ABSTRACTCrenarchaeotal genomes encode the 3-hydroxypropionate/4-hydroxybutyrate (3-HP/4-HB) cycle for carbon dioxide fixation. Of the 13 enzymes putatively comprising the cycle, several of them, including methylmalonyl-coenzyme A (CoA) epimerase (MCE) and methylmalonyl-CoA mutase (MCM), which convert (S)-methylmalonyl-CoA to succinyl-CoA, have not been confirmed and characterized biochemically. In the genome ofMetallosphaera sedula(optimal temperature [Topt], 73°C), the gene encoding MCE (Msed_0639) is adjacent to that encoding the catalytic subunit of MCM-α (Msed_0638), while the gene for the coenzyme B12-binding subunit of MCM (MCM-β) is located remotely (Msed_2055). The expression of all three genes was significantly upregulated under autotrophic compared to heterotrophic growth conditions, implying a role in CO2fixation. Recombinant forms of MCE and MCM were produced inEscherichia coli; soluble, active MCM was produced only if MCM-α and MCM-β were coexpressed. MCE is a homodimer and MCM is a heterotetramer (α2β2) with specific activities of 218 and 2.2 μmol/min/mg, respectively, at 75°C. The heterotetrameric MCM differs from the homo- or heterodimeric orthologs in other organisms. MCE was activated by divalent cations (Ni2+, Co2+, and Mg2+), and the predicted metal binding/active sites were identified through sequence alignments with less-thermophilic MCEs. The conserved coenzyme B12-binding motif (DXHXXG-SXL-GG) was identified inM. sedulaMCM-β. The two enzymes together catalyzed the two-step conversion of (S)-methylmalonyl-CoA to succinyl-CoA, consistent with their proposed role in the 3-HP/4-HB cycle. Based on the highly conserved occurrence of single copies of MCE and MCM inSulfolobaceaegenomes, theM. sedulaenzymes are likely to be representatives of these enzymes in the 3-HP/4-HB cycle in crenarchaeal thermoacidophiles.
Interaction between a metal-binding motif of Hammerhead Ribozyme and divalent cations
