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Ch18.1 - C h a p te r 1 8 M e ta b o lis m o f a m in o g r...

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Chapter 18 Metabolism of amino groups and the Urea Cycle
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If a person's urine contains unusually high concentrations of urea, which one of the following diets has he or she probably been eating recently? A) High carbohydrate, very low protein B) Very high carbohydrate, no protein, no fat C) Very very high fat, high carbohydrate, no protein D) Very high fat, very low protein E) Very low carbohydrate, very high protein
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Figure 3-5
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The metabolism we have discussed so far has been biosynthetic metabolism (biosynthesis of carbohydrates and lipids) and that will be the primary focus this semester. BUT, one last catabolic pathway you have not studied -- Amino Acid Degradation , which… •can be a significant source of metabolic energy depending on diet •provides metabolites for other biosynthetic pathways
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Animals degrade amino acids when… • Normal protein turnover (syn. and degrad. of cellular proteins) releases amino acids that are not needed for new protein synthesis Protein-rich diet is consumed & the ingested amino acids exceed what is needed for protein synthesis (excess amino acids not stored…must be catabolized) Carbohydrates are not available (e.g. starvation or untreated diabetes) so cellular proteins are catabolized as an alternative fuel
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First, how are dietary proteins degraded to amino acids and how do these amino acids get to the liver? Dietary protein in stomach secretion of hormone gastrin HCl and pepsinogen secretion HCl -- denatures (unfolds) proteins Pepsinogen -- zymogen of pepsin hydrolyzes peptide bonds on amino (N-) side of aromatic amino acids Peptides (low pH) enter small intestine secretion of hormone secretin HCO 3 - secretion, neutral pH Zymogens of trypsin, chymotrypsin and carboxypeptidases A and B are secreted from pancreatic cells. Enteropeptidase converts trypsinogen to trypsin, which converts other proteases (these proteases all have pH optimum near neutral pH). Why are inactive zymogens secreted instead of active proteases? See Figure 18-3
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Note trypsin, chymotrypsin and pepsin amino acid specificities: These enzymes hydrolyze peptides from stomach even further in small intestine Then the short peptides are completely degraded to amino acids by: Carboxypeptidase A & B - cleave successive C-term. residues Aminopeptidase - successive N-term.
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