Two different aa3-type cytochromes can be purified from the bacterium Bacillus cereus

Two aa3-type cytochromes were purified from membranes of sporulating Bacillus cereus. One of them, an aa3 complex, was found to be composed of two subunits (51 and 31 kDa), two a hemes and three copper atoms, thus being similar to the cytochrome aa3 previously purified from vegetative B. cereus [García-Horsman, J. A., Barquera, B., González-Halphen, D. & Escamilla, J. E. (1991) Mol. Microbiol. 5, 197-205]. The second isoform, a caa3 complex, was expressed in sporulating cells only, and was found to be composed of two subunits (51 and 37 kDa). The 37-kDa subunit (subunit II) is a heme-c-containing polypeptide as shown by its peroxidase activity in SDS/PAGE gels and by its spectral features. Both subunits of the caa3 complex immunologically cross-reacted with antiserum raised against B. cereus cytochrome aa3, suggesting homology between the two enzymes. Also, the heme-c-containing subunit of the caa3 complex was reactive with anti-(bovine cytochrome c) antiserum, but not with anti-(bovine cytochrome c1) antiserum. In addition to one heme c and two hemes a, the caa3 complex contained three copper atoms. Kinetic comparison of aa3 and caa3 complexes revealed that the latter is slightly more active (k = 150 s-1) and has a lower affinity to yeast cytochrome c (Km = 76 microM) and to oxygen (Km = 2 microM) as compared with cytochrome aa3 (100 s-1, 10 microM, and 5 microM, respectively).