LEHNINGER PRINCIPLES OF BIOCHEMISTRY Fifth Edition David L. Nelson and Michael M. Cox © 2008 W. H. Freeman and Company CHAPTER 18 Amino Acid Oxidation and the Production of Urea

Introduction 1. In animal, AA undergo oxidative degradation in three different metabolic circumstance - The normal degradation of cellular protein - A diet is rich in protein - When carbohydrates are either unavailable, cellular proteins are used as fuel 2.AA lose their amino groups to form  -keto acid  -keto acid undergo oxidation to CO2 and H2O  -kteo acid can converted by gluconeogenesis into glucose, the fuel for brain, skeletal muscle, and other tissue 5.AA acid contains an amino group and thus aa degradation include a key step in which the  -amono group is shunted into the pathways of amino group metabolism

Overview of aa catabolism in mammal

1.Most aa are metabolized in the liver 2.The ammonia generated in this process is recycled and used in a variety of biosynthetic pathway 3.The excess is either excreted directly or converted to urea or uric acid for excretion 4.Excess ammonia generated in other tissues travels to the liver for conversion to the excretory form 5.In cytosol of hepatocytes, amino groups are transferred to a-KG to form Glu, which enter to Mito. To form the ammonia 6.Excess ammonia from most other tissue is converted to the amide nitrogen of Gln which pass to liver 7.In skeletal muscle, excess amino groups are tranffered to pyruvate to form alanine Amino group catabolism

Gastrin : hormone which stimulate the secretion of HCl and pepsinogen Secretin: hormone which stimulate secretion of bicarbonate by pancreas Cholecystokinin: hormone which stimulate trypsinogen, chymotrypsinogen, procarbosypeptidases Why zymogens (inactive precursors)?

1.First step is removal of the  -amino groups by aminotranferase 2. In this transamination reaction, the  - amino group is transferred to the  -carbon atom of  -ketoglutarate 3. The glutamate then functions as the amino group donor for biosynthetic pathways or for excretion pathway 4. Amino transferase contain the prosthetic group, pyridoxal phosphate (PLP) 5. PLP is covalently bound to lysine through aldimine (Schiff base) 6. Aminotransferases are classic example of ping-pong reaction.

1.In hepatocyte, glutamate is transported from the cytosol into mitochondria, where it undergoes oxidative deamination catalyzed by glutamate dehydrogenase. 2. Combined action of an aminotransferase and glutamate dehydregenase is referred to as transdeamination. 3.  -kg can be used in citric acid cycle.

1.Ammonia is quite toxic to animal tissue and thus much of free ammonia is converted to a nontoxic compound before export from the extrahepatic tissues into the blood. 2. The free ammonia in tissues is combined with glutamate to yield glutamine by glutamine synthetase. 3. Glutaminase

1.Alanine also plays a special role in transporting amino group to the liver in a nontoxid from via a pathway called the glucose-alanine cycle. 2. Alanine aminotransferase 3. Example of the intrinsic economy of living organism.

1.The ammonia deposited in the mitochondria of hepatocytes is converted to urea in the urea cycle 2. Urea production occurs almost exclusively in the liver. 3. The urea passes into the blood stream and thus to the kidneys and is excreted into the urine 4. Five enzymatic steps: two steps in mito. And three steps in cyto. 5. Carbamoyl phosphate synthetase I 6. Orninthin transcarbamoylase 7. Arginosuccinate synthetase 8. Argrinosuccinase 9. Arginase

Nitrogen-acquiring reactions in the synthesis of urea.

1.The citric acid cycle and urea cycle are interconnected through common intermediate, fumarate. 2.Communication depends on the transport of key intermediates between Mito. and Cyto. 3.Fumarate from the urea cycle is transported to Mito. and can be enter the citric acid cycle. 4.Aspartate from the citric acid cycle is transported to cytosol and can transfer amino group to citrulline

Regulation of the urea cycle 1.When the dietary intake is primarily protein, the carbon skeletons of amino acid are used for fuel, producing much urea from the excess amino group 2.During prolong starvation, when breakdown of muscle protein begins to supply much of the organism’s metabolic energy, urea production increases 3.The activity of the urea cycle is regulated at two level: long term and short term regulation 4.Long term by regulation of the rates of synthesis of urea cycle enzyme 5. Short term by allosteric activation of carbamoyl phosphate synthetase I by N-acetylglutamate

Pathway interconnections reduce the energetic cost of urea synthesis 1.The synthesis of urea requires four high-energy phosphate groups. - Two ATP molecules are required to make carbamoyl phosphate - Two for making arginosuccinate 2.The urea cycle also causes a net conversion of oxaloacetate to fumarate (via aspartate), and the regeration of axaloacetate produces NADH. 3.Each NADH can generate up to 2.5 ATP, greatly reducing the overall energetic cost of urea synthesis.

1.People with genetic defects in any enzyme involved in urea formation cannot tolerate protein-rich diets 2.A protein-free diet is not treatment option because humans are incapable of synthesizing all a.a. and these essential a.a must be provided in the diet.

1.Careful administration of the aromatic acids benzoate or phenylbutyrate in the diet can help lower the level of ammonia in the blood 2.Benzate is converted to benzyol- CoA, which combines with glycine to form benzoylglycine 3.Phenylacetyl-CoA combines with glutamin to form phenyacetylglutamine. 4.Both products are nontoxic compounds that are excreted in the urine.

1.20 catabolic pathways converge to form only six major products 2. The carbon skeletons are diverted to gluconeogenesis, ketogenesis or oxidation 3. Trp, Phe, Tyr, Thr, Ile are both ketogenic and transgenic.

1.Two reactions are important for a.a catabolism 2.Transamination: PLP 3.One carbone transfer: Biotin (CO2), THF (intermediate oxidation state), SAM (most reduced state, methyl group)

Six a.a are degraded to pyruvate

Seven a.a are degraded to acetyl-CoA

Phenylalanin Catabolism is genetically defected in some people

1.Phenylalanine hydroxylase: mixed function oxidase catalyze simultaneous hydroxylation of a substrate by an oxygen atom of O2 and reduction of other oxygen atom to H2O 2.Tetrahydrobiopterin : electron from NADPH to O2

Five a.a are degraded to a-ketoglutarate

Four a.a are degraded to succinyl-CoA

Two a.a are degraded to succinyl-CoA