CRYSTAL-STRUCTURE OF PROTEUS-MIRABILIS PR CATALASE WITH AND WITHOUT BOUND NADPH

A catalase from a peroxide resistant mutant of Proteus mirabilis binds NADPH tightly. Interestingly, this enzyme can be stripped of NADPH wi thout loss of the catalatic activity It is the only known non-mammalia n catalase able to bind NADPH. The structure without cofactor was solv ed by molecular replacement using the structure of beef liver catalase as a model. The structure was refined to an R-factor of 19.3% in the range 8 to 2.2 Angstrom resolution. According to the sequence, a methi onine sulphone was positioned in the haem active site. This oxidized f orm of methionine is particular to Proteus mirabilis catalase and like ly to produce some steric hindrance in the active site. Two important water molecules are positioned in the haem distal site. These two wate r molecules are not located in the structure of beef liver catalase, b ut are supposed to account for the catalytic mechanism. The liganded f orm was obtained by soaking crystals of the unliganded form into an NA DPH solution. The structure was refined to an X-factor of 15.9% in the range of 8 to 3.1 Angstrom resolution using the unliganded structure as a model. The NADPH was clearly located in the electron density map with the same conformation as in beef liver catalase. The NADPH bindin g induces slight structural changes. However, the imidazole ring of a histidine residue (His284) rotates about 50 degrees to accommodate the cofactor. The electron transfer from NADPH to the haem molecule was e xamined and several pathways are proposed.