Identification of an Archaeal Type II Isopentenyl Diphosphate Isomerase in Methanothermobacter thermautotrophicus

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Abstract
Isopentenyl diphosphate (IPP):dimethylallyl diphosphate isomerase catalyzes the interconversion of the fundamental five-carbon homoallylic and allylic diphosphate building blocks required for biosynthesis of isoprenoid compounds. Two different isomerases have been reported. The type I enzyme, first characterized in the late 1950s, is widely distributed in eukaryota and eubacteria. The type II enzyme was recently discovered in Streptomyces sp. strain CL190. Open reading frame 48 (ORF48) in the archaeon Methanothermobacter thermautotrophicus encodes a putative type II IPP isomerase. A plasmid-encoded copy of the ORF complemented IPP isomerase activity in vivo in Salmonella enterica serovar Typhimurium strain RMC29, which contains chromosomal knockouts in the genes for type I IPP isomerase ( idi ) and 1-deoxy- d -xylulose 5-phosphate ( dxs ). The dxs gene was interrupted with a synthetic operon containing the Saccharomyces cerevisiae genes erg8 , erg12 , and erg19 allowing for the conversion of mevalonic acid to IPP by the mevalonate pathway. His 6 -tagged M. thermautotrophicus type II IPP isomerase was produced in Escherichia coli and purified by Ni 2+ chromatography. The purified protein was characterized by matrix-assisted laser desorption ionization mass spectrometry. The enzyme has optimal activity at 70°C and pH 6.5. NADPH, flavin mononucleotide, and Mg 2+ are required cofactors. The steady-state kinetic constants for the archaeal type II IPP isomerase from M. thermautotrophicus are as follows: K m , 64 μM; specific activity, 0.476 μmol mg −1 min −1 ; and k cat , 1.6 s −1 .