synthetase 2ADP + Pi

Carbamoyl phosphate  Ornithine transcarbamoylase Citrulline Pi

Citrulline

-

2ATP + HCO3- + NH4+

CYTOPLASM

MITOCHONDRIA

 CH2COO-

 argininosuccinate synthetase

 argininosuccinase

arginase

Figure 7. Carbamoyl phosphate synthetase reaction and the urea cycle. Overall: 3ATP+HCO3-+NH4++asp  2ADP+AMP+2Pi+PPi+fumarate+urea

UREA CYCLE FACTS  Found primarily in liver and lesser extent in kidney  Nitrogen added to the urea cycle via carbamoyl phosphate aspartate

and

 Carbamoyl phosphate synthetase is allosterically activated by N-acetylglutamate (acetyl CoA + glutamate → N-acetylglutamate)  Arginine stimulates the formation of N-acetylglutamate

HYPERAMMONEMIAS Acquired = Liver disease leads to portal-systemic shunting Inherited = Urea cycle enzyme defects of CPS I or ornithine transcarbamoylase lead to severe hyperammonemia

Fatty liver can lead to cirrhosis

primary defect in phenylketonuria

Phenylalanine hydroxylase O2

Phenylpyruvate

Phenyllactate

Phenylalanine

H2O

X

Tetrahydrobiopterin

Tyrosine Dihydrobiopterin

Phenylacetate NADP+

NADPH

Figure 8. Unusual compounds produced from phenylalanine in phenylketonuria. The phenylalanine hydroxylase reaction (or regeneration of the tetrahydrobiopterin cofactor) are defective in phenylketonuria.