Lect_16_AAIII

Lect_16_AAIII - Amino acid catabolism and the urea cycle...

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Amino acid catabolism and the urea cycle Products of amino acid catabolism carbon skeletons and nitrogen Major catabolic enzymes transaminases glutamic dehydrogenase decarboxylases Secretion of nitrogen ammonotelic, ureotelic, uricotelic organisms NH 3 , urea, uric acid Urea cycle ornithine, carbamyl phosphate arginine, urea
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AA catabolism Page 71 Ingested proteins digestion, absorption protein synthesis 400 g/day Proteins Amino acids catabolism in the liver and kidney 100 g/day NH 3 carbon skeleton Intracellular protein degradation by proteases Precursor to cellular compounds, e.g. purines, pyrimidines, porphyrins, vitamins, hormones, neurotransmitters, amines, vasodilators Urea cycle (liver, kidney) urea is secreted in urine TCA cycle Fatty acids Glucose acetyl-CoA
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Excess amino acids are degraded when there is an excess of organic nitrogen, because unlike carbohydrates and fats, proteins cannot be stored. Free amino acids cannot be excreted. The degraded amino acids are converted to carbon skeletons which can be used as metabolic fuel and to NH 3 . Blood plasma contains about 0.5 mg/liter of NH 3 , which is produced primarily during amino acid degradation; 2-3 times this level is toxic for animals. The excess NH 3 is converted to urea, a relatively non- toxic substance and secreted; adult humans excrete about 20 g of urea N per day.
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Carbon skeleton products Glucogenic amino acids 14 amino acids are pure glucogenic amino acids and produce such products as pyruvate, α -KG, succinyl-CoA, OAA, fumarate, acetyl-CoA, and acetoacetyl-CoA 5 amino acids are both ketogenic and glucogenic: Ile, Lys, Phe, Trp, Tyr Glucogenic and ketogenic amino acids Ketogenic amino acid 1 amino acid is a pure ketogenic amino acid : Leu Catabolism or degradation of amino acids
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pyruvate, α -KG, succinyl-CoA, OAA, fumarate, acetyl-CoA, and acetoacetyl-CoA Ketogenic amino acids produce ketone bodies such as acetoacetate, acetone, and β -hydroxybutyrate from acetyl-CoA. Most amino acids are glucogenic and can be used to produce glucose from amino acids fatty acids glucose acetone β -hydroxybutyrate OAA amino acids acetyl-CoA citrate (TCA) acetoacetate
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Ketone bodies are formed during degradation of fatty acids and amino acids. CH 3 -C-CH 3 O CH 3 -C-CH 2 -C O O O - CH 3 -C-CH 2 -C OH O O - H acetone acetoacetate β -hydroxybutyrate exhaled Converted to acetyl-CoA and oxidized in TCA cycle
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There are 3 major reactions for the degradation of amino acids : 1. Transamination - transaminases 2. Glutamic dehydrogenase 3. Decarboxylation - decarboxylases The reactions for amino acid catabolism are very different from the pathways for biosynthesis of amino acids. The degradation steps are usually not reversals of synthesis steps.
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BIS 103 - 3 basic catabolic steps COOH l H 2 N-C-H l R 1 + COOH l C=O l CH 2 CH 2 l COOH l COOH l C=O l R 1 + COOH l H 2 N-C-H l CH 2 CH 2 l COOH l PLP glutamic transaminase α -ketoglutarate glutamic acid COOH l H 2 N-C-H l CH 2 CH 2 l COOH l glutamic acid + NAD + + H
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This note was uploaded on 04/19/2008 for the course BIS 103 taught by Professor Abel during the Spring '08 term at UC Davis.

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Lect_16_AAIII - Amino acid catabolism and the urea cycle...

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