Beschreibung:
<jats:title>Abstract</jats:title><jats:p>To investigate the unknown stereochemical course of the reaction catalyzed by the type‐II isomerase, which interconverts isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), a sample of [1,2‐<jats:sup>13</jats:sup>C<jats:sub>2</jats:sub>]‐IPP stereospecifically labelled with <jats:sup>2</jats:sup>H at C(2) was prepared by incubating a D<jats:sub>2</jats:sub>O solution of (<jats:italic>E</jats:italic>)‐4‐hydroxy‐3‐methyl[1,2‐<jats:sup>13</jats:sup>C<jats:sub>2</jats:sub>]but‐2‐enyl diphosphate with a recombinant IspH protein of <jats:italic>Escherichia coli</jats:italic> in the presence of NADH as a reducing agent and flavodoxin as well as flavodoxin reductase as auxiliary proteins. As monitored by <jats:sup>13</jats:sup>C‐NMR spectroscopy, treatment of the deuterated IPP with either type‐I or type‐II IPP isomerase resulted in the formation of DMAPP molecules retaining all the <jats:sup>2</jats:sup>H label of the starting material. From the known stereochemical course of the type‐I isomerase‐catalyzed reaction, one has to conclude that the label introduced from D<jats:sub>2</jats:sub>O in the course of the IspH reaction resides specifically in the H<jats:sub><jats:italic>Si</jats:italic></jats:sub>C(2) position of IPP and that the two isomerases mobilize specifically the same H<jats:sub><jats:italic>Re</jats:italic></jats:sub>C(2) ligand of their common IPP substrate. The outcome of an additional experiment, in which unlabelled IPP was incubated in D<jats:sub>2</jats:sub>O with the type‐II enzyme, demonstrates that the two isomerases also share the same preference in selecting for their reaction the (<jats:italic>E</jats:italic>)‐methyl group of DMAPP.</jats:p>