Studies on the physiological significance of the lack of a pyruvate dehydrogenase complex in Hyphomicrobium sp.

Hyphomicrobium X was grown in media containing either methanol or ethanol as a carbon and energy source, with or without additional organic carbon sources. The organism transported pyruvate, malate and succinate into the cells, and incorporated their carbon skeletons into cellular material, but when each of these compounds was added as sole carbon and energy source none supported growth of the organism. Enzymic analysis of crude cell-free extracts failed to detect either a complete pyruvate dehydrogenase complex or an active E1 component. Furthermore, oxygen uptake experiments with whole cell suspensions did not show any oxidation of pyruvate, succinate or malate. The distribution of radioactivity amongst the amino acids in hydrolysates of cell protein obtained from organisms grown in the presence of [14C]pyruvate, [14C]acetate or [14C]succinate indicated that the organism is limited in its ability to metabolize pyruvate. Growth in the presence of [14C]pyruvate resulted in 93% of the total radioactivity recovered being associated with amino acids derived directly from pyruvate. In contrast, growth in the presence of [14C]acetate or [14C]succinate resulted in more-or-less uniform labelling of all biogenic classes of amino acids. These results are consistent with the lack of an active pyruvate dehydrogenase complex which would make it impossible for Hyphomicrobium X to convert pyruvate into acetyl-CoA and to generate energy from carbon compounds for which the energy metabolism relies on oxidation through tricarboxylic acid (TCA) cycle intermediates.