Comparison of isocitrate dehydrogenase from three hyperthermophiles reveals differences in thermostability, cofactor specificity, oligomeric state, and phylogenetic affiliation

With the aim of gaining insight into the molecular and phylogenetic relatio nships of isocitrate dehydrogenase (IDH) from hyperthermophiles, we carried out a comparative study of putative IDHs identified in the genomes of the eubacterium Thermotoga maritima and the archaea Aeropyrum pernix and Pyroco ccus furiosus. An optimum for activity at 90 degreesC or above was found fo r each IDH. PfIDH and ApIDH were the most thermostable with a melting tempe rature of 103.7 and 109.9 degreesC, respectively, compared with 98.3 and 98 .5 degreesC for TmIDH and AfIDH, respectively. Analytical ultracentrifugati on revealed a tetrameric oligomeric state for TmIDH and a homodimeric state for ApIDH and PfIDH. TmIDH and ApIDH were NADP-dependent (K-m(NADP) of 55. 2 and 44.4 muM, respectively) whereas PfIDH was NAD-dependent (K-m(NAD) of 68.3 muM). These data document that TmIDH represents a novel tetrameric NAD P-dependent form of IDH and that PfIDH is a homodimeric NAD-dependent IDH n ot previously found among the archaea. The homodimeric NADP-IDH present in A. pernix is the most common form of IDH known so far. The evolutionary rel ationships of ApIDH, PfIDH, and TmIDH with all of the available amino acid sequences of di- and multimeric IDHs are described and discussed.