Purification and characterization of a hydroperoxidase from the cyanobacterium Synechocystis PCC 6803: identification of its gene by peptide mass mapping using matrix assisted laser desorption ionization time‐of‐flight mass spectrometry

A cytosolic catalase‐peroxidase from the cyanobacterium Synechocystis PCC 6803 was purified to homogeneity by a six‐step purification procedure. It is a homodimeric enzyme with a subunit molecular mass of 85 kDa. The isoelectric point of the protein is at pH 5.5; Michaelis constant, turnover number, and catalytic efficiency of the catalase activity for H 2 O 2 were measured to be 4.8 mM, 3450 s −1 , and 7.2×10 5 M −1 s −1 , respectively. Preparation and spectroscopy of the pyridine ferrohemochrome identified an iron protoporphyrin IX as the prosthetic group. The enzyme was shown to exhibit both catalase and peroxidase activities, both of which were inhibited by cyanide, leading to a high‐spin to low‐spin transition of the heme iron center as detected by a shift of the Soret peak from 405 to 421 nm. The catalase‐specific inhibitor 3‐amino‐1,2,4‐triazole proved ineffective. o ‐Dianisidine, pyrogallol and guaiacol functioned as a peroxidatic substrate, but no reaction was detected with NADH, NADPH, glutathione, and ascorbate. Peptide mass mapping using matrix assisted laser desorption ionization time‐of‐flight mass spectrometry showed the identity between the purified protein and a putative katG gene derived from the genome of Synechocystis PCC 6803. A comparison of amino acid sequences of the catalase‐peroxidase from Synechocystis PCC 6803 and those from other bacteria showed a high homology around the assumed distal and proximal histidine residues, suggesting a highly conserved histidine as the fifth ligand of the heme iron.