Urea cycle of ICBB - Overall view of catabolic pathways for...

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1 Diet Proteins Carbohydrates Triglycerides L-Amino acid Monosaccharides D-Glucose Glycerol Fatty acids α -Keto acids NH 4 + Pyruvate Acetyl coenzyme A Urea H 2 O, CO 2 ATP Urea cycle Krebs cycle Overall view of catabolic pathways for dietary nutrients β - o x i d a t n 2 Amino acids catabolism source of a.a. food intake degradation of intracellular proteins a.a. structure H - COO-C-NH3 + R
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3 Key concepts catabolism of amino acids often begins with deamination, resulting in the production of ammonia, followed by degradation of the remaining carbon chains urea cycle converts ammonia into urea which can then be excreted catabolism pathway of a.a. varies according to structure of a.a. But most of the metabolites converge with major pathways of metabolism (e.g. TCA cycle) 4 2 steps of a.a. catabolism transamination : transfer of α amino group from α amino acid glutamate to become NH 4 + (toxic) to become non-toxic urea (urea cycle) excreted in urine basic mechanism common to all a.a. carbon skeleton => altered and enter various pathways of carbon metabolism (eventually entering TCA cycle) specific to different a.a (i.e. will enter TCA cycle via different intermediates)
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5 Transamination occurs in liver mitochondria catalyzed by transaminase transfer of α -amino group from α -amino acid onto α -ketoglutarate to become α -keto acid and glutamate glutamate dehydrogenase release of amino group as ammonia with the concomitant oxidation of glutamate by NAD + to become α -ketoglutarate
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NH 3 + C H COO- R O C COO- CH 2 2 COO- N H 2 C O NH 2 O C R COO- NH 3 + C H CH 2 CH 2 COO- COO- α - amino acid α - keto acid α - ketoglutarate glutamate Glutamate dehydrogenase NADH + NH 4 + NAD+ + H 2 O ammonia toxic Urea non-toxic HCO 3 - aspartate Urea cycle excreted Trans-amination Transaminase
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6 Excretion of ammonia ammonia: highly toxic, kept at a low level in body aquatic organisms excrete ammonia directly across cell membranes of gills and released into surrounding water birds and reptiles convert ammonia into uric acid (non toxic, insoluble) terrestrial vertebrates convert ammonia into urea
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Urea cycle of ICBB - Overall view of catabolic pathways for...

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