Catalytic and Structural Role of a Metal-Free Histidine Residue in Bovine Cu−Zn Superoxide Dismutase

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Abstract
Cu−Zn superoxide dismutase (SOD) contains a conserved, metal-free His residue at an opening of the backbone β-barrel in addition to six Cu- and/or Zn-bound His residues in the active site. We examined the protonation and hydrogen bonding state of the metal-free His residue (His41) in bovine SOD by UV Raman spectroscopy. Analysis of the His Raman intensity at 1406 cm-1 in a D2O solution has shown that His41 has a pKa of 9.4, consistent with the NMR and X-ray structures at acidic to neutral pH, in which two imidazole nitrogen atoms of cationic His41 are hydrogen bonded to the main chain CO groups of Thr37 and His118. Upon deprotonation of His41 at pH 9.4, the Thr37−His41−His118 hydrogen bond bridge breaks on the His118 side and SOD loses 70% of its activity. Concomitantly, hydrogen−deuterium exchange is accelerated for amide groups of β-strands, indicating an increased conformational fluctuation of the β-barrel. Thr37 and His41 are in direct contact with Leu36, whose hydrophobic side chain closes off the opening of the β-barrel, while His118 is indirectly connected to Arg141 that assists the docking of superoxide to Cu. These Raman findings strongly suggest that the His41-mediated hydrogen bond bridge plays a crucial role in keeping the protein structure suitable for highly efficient catalytic reactions. The catalytic and structural role of His41 is consistent with the observation that the mutation of His43 in human SOD (equivalent to His41 in bovine SOD) to Arg largely reduces the dismutase activity and the protein structural stability.