metabolism_lecture_9 - Howproteinsaredigestedinanimals

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Chapter 18: Amino acid oxidation and production of urea How proteins are digested in animals How amino acids are degraded in animals How urea is made and excreted
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Oxidation of amino acids is a significant energy-yielding pathway in  carnivores Not all organisms use amino acids as the source of energy ~ 90% of energy needs in carnivores can be met by amino acids  immediately after a meal  Only a small fraction of energy needs of herbivores are met by amino acids Microorganisms scavenge amino acids from their environment for fuel Plants  rarely oxidize amino acids for energy production (almost exclusively used for biosynthesis of protein, nucleic acids, etc., amino acid catabolism only to produce metabolites for other pathways) Oxidative catabolism of amino acids under 3 circumstances: 1)Leftover amino acids during normal protein turnover (synthesis + degradation) 1)Dietary amino acids that exceed need for biosynthesis (no storage possible) 1)During starvation or in uncontrolled diabetes mellitus
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Pepsin  cleaves proteins into peptides in  the stomach Trypsin  and  chymotrypsin  hydrolyze  proteins and larger peptides into smaller  peptides in the small intestine Aminopeptidase  and  carboxypeptidases  A  and  B  degrade peptides into amino  acids in the small intestine Enzymatic hydrolysis of dietary proteins many proteases are secreted as  inactive  zymogen precursors -> activation by proteolytic cleavage (autocatalysis or enteropeptidase)  Human gastrointestinal tract stimulated by dietary proteins Also: self-protection of pancreas against  pre-mature peptidase activation  -> pancreatic trypsin inhibitor
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Overview of amino acid catabolism (mammals) 1 st step after amino acids reached the liver : Removal of the α -amino group by aminotransferases  or transaminases Separate, but interconnected routes for the  amino groups and the carbon skeletons 
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transaminase Catabolism of amino groups in vertebrate  liver
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(Plants conserve almost all the nitrogen) Many aquatic vertebrates release  ammonia  to their environment Passive diffusion from epithelial cells Active transport via gills Many terrestrial vertebrates and sharks excrete nitrogen in the form of  urea Urea is far less toxic than ammonia Urea has very high solubility Some animals, such as birds and reptiles excrete nitrogen as  uric acid Uric acid is rather insoluble
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metabolism_lecture_9 - Howproteinsaredigestedinanimals

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