Studies on the biosynthesis of porphyrin and bacteriochlorophyll by Rhodopseudomonas spheroides. 3. The effect of threonine on the biosynthesis of homoserine and methionine

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Abstract
The presence of [beta]-aspartokinase, aspartate [beta]-semialdehyde dehydrogenase and homoserine dehydrogenase was demonstrated in extracts of R. spheroides. The properties of these enzymes are very similar to those found in yeast, except that homoserine dehydrogenase in R. spheroides is specific for NADP. Threonine has no significant inhibitory effects on 5-aspartokinase or aspartate 5-semialdehyde dehydrogenase. It competitively inhibits both the forward and reverse reactions catalysed by homoserine dehydrogenase. At 37[degree] and at pH 6- 7 the Km for L-aspar-tate B-semialdehyde is 0-125 m[image] and the Ki for L-threonine is 0- 15 m[image]. Threonine does not repress the synthesis of any of the three enzymes which convert aspartic acid into homoserine. Methionine, lysine and diaminopimelic acid neither inhibit nor repress any of these enzymes. When organisms were illuminated in the presence of [14C] -aspartic acid, there was considerable incorporation of radioactivity into the intracellular unbound methionine. This incorporation was markedly decreased when threonine was added to the illumination medium. It is concluded that threonine inhibits the synthesis of methionine in R. spheroides by competitively inhibiting homoserine dehydrogenase. This is the most probable explanation for the previous observation that threonine inhibits the biosynthesis of bacteriochlorophyll and leads to an accumulation of porphyrins (Gibson et al. 1962b).