Using neutron and x-ray and scattering, we have observed a variety of the subunits alone and in partial complexes.  Magnesium chelatase catalyses the insertion of Mg(II) into protoporphyrin IX to make magnesium protoporphyrin, the first committed step in the biosynthesis of chlorophyll.  The catalytic activity is driven by AAA(+) ATPase subunit “I”.  The core structure is composed of hexameric I and a related, but non-ATPase hydrolysing hexameric “D”.  Subunit I exists in oligomeric from or hexameric form depending on nucleotide binding.  The D subunit exists only as the hexamer.  Scattering results indicate that the hexameric ID core structure from Chlamydomonas reinhardtii is significantly larger than expected, possibly a dynamic structure containing additional loosely bound I subunits attached.

The protoporphryn IX molecule is bound jointly by a subunits H and GUN4.  H and GUN4 are well characterised in isolation, but they bind ambiguously to the core structure, as observed in SAXS and SANS experiments.

A more recently discovered highly homologous “I2” subunit substitutes for I subunits in the complex.  SANS data indicates that I2 has zero affinity for itself, but activity assays reveal a dramatic modulation of the catalytic activity due to the additional presence of I2.