This model represents the H subunit of the magnesium chelatase complex responsible for magnesium insertion into the protoporphyrin IX ring in the biosynthesis of both chlorophyll and bacteriochlorophyll. In chlorophyll-utilizing species, this gene is known as ChlH, while in bacteriochlorophyll-utilizing spoecies it is called BchH. Subunit H is the largest (~140kDa) of the three subunits (the others being BchD/ChlD and BchI/ChlI), and is known to bind protoporphyrin IX. Subunit H is homologous to the CobN subunit of cobaltochelatase and by anology with that enzyme, subunit H is believed to also bind the magnesium ion which is inserted into the ring. In conjunction with the hydrolysis of ATP by subunits I and D, a conformation change is believed to happen in subunit H causing the magnesium ion insertion into the distorted protoporphyrin ring. [Biosynthesis of cofactors, prosthetic groups, and carriers, Chlorophyll and bacteriochlorphyll]
Jiyao W et al.(2023). "The conserved domain database in 2023.",