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Download Fate of excess amino acids Fate of amino groups
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Transcript
Nitrogen metabolism • N itrogen-containing compounds are handled in pathways, separate from paths. of Carbon Skeleton • Humans can only use Nitrogen if it is in form of -NH 2 Fate of excess amino acids 1. amino group removed -transamination -deamination COO HC-NH3 R 2. carboxyl group removed -decarboxylation 3. Degradation of carbon skeleton Excess amino acids • amino group is removed and recycled or excreted as urea • carboxyl group is removed and excreted as CO2 • remaining carbons are returned to carbon skeleton Fate of amino groups • amino groups are removed from excess a.a. (those not needed for protein synthesis) • 2 kinds of enzymes remove - NH3 a. transaminases b. deaminases Exchange of -NH3 a. transamination (transaminases) -many transaminases bind excess amino acids and exchange the -NH 3 for a C=O group - the keto acid acceptors are: pyr, OAA or KG Transamination phe + OAA O=C-O O=C-O HC-NH3 C=O H-C-H H-C-H O=C-O amino acid keto acid phenyl + asp pyruvate O=C-O C=O H-C-H O=C-O HC-NH3 H-C-H O=C-O keto acid amino acid Deamination Removal of -NH3 NADP+ NADP:H - deaminases remove amino groups from excess amino acids - products of deamination are: keto acid and NH 4 ions O=C-O HC-NH3 H-C-H HOH O-C=O C=O H-C-H NH4+ amino acid keto acid Fate of NH4 ions Describe the fate of NH4 ions in the human body. - ammonium ions are extremely toxic to mammals - must be removed immediately (usually by liver)to prevent damage to nervous tissue Fate of NH4 ions a. Recycle used in amination reactions to form i) glu or ii) glutamine (gln) b. Excrete used to form carbamyl P and then urea Amination reactions for recycling i. (aminase) glu from KG KG glutamic NH4 ii. formation of gln (glutamine synthase) NH4 glu glutamine Degradation of carbon backbones in amino acids glutamine synthesis glutamine (gln) stores and transports excess amino groups through blood stream glu glucogenic a.a. gly ser cys ala COO COO gln HC-NH3 HC-NH3 NH4+ CH CH CH CH O=C - NH2 O=C-O PEP asp Degradation of amino acids thr pyr OAA pyr acetyl CoA Degradation of amino acids gln ketogenic a.a. glu isocitrate leu lys phe tyr trp his arg KG acetoacetyl CoA pro HMG~CoA succinyl CoA acetyl CoA ketone bodies (acetoacetate, OH-butyrate ) ile met val urea cycle urea cycle + • Main function: remove free NH 4 ions from cells and blood stream • Substrates: carbamyl PO4 and asp O=P-O-C=O COO O NH HC-NH3 H HCH COO • Endproduct: • Location: urea HN-C-NH HO H mitochondria • Regulation: not well understood urea cycle O-C=O H-C-NH3 H-C-H argininosuccinate O-C=O citrulline fumarate arginine carbamyl P ornithine gln ATP CO 2 O urea NH2 - C - NH2 Show how urea could be labelled from C14-alanine • how to get carbon from alanine into CO2 • first ala must enter carbon skeleton • then go through reaction where CO2 is product • ala pyr acetyl CoA CO2 citrulline urea NH4+ Toxicity Reyes syndrome • NH4+ ions are made into urea in mitochondria of the liver • urea is put into blood and excreted by kidneys • if [NH4+] exceeds 70 µM in blood then confusion, dizziness, nausea and vomitting result • confusion, dizziness,nausea and vomitting occasionally result in children given aspirin following the flu or other viral illness • it appears that the viral infection may have damaged liver cells, impairing its ability to make urea NO synthase NO action • NO is formed from arginine by Nitric Oxide sythase in several different tissues • NO causes relaxation of smooth muscle in artery walls • NO is released into the blood and acts as a hormone entering smooth muscle cells in artery walls, causing relaxation • this relaxation results in vasodilation and decreased pressure NO stimulates immune system • macrophage cells of the immune system produce NO after binding to foreign cells or tumor cells • NO helps kill foreign cells, viruses and tumor cells nitroglycerin H H-C-O-N=O H-C-O-N=O H-C-O-N=O H NO H H-C-O-H H-C-O-N=O H-C-O-N=O H Bio-molecules in diet Polymers monomers linkage Proteins amino acids peptide Carbohydrates glucose glycosidic Lipids fatty acids ester (triglycerides) glycerol Degradation of amino acids glucogenic a.a. gly ser ala PEP What are the glucogenic amino acids and why are they termed glucogenic? Degradation of amino acids ketogenic a.a. leu lys phe tyr trp cys thr acetoacetyl CoA HMG~CoA pyr acetyl CoA asp OAA pyr acetyl CoA ketone bodies (acetoacetate, OH-butyrate )
