18.2 Nitrogen Excretion and the Urea Cycle

18.2 Nitrogen Excretion and the Urea Cycle

Produced in liver

Blood

Kidney urine

Urea Cycle in Mitochondria

Formation of carbamoyl phosphate; preparatory step

NH4+ + HCO3

- + 2 ATP carbamoyl phosphate + 2 ADP + Pi

Carbamoyl phosphate synthetase I

- ATP-dependent reaction

1st step in the urea cycle;

Ornitine + carbamoyl phosphate citrulline + Pi

Ornitine transcarbamoylase

Urea Cycle in Cytosol

2nd step; formation of argininosuccinate Incorporation of the second N from aspartate Argininosuccinate synthetase ATP requirement Citrullyl-AMP intermediate

3rd step; formation of arginine & fumarate Arginosuccinase; only reversible step in the cycle

4th step; Cleavage of arginine to urea & ornithine Arginase

Asparatate-argininosuccinate shunt

Metabolic links between citric acid and urea cycles In cytosol

Fumarate to malate citric acid cycle in mitochondria In mitochondria

OAA + Glu -ketoglutarate + Asp urea cycle in cytosol

Energetic cost

• Consumption

3 ATP for urea cycle

• Generation

Malate to OAA

1 NADH = 2.5 ATP

Regulation of the Urea Cycle

Long term regulation

Regulation in gene expression

Starving animals & very-high protein diet Increase in synthesis of enzymes

in urea cycle

Short term regulation

Allosteric regulation of a key enzyme

Carbamoyl phosphate synthetase I Activation by N-acetylglutamate

Treatment of genetic defects in the urea cycle

Genetic defect in the urea cycle

ammonia accumulation; hyperammonemia Limiting protein-rich diet is not an option Administration of aromatic acids; benzoate or phenylbutyrate Administration of carbamoyl glutamate Supplement of arginine

18.3 Pathways of Amino Acid Degradation

Amino Acid Catabolism

Carbon skeleton of 20 amino acids

Conversion to 6 major products

- pyruvate

- acetyl-CoA

- -ketoglutarate

- succinyl-CoA

- fumarate

- oxaloacetate

Glucogenic or Ketogenic Amino Acids

Ketogenic amino acids

Conversion to acetyl-CoA or acetoacetyl-CoA

ketone bodies in liver

Phe, Tyr, Ile, Leu, Trp, Thr, Lys Leu : common in protein

Contribution to ketosis under starvation conditions

Glucogenic amino acids

Conversion to pyruvate, -ketoglutarate, succinyl-CoA, fumarate, and OAA

glucose/glycogen synthesis

Both ketogenic and glucogenic Phe, Tyr, Ile, Trp, Thr

Enzyme cofactors in amino acid catabolism

One-carbon transfer reactions ; common reaction type, involvement of one of 3 cofactors

Biotin ; one-carbon tranfer of most oxidized state, CO2

Tetrahydrofolate (H4 folate) ; One-carbon transfer of intermediate oxidation states or methyl groups S-adenosylmethionine ; one-carbon transfer of most reduced state, -CH3

Tetrahydrofolate

folate (vitamin) to H4 folate Dihydrofolate reductase

Primary source of one-carbon unit

Carbon removed in the conversion of Ser to Gly

Oxidation states of H4 folate ; One-carbon groups bonded to

N-5 or N-10 or both- Methyl group (most reduced)- Methylene group- Methenyl, formyl, formimino group

(most oxidized) Interconvertible & donors of one-

carbon units (except N5-methyl-tetrahydrofolate)

S-adenosylmethionine (adoMet)

Cofactor for methyl group transfer Synthesized from Met and ATP

Methionine adenosyl transferase Unusual displacement of triphosphate from ATP

Potent alkylating agent Destabilizing sulfonium ion inducing nucleophilic attack on methyl group

Six amino acids are degraded to pyruvate

Ala, Trp, Cys, Ser, Gly, Thr pyruvate acetyl-CoA citric acid cycle or gluconeogenesis

Interplay of PLP and H4folate in Ser/Gly metabolism

3rd pathway of glycine degradation - D-amino acid oxidase detoxification of D-amino acid high level in kidney

- Oxalate crystals of calcium oxalate (kidney stones)

Seven Amino Acids Are Degraded to Acetyl-CoA

Trp, Lys, Phe, Tyr, Leu, Ileu, Thr acetoacetyl-CoA acetyl-CoA

Intermediates of Trp catabolism be precusors for other biomolecules

Catabolic pathways for Phe & Tyr

Phe & Tyr are precusors dopamine norephinephrine, epinephrine melanin