Chapter17Fall2005 - Chapter 17 Amino Acid Metabolism Will...

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Chapter 17 – Amino Acid Metabolism Will be interested in two things: 1) origin of nitrogen atoms and their incorporation into amino group 2) origin of carbon skeletons AMINO ACID SYNTHESIS Nitrogen fixation Gaseous nitrogen is chemically unreactive due to strong triple bond. To reduce nitrogen gas to ammonia takes a strong enzyme --> reaction is called nitrogen fixation. Only a few organisms are capable of fixing nitrogen and assembling amino acids from that. Higher organisms cannot form NH 4 + from atmospheric N 2 . Bacteria and blue-green algae (photosynthetic procaryotes) can because they possess nitrogenase . Enzyme has two subunits: 1) strong reductase - has Fe-S cluster that supplies e - to second subunit 2) two re-dox centers, one of which is a nitrogenase Composed of iron and molybdenum that reduces N 2 to NH 4 + Reaction is ATP-dependent, but unstable in the presence of oxygen. Enzyme is present in Rhizobium, symbiotic bacterium in roots of legumes (i.e. soybeans) Nodules are pink inside due to presence of leghemoglobin (legume hemoglobin) that binds to oxygen to keep environment around enzyme low in oxygen (nitrogen fixation requires the absence of oxygen) Plants and microorganisms can obtain NH 3 by reducing nitrate (NO 3 - ) and nitrite (NO 2 - ) --> used to make amino acids, nucleotides, phospholipids. Assimilation of Ammonia Assimilation into amino acids occurs through glutamate and glutamine. α -amino group of most amino acids comes from α -amino group of glutamate by transamination. Glutamine contributes its side-chain nitrogen in other biosynthetic reactions. Reaction: NADPH +H + NADP + NH 4 + + α -ketoglutarate glutamate + H 2 O glutamate dehydrogenase Another reaction that occurs in some animals is the incorporation of ammonia into glutamine via glutamine synthetase : 1
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glutamate + NH 4 + + ATP glutamine + ADP + P i + H + When ammonium ion is limiting, most of glutamate is made by action of both enzymes to produce the following (sum of both reactions): NH 4 + + α -ketoglutarate + NADPH + ATP glutamate + NADP + + ADP + P i Transamination Reactions Having assimilated the ammonia, synthesis of nearly all amino acids is done via tranamination reactions. Glutamate is a key intermediate in amino acid metabolism
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Chapter17Fall2005 - Chapter 17 Amino Acid Metabolism Will...

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