The optical absorbance spectrum of reduced myeloperoxidase shows an unusual red-shifted Soret band at 472 nm and an alpha band at 636 nm. It has been speculated that this red-shift is due to interaction of the protein matrix with the chromophore. The carboxylate side chain of Glu242 is in close proximity of the prosthetic group of the enzyme, and we have examined the effect of the Glu242 to Gln mutation on the spectroscopic properties and catalytic activity of the enzyme. The mutation shifts the Soret band in the optical absorption spectrum of the reduced mutated enzyme from 472 to 458 nm. The EPR spectrum was hardly affected and was typical of a rhombic high-spin system (g(x) = 6.6, g(y) = 5.2). The alkaline pyridine hemochrome spectrum of the mutant was nearly identical to that of native myeloperoxidase. The resonance Raman spectrum, however, was drastically affected in the mutant. The symmetry-reducing effects were lifted by the mutation and the resonance Raman spectrum was indicative of an iron-porphyrin-like chromophore with a singlet nu(4) line at 1367 cm(-1). The mutant enzyme was not able to peroxidize chloride to hypochlorous acid. We conclude that the interaction of residue Glu242 with the prosthetic group in native myeloperoxidase is partly responsible for the red-shifted Soret band in the optical spectrum and that this interaction is the origin of the symmetry-reducing effects in the resonance Raman spectrum of the native enzyme. This residue also plays a pivotal role in the ability of the enzyme to peroxidize chloride.