Glucose-6-phosphate isomerase from the hyperthermophilic archaeon Methanococcus jannaschii: characterization of the first archaeal member of the phosphoglucose isomerase superfamily

ORF MJ1605, previously annotated as pgi and coding for the putative glucose-6-phosphate isomerase (phosphoglucose isomerase, PGI) of the hyperthermophilic archaeon Methanococcus jannaschii, was cloned and functionally expressed in Escherichia coli. The purified 80-kDa protein consisted of a single subunit of 45 kDa, indicating a homodimeric (α2) structure. The Km values for fructose 6-phosphate and glucose 6-phosphate were 0.04 mM and 1 mM, the corresponding Vmax values were 20 U/mg and 9 U/mg, respectively (at 50 °C). The enzyme had a temperature optimum at 89 °C and showed significant thermostability up to 95 °C. The enzyme was inhibited by 6-phosphogluconate and erythrose-4-phosphate. RT-PCR experiments demonstrated in vivo expression of ORF MJ1618 during lithoautotrophic growth of M. jannaschii on H2/CO2. Phylogenetic analyses indicated that M. jannaschii PGI was obtained from bacteria, presumably from the hyperthermophile Thermotoga maritima.