Preference for threonine over serine near the active site metal of superoxide dismutase in Porphyromonas gingivalis:Effect of Gly 155 to Ser mutation
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Summary In this study, we analyzed the cambialistic superoxide dismutases (SODs) of Porphyromonas gingivalis (Pg–SODs) with a mutation directed at glycine position 155 to introduce serine. Glycine 155 is a highly conserved outer sphere in manganese–containing SODs (Mn–SODs), even though threonine is substituted at this position in most iron–containing SODs (Fe–SODs). Conversion of glycine 155 may affect the metal–specific activity of SODs, including that of cambialistic Pg–SODs.Previously, we reported that a Pg–SOD Gly155Thr mutant exhibited a substantially changed metalspecific activity from that of a cambialistic type to an Fe–specific type. Although serine and threonine equally contribute to protein function, serine has never been observed at position 155 in SODs. In order to elucidate this phenomenon, we created a Pg–SOD mutant Gly155Ser. The specific Fedependent activity of this mutant was almost identical to the wild–type SOD, whereas the Mndependent activity exhibited a 60% reduction. The ultraviolet–visible absorption of Fe– and Mnreconstituted mutant SODs did not exhibit characteristic absorption spectra. Similar to the wild–typeSODs, the mutant SODs exhibited a single band with identical mobilities after separation by polyacrylamide gel electrophoresis. However, their behavior after anion–exchange chromatography differed from that of the wild–type SODs. Thus, Gly155 is considered to be an essential residue for maintaining the hydrogen–bond network for Mn–specific and Fe/Mn–tolerant activity. Gly155 was possibly retained instead of Ser during the evolution of SODs owing to its increased efficiency in maintaining a dimeric structure.
Preference for threonine over serine near the active site metal of superoxide dismutase in Porphyromonas gingivalis:Effect of Gly 155 to Ser mutation
