Acidophilic adaptations in the structure of Acetobacter aceti N⁵-carboxyaminoimidazole ribonucleotide mutase (PurE)

The crystal structure of Acetobacter aceti PurE was determined to a resolution of 1.55 Å and is compared with the known structures of the class I PurEs from a mesophile, Escherichia coli, and a thermophile, Thermotoga maritima. Analyses of the general factors that increase protein stability are examined as potential explanations for the acid stability of A. aceti PurE. Increased inter-subunit hydrogen bonding and an increased number of arginine-containing salt bridges appear to account for the bulk of the increased acid stability. A chain of histidines linking two active sites is discussed in the context of the proton transfers catalyzed by the enzyme.