Blue Type I Copper Centers vs Purple Cu A Centers 4439 5. Enzymes Employing a Combination of Different Types of Electron Transfer Centers 4439 5.1. Enzymes Using Both Heme and Cu as Electron Transfer Centers 4439 5.1.1. Cytochrome c and Cu A as Redox Partners to Cytochrome c Oxidases 4439 5.1.2. Cu A and Heme b as Redox Partners to Nitric Oxide Reductases 4440 5.1.3. Cytochrome c and Cu A as Redox Partners to Nitrous Oxide Reductases 4440 5.2. Enzymes Using Both Heme and Iron−Sulfur Clusters as Electron Transfer Centers 4440 5.2.1. As Redox Partners to the Cytochrome bc 1 Complex 4440 5.2.2. As Redox Partners to the Cytochrome b 6 f Complex 4442 5.2.3. As Redox Centers in Formate Dehydrogenases 4443 5.2.4. As Redox Centers in Nitrate Reductase 4443 6. Summary and Outlook 4443 Author Information 4445 Corresponding Author 4445 Author Contributions 4445 Notes 4445 Biographies 4445 Acknowledgments 4447 Abbreviations 4447 References 4447