Kim HJ, Zatsman A, Upadhyay AK, Whittaker M, Bergmann D, Hendrich MP, Hooper AB. Membrane Tetraheme Cytochrome cm552 of the Ammonia-Oxidizing Nitrosomonas europaea: A Ubiquinone Reductase. Biochemistry. 2008 May 28;47(25):6539–51. doi: 10.1021/bi8001264.