Combined Microspectrophotometric and Crystallographic Examination of Chemically Reduced and X-ray Radiation-Reduced Forms of Cytochrome ba3 Oxidase from Thermus thermophilus: Structure of the Reduced Form of the Enzymes

Three paths for obtaining crystals of reduced (II-E4Q/I-K258R) cytochrome ba(3) are described, and the structures of these are reported at similar to 2.8-3.0 angstrom resolution. Micro spectrophotometry of single crystals of Thermus ba3 oxidase at 100 K was used to show that crystals of the oxidized enzyme are reduced in an intense X-ray (beam line 7-1 at the Stanford Synchrotron Radiation Laboratory), being nearly complete in 1 min. The previously reported structures of ba3 (Protein Data Bank entries and ), having a crystallographically detectable water between the CUB and Fe,,3 metals of the dinuclear center, actually represent the X-ray radiation-reduced enzyme. Dithionite-reduced crystals or crystals formed from dithionite-reduced enzyme revealed the absence of the above-mentioned water and an increase in the Cu-B-Fe-a3 distance of similar to 0.3 A. The new structures are discussed in terms of enzyme function. An unexpected optical absorption envelope at similar to 590 nm is also reported. This spectral feature is tentatively thought to arise from a five-coordinate, low-spin, ferrous heme 613 that is trapped in the frozen crystals.