Amino acid metabolism in Acetobacter suboxydans Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2j62s725b

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  • Nutritional studies with Acetobacter suboxydans indicated that when isoleucine, lysine, methionine, serine and arginine were singly omitted from a complete amino acid mixture, no growth was obtained. However, the organism could grow on single amino acids like glutamic acid, histidine and proline. No growth was obtained with ammonium sulfate as the sole nitrogen source. An investigation on the cause for the apparent requirement for isoleucine was made. When valine was added to a synthetic medium containing histidine, glutamic acid, proline and ammonium sulfate no growth was obtained. Valine appeared to inhibit growth, an effect which could be reversed by addition of isoleucine. Cell-free extracts of the organism synthesize both valine and isoleucine from acetolactate and acetohydroxybutyrate respectively. The amounts of the two amino acids synthesized from different intermediates were determined. A study of the mechanism of growth inhibition due to valine was initiated. U-C¹⁴ threonine was incorporated into isoleucine by growing cells. The first step in the biosynthesis of isoleucine from threonine is the deamination of the latter to ?-ketobutyrate. This enzyme, threonine deaminase, was precipitated from cell-free extracts at 50 percent saturation with ammonium sulfate. Using this fraction, it was found that the activity of the deaminase was competitively inhibited by valine and isoleucine. Although isoleucine repressed the synthesis of the deaminase, valine did not. Therefore one mechanism by which valine inhibits growth of the organism is by feed-back inhibition of the threonine deaminase thereby limiting isoleucine biosynthesis. This is evident by an apparent requirement for isoleucine when valine is present in the growth medium. No inhibition of the transport of isoleucine across the cell wall was observed. Mutants of Acetobacter suboxydans have been isolated which are prototrophic towards isoleucine and resistant to growth inhibition by valine. Some of these mutants possess a threonine deaminase which is not sensitive to valine or isoleucine.
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