Supporting information is at the end of the main manuscript. AbstractMetalloporphyrins play important roles in areas ranging from biology to nanoscience.Biology uses a narrow set of metal centres comprising mainly of iron and magnesium. Here, we convert metalloporphyrin specificity of cytochrome b562 from iron (haem) to fluorogenic zinc protoporphyrin IX (ZnPP). Through a computationally guided iterative design process, a variant with a near total preference for ZnPP was generated representing a switch in specificity. The new variant greatly enhanced (≥60 fold) the negligible aqueous fluorescence of free ZnPP in vitro and in vivo. ZnPP Designed interactions 0 10 20 30 40 50 60 570 600 630 660 690 Emission Intensity Wavelength (nm) +cyt b 562 ZnPP