pob5e_solutions_ch22 - 2608T_ch22sm_S258-S265 2/23/08...

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Biosynthesis of Amino Acids, Nucleotides, and Related Molecules chapter 22 S-258 1. ATP Consumption by Root Nodules in Legumes Bacteria residing in the root nodules of the pea plant consume more than 20% of the ATP produced by the plant. Suggest why these bacteria consume so much ATP. Answer Bacteria in the root nodules maintain a symbiotic relationship with the plant: the plant supplies ATP and reducing power, and the bacteria supply ammonium ion by reducing atmospheric nitrogen. This reduction requires large quantities of ATP. 2. Glutamate Dehydrogenase and Protein Synthesis The bacterium Methylophilus methylotro- phus can synthesize protein from methanol and ammonia. Recombinant DNA techniques have im- proved the yield of protein by introducing into M. methylotrophus the glutamate dehydrogenase gene from E. coli. Why does this genetic manipulation increase the protein yield? Answer The synthesis of protein requires the synthesis of amino acids. The transfer of nitro- gen from an ammonium ion to carbon skeletons—that is, amino acid synthesis—can be carried out in two ways: (1) combination of the NH 3 with glutamate to form glutamine, catalyzed by glutamine synthetase and (2) reductive amination of a -ketoglutarate to form glutamate, cat- alyzed by glutamate dehydrogenase. The latter process, which is promoted by the introduc- tion of the E. coli enzyme, is especially important because glutamate is the amino group donor in all transamination reactions. 3. PLP Reaction Mechanisms Pyridoxal phosphate can help catalyze transformations one or two car- bons removed from the ± carbon of an amino acid. The enzyme threonine synthase (see Fig. 22–15) promotes the PLP-dependent conversion of phosphohomoserine to threonine. Suggest a mechanism for this reaction. Answer A link between enzyme-bound PLP and the phosphohomoserine substrate is first formed, with rearrangement to generate the ketimine at the ± carbon of the substrate. This activates the ² carbon for proton abstraction, leading to displacement of the phosphate and formation of a double bond between the ² and ³ carbons. A rearrangement (beginning with proton abstraction at the pyridoxal carbon adjacent to the substrate amino nitrogen) moves the double bond between the ± and ² carbons, and converts the ketimine to the aldimine form of PLP. Attack of water at the ² carbon is then facilitated by the linked pyridoxal, followed by hydrolysis of the imine link between PLP and the product, to generate threonine. 4. Transformation of Aspartate to Asparagine There are two routes for transforming aspartate to asparagine at the expense of ATP. Many bacteria have an asparagine synthetase that uses ammonium ion as the nitrogen donor. Mammals have an asparagine synthetase that uses glutamine as the nitrogen donor. Given that the latter requires an extra ATP (for the synthesis of glutamine), why do mammals use this route?
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This note was uploaded on 09/24/2011 for the course CHEM 369 taught by Professor Wang during the Spring '11 term at University of Houston.

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pob5e_solutions_ch22 - 2608T_ch22sm_S258-S265 2/23/08...

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