A pH Switch Controls Zinc Binding in Tomato Copper‐Zinc Superoxide Dismutase

Abstract

Copper‐zinc superoxide dismutase (SOD1) is a major antioxidant metalloenzyme that protects cells from oxidative damage by superoxide anions (O2−). Structural, biophysical and other characteristics have in the past been compiled for mammalian SOD1s and for the highly homologous fungal and bovine SOD1s. Here, we characterize the biophysical properties of a plant SOD1 from tomato chloroplasts and present several of its crystal structures. The most unusual of these structures is a structure at low pH in which tSOD1 harbors zinc in the copper‐binding site but contains no metal in the zinc‐binding site. The side chain of D83, normally a zinc ligand, adopts an alternate rotameric conformation to form an unusual bidentate hydrogen bond with the side chain of D124, precluding metal binding in the zinc‐binding site. This alternate conformation of D83 appears responsible for the previously observed pH‐dependent loss of zinc from the zinc‐binding site of SOD1. In addition, Analytical ultracentrifugation data suggest that it is a weaker dimer relative to human SOD1.Support or Funding InformationSt. Mary’s Faculty Grant and Biaggini Fellowship