Enzymatic metal insertion into tetrapyrrole derivatives

Mg-chelatase catalyses the insertion of Mg(II) into protoporphyrin IX, the first committed step in chlorophyll biosynthesis. It is consisted of three polypeptides of around 40 kDa, 70 kDa and 140 kDa (BchI, BchD and BchH, in bacteriochlophyll synthesis and ChlH-I-D in plants). Mg2+ insertion proceeds in a two-step reaction consisting of a Mg:ATP-dependent activation involving the I and D subunits, followed by a chelation step during which ATP is hydrolysed. The H subunit non-covalently binds protoporphyrin IX. Earlier we demonstrated that the three-dimensional structure of subunit I belongs to the class of AAA+ ATPases (Fodje et al., 2001), known to form large chaperone-like oligomeric complexes. In the subsequent work, using single-particle electron microscopy we demonstrated that subunits I and D form a complex, composed of two hexameric rings formed by the AAA+ domains of I and D (Elmlund et al., 2008). Using single-particle cryo-electron microscopy, we have obtained 7.5 Å resolution structure of the complex between subunits BchI and BchD in the presence of ADP and an ATP analogue (Lundqvist et al., 2010). These reconstructions show differences in conformation between the two states of the complex. We have also obtained the complex of the corresponding subunits of the Co-chelatase (Lundqvist et al., 2009). Co-chelatase is essential for the synthesis of vitamin B12. Currently we focus on obtaining higher resolution structures of these complexes as well as separate subunits of the enzymes.