Chlorophyll (and bacteriochlorophyll) and heme biosynthesis diverge at the point where a metal atom is inserted into the middle of the porphyrin skeleton. Ferrochelatase inserts an iron atom into the nascent heme, whereas magnesium chelatase introduces magnesium. Hansson et al. have examined the activity of BchI, one of three proteins (BchD and BchH being the other two) that support the synthesis of bacteriochlorophyll. BchI is a member of a family of ATPases associated with various cellular activities (termed AAA+) and has been shown to form a hexameric structure as have other AAA ATPases involved in protein degradation and DNA repplication. Binding ATP is sufficient to promote formation of the hexamer and the interaction between BchI and BchD, but for BchI to catalyze the insertion of magnesium into protoporphyrin IX, which is bound by BchH, ATP must be hydrolyzed. Mixing wild-type and ATPase-deficient monomers did not yield active hexamers, suggesting that the conformational changes attendant on ATP hydrolysis must occur across all six monomer-monomer interfaces before metal atom insertion can take place. -- GJC
Proc. Natl. Acad. Sci. U.S.A. 99, 13944 (2002)
