Studies in protein structure : the structure and properties of the iron superoxide dismutase from Methanobacterium thermoautotrophicum : the structures of [beta]-lactoglobulin in two new crystal forms : a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Institute of Fundamental Sciences at Massey University
The crystal structure of Methanobacterium thermoautotrophicum iron superoxide dismutase (Mt-FeSOD) has been determined by X-ray diffraction to a resolution of 2.6 Å. The crystals were grown from PEG 6000 at a pH of 5.5, and the structure was solved by molecular replacement. The structure, in concert with structural and functional data from other Fe and Mn SODs, provides insights into aspects of metal specificity, reactivity of superoxide dismutase towards toward the inhibitor azide and deactivator hydrogen peroxide, and how the primary structure is involved in subtle tuning of these properties. The structure reveals how the protein is designed for thermal and chemical stability, yet retains moderate superoxide dismutase activity at ambient temperature. Bovine β-lactoglobulin (BLG) has been studied for many decades; numerous X-ray and NMR structures are available. Here we present two new X-ray structures, one from a crystal grown at very low ionic strength, and at the lowest pH (~5.2) of any X-ray structure. This structure provides validation of the other, high ionic strength X-ray structures. The core elements of BLG, an eight-stranded β-barrel, a three-turn α-helix external to the barrel, and an external β-strand (that forms the dimeric interface), are almost invariant across all structures. Four flexible loops have a variety of positions in the known structures and this represents a set of snapshots of the in vivo states of BLG. These flexible loops play an important role in the entropic stabilization of the β-barrel.