Zhou, Y.-l.; Yuan, H.; Wu, Y.-c.; Wang, J.; Chen, H.; Yao, L.; Wang, M.; Wang, X.; Wang, J.; He, C.; Chen, X.; Liu, L.

Nature has evolved two distinct chelatase families to catalyze the insertion of metal ions into tetrapyrrole macrocycles. Whereas the single-subunit ATP-independent chelatases have been widely investigated, little is known about the three-subunit ATP-driven chelatases. Here we show step-wise assembly of the ATP-driven cobalt chelatase CobSTN that is essential for aerobic vitamin B12 biosynthesis. The motor subunit CobS fits into a hexameric or dodecameric spiral, and forms complex with the adaptor subunit CobT. Upon binding to adenine nucleotide, the spiral transforms to an asymmetrical ring and CobT synergistically rotates and inserts a distinctive shaft into the ring hole. The largest subunit CobN interacts with the opposite side of CobT from the CobS ring, and hence the holoenzyme is assembled.