Comparative study of manganese catalase monomers, interfaces and cage architecture within the ferritin superfamily
Manganese catalase protein is an example of a protein cage within the ferritin superfamily that focuses on enzymatic catalysis rather than on storage which other ferritin proteins are known for. Formed of 6 homomeric chains, it shows large subunit-subunit sidewise contacts and special interfacial interactions ensuring cage generation with only 6 subunits. We aim to explore manganese catalase at the monomer, subunit-subunit and cage level. As compared to other ferritin subtypes, we found manganese catalase monomers to have greater fraction of {beta}-turns at the secondary structure level and thermophilic manganese catalase monomers to have greater non-polar to polar residue ratio. At the interface level, the placement of subunits in manganese catalase cage with sidewise and angular interface was found to be distinctly different from the parallel and perpendicular interfaces present in other ferritin subtypes. Our study highlights the contribution of sidewise interfaces and terminal extensions in making the cage architecture possible in 6-mer manganese catalases. We also studied the quaternary structure of manganese catalase cage with respect to Classical ferritin (C-ferritin) and found manganese catalase to have smaller cavity volume and cavity surface area than C-ferritin but higher cavity surface to volume ratio. This observation along with the smaller distance between substrate entry point and active site as compared to C-ferritin highlights the structural distinction between catalytic enzymes like manganese catalase and storage proteins like ferritins. Thus, the study gives structural insight into manganese catalase protein cage with focus on its ability to form a cage architecture and show efficient catalytic activity.