BC368 Biochemistry of the Cell II Nitrogen Metabolism I (Ch 18) Amino Acid Oxidation and the Production of Urea May 1, 2015

Overview of amino acid metabolism  Proteins constantly undergo turnover.

Overview of amino acid metabolism  No protein stores, so essential amino acids must come from diet.  Amino acids are also used to synthesize some non-protein metabolites.  Proteins constantly undergo turnover.

Amino acid catabolism in humans  Proteins are broken down in stomach and small intestine to constituent amino acids.  Amino acids are either used as building blocks or burned for energy (~10% of our energy needs).  Catabolism of amino acids increases  for use in gluconeogenesis when glucose is unavailable (e.g., starvation/diabetes)  when protein content of diet exceeds need for building blocks  during times of stress

Overview of amino acid catabolism  Special role for four amino acids:

 Digestive events are triggered of the hormone gastrin, released when food enters the stomach. Digestion and Absorption

 Low pH activates digestive enzymes; e.g., pepsin.  Resulting amino acids are absorbed by the intestinal mucosa, enter the capillaries, travel to the liver. Digestion and Absorption  Liver can degrade all amino acids but Leu, Ile, Val.

Fig 18-1 Divergent pathways of NH 3 groups and carbon skeletons Fig 18-1

Removal of amino group via transamination  Amino groups can be removed by transamination.  In liver cytosol, amino groups are passed to α- KG, forming glutamate.  Transaminases (aka aminotransferases) require pyridoxal phosphate cofactor.

Pyridoxal phosphate and transamination

Fig 18-1  Glutamate in the liver cytosol enters the mitochondrial matrix, where its amino group is removed by glutamate dehydrogenase. Removal of amino group via oxidative deamination  Amino group must be processed for excretion or recycled. Fig 18-7

Transport of amino groups as glutamine  Peripheral tissues may send their amino groups as glutamine through the bloodstream to the liver for processing. Fig 18-8 To liver via bloodstream

Fig 18-1 Transport of amino groups as alanine  In concert with the Cori cycle, skeletal muscle may send pyruvate through bloodstream as alanine (the glucose-alanine cycle).  Operates when muscle proteins are undergoing catabolism. Fig 18-9

Fig 18-1 Summary of paths of amino groups Fig 18-2

Fig 18-1 Fate of NH 4 + excreted as NH 3 (ammonotelic)

Fate of NH 4 + excreted as uric acid (uricotelic)

Fate of NH 4 +

excreted as urea in H 2 O (ureotelic)

Case Study EM, the third child of parents unrelated by blood, had one healthy sister and one brother who demonstrated spasticity. EM appeared normal at birth with good Apgar scores. Hypotonia was observed after the third month of life. At 7 months of age (weight, 6.0 kg; height, 67 cm), he was admitted for evaluation of painful swollen joints. Neurological examination revealed hyperreflexia and an inability to lift his head. Laboratory tests revealed the following: What is wrong with EM? What treatment would you recommend?

Gout, the evil demon The Disease of Kings and the King of Diseases… The Gout by James Gilray, 1799 Alternate fate of NH 4 + = uric acid from purines  Podagra (swelling of the big toe) results from crystals of uric acid in the synovial fluid

XO= xanthine oxidase Normal pathway of purine degradation GMP AMP

Treatment for gout Trojan horse inhibitor of xanthine oxidase Fig 22-47

Treatment for gout Trojan horse inhibitor of xanthine oxidase Gertrude Elion, Fig 22-47

Treatment for gout Trojan horse inhibitor of xanthine oxidase Fig 22-47

Purine Salvage Defect  Purines are recycled through the purine salvage pathway.  Key enzyme is HGPRT (hypoxanthine-guanine phosphoribosyltransferase).  Defect in HGPRT leads to Lesch-Nyhan syndrome.

Purine Salvage Defect  Purines are recycled through the purine salvage pathway.  Key enzyme is HGPRT (hypoxanthine-guanine phosphoribosyltransferase).  Defect in HGPRT leads to Lesch-Nyhan syndrome.

Urea cycle  Spans two compartments: matrix and cytosol  Occurs in the liver

Preparatory step: carbamoyl phosphate synthetase I Occurs in the matrixFirst N of urea Fig 18-11

Step 1: Ornithine transcarbamoylase Also occurs in the matrix, but citrulline is transported to cytosol Ornithine is analogous to OA

Step 2: Argininosuccinate synthetase Second N of urea!

Step 3: Argininosuccinase

Step 4: Arginase Ornithine is transported back to the matrix.

Krebs’ bicycle Fig 18-12

Twenty-four hours after birth, a formula-fed male infant becomes somnolent and feeds poorly. Soon he begins to vomit and then goes into a coma. It looks like sepsis, but he has no risk factors and his sepsis work-up is negative. His serum ammonia and ornithine are elevated while his citrulline levels are undetectable. The maternal grandmother tells you that she had a son who died as a baby from the same symptoms. Which of the following enzymes is defective? 1)Carbamoyl phosphate synthetase 2)Ornithine transcarbamoylase 3)Arginase 4)Alanine aminotransferase 5)Pyruvate carboxylase

Fates of carbon skeletons  Glucogenic amino acids are degraded to pyruvate or TCA intermediate.  Ketogenic amino acids are degraded to acetoacetyl-CoA or acetyl-CoA.  Some amino acids are both. Fig 18-15