AMMONIA METABOLISMUREA CYCLE

Compiled by:-PRATEEK CHOPRA

BT/BIO/05/310022AMITY 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 Nonessential

Argininea Methionineb Alanine Glutamine

Histidine Phenylalaninec Aspartate Glycine

Isoleucine 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 supplied by the diet.

CatabolismUrea + CO2Amino Acid Pool

Carbon compounds + nitrogen

De novo synthesis

Dietary amino acids

Porphyrins, creatine, carnitine, hormones, nucleotides

Biosynthesis of nitrogen compounds

Fates of amino acidsAmino acid sources

Figure 1. Sources and fates of amino acids

BODY PROTEIN

Proteolysis Protein synthesis

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

-Amino acid

-Keto acid

NH2

HOOC-CH-CH2CH2COOH

O

HOOC-C-R

NH2

HOOC-CH-R

O

HOOC-C-CH2CH2COOH

-Ketoglutarate

Glutamate

Cofactor = pyridoxal phosphate

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

+ -ketoglutarate+ glutamate

Aspartate aminotransferase (glutamate-oxaloacetate transaminase)

NH2 Aspartate

HOOC-CH-CH2COOH

O Oxaloacetate

HOOC-C-CH2COOH

Alanine aminotransferase (glutamate-pyruvate transaminase)

+ -ketoglutarate+ glutamate

NH2 Alanine

HOOC-CH-CH3

O Pyruvate

HOOC-C-CH3

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

NADH NAD+

-Ketoglutarate + NH4

+

Glutamate

Glutamate dehydrogenase

Glutamine

Glutamine synthetase

NH3 + ATP

ADP + Pi

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.

Glutaminase

Glutamate

Glutamine

NH4+

Glutamate dehydrogenase

-Ketoglutarate + NH4

+

NAD+

NADH

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

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

-Amino acid

-Keto acid

-Ketoglutarate

Glutamate

Aminotransferase

NAD+ + H2O

Glu dehydrogenase

-Ketoglutarate

Glutamate

NADH + NH4+NH4

+

UREA

Urea cycle

CYTOPLASM MITOCHONDRIA

Figure 7. Carbamoyl phosphate synthetase reaction and the urea cycle. Overall: 3ATP+HCO3

-+NH4++asp 2ADP+AMP+2Pi+PPi+fumarate+urea

Ornithine

Citrulline

argininosuccinate synthetase argininosuccinase arginase

AMP+PPi

-Aspartate

Argininosuccinate

ATP

Arginine

Fumarate(returns to TCA cycle)

Pi

Ornithine

Citrulline

Ornithine transcarbamoylase

Carbamoyl phosphate

2ATP + HCO3- + NH4

+

2ADP + Pi

Carbamoyl phosphate synthetase

UREAO

H2N-C- NH2

Ornithine

-OOC-CH-NH3+

CH2COO-

UREA CYCLE FACTS

Found primarily in liver and lesser extent in kidney

Nitrogen added to the urea cycle via carbamoyl phosphate and aspartate

Carbamoyl phosphate synthetase is allosterically activated by N-acetylglutamate

(acetyl CoA + glutamate N-acetylglutamate)

Arginine stimulates the formation of N-acetylglutamate

Fatty liver can lead to cirrhosis

HYPERAMMONEMIASAcquired = Liver disease leads to portal-systemic shunting

Inherited = Urea cycle enzyme defects of CPS I or ornithine transcarbamoylase lead to severe hyperammonemia

O2

Tyrosine

H2O

Dihydrobiopterin

Phenylalanine hydroxylase

Phenylalanine

NADP+ NADPH

Tetrahydrobiopterin

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

primary defect in phenylketonuria

Phenylpyruvate

Phenylacetate

Phenyllactate

X