Early steps of isoprenoid biosynthesis in Escherichia coli

The incorporation of 2H- and 13C-labelled precursors into ubiquinone-8 (Uq-8) by strains of Escherichia coli was measured in order to define the pathway for the early steps in the biosynthesis of isoprenoids in these eubacteria. Cells grown with DL-[methyl-2H6]valine were found to label both the alpha-oxoisovaleric ('alpha-ketoisovaleric') acid alpha-oxoisohexanoic ('alpha-ketoisocaproic') acid, but not the Uq-8. Since these acids are required for the biosynthesis of isoprenoids by the acetolactate pathway, the operation of this pathway in the biosynthesis of Uq-8 is excluded. Cells grown with [1,2-13C2]acetate and non-labelled glucose readily incorporated 13C2 units into fatty acids, but failed to incorporate any label into the Uq-8. Cells grown with [U-13C6]glucose and non-labelled acetate, however, were found to label both the fatty acids and the Uq-8. Oxidative cleavage with periodate/permanganate of the Uq-8 isolated from cells grown with U-13C6-labelled glucose produced laevulinic acid, which was shown to be derived from two C2 units and one C1 unit of the labelled glucose by mass-spectral analysis of the 4,5-dihydro-6-methyl-2-phenylpyridazin-3(2H)-one derivative. The results of this work indicate that the C-2 and C-3 carbon unit of pyruvate, not acetyl-CoA, is the precursor to isopentenyl pyrophosphate (IPP) in these cells; however, the labelling pattern observed is consistent with the established acetoacetate pathway of isoprenoid biosynthesis. These data, coupled with the observed lack of inhibition of the growth of E. coli by mevinolin, a specific inhibitor of 3-hydroxy-3-methylglutaryl-CoA, can be best rationalized by the biosynthesis of IPP occurring in E. coli through a series of bound intermediates.