In prior work, we surveyed over 300 structures for the presence of leucines with Cα atom within 7 Å of a vinyl Cα atom and Cβ atom within 6 Å of the same vinyl Cα atom. These criteria identified L79 in GlbN and L75 in CtrHb as candidates for histidine replacement and heme modification. Many other leucines were found. Examples include L106 in nitrophorin 2 (1PEE), L211 in chlorite dismutase (2VXH), L37 in ascorbate peroxidase (1APX), and L71 in cytochrome b 5 (3NER), chosen to highlight proteins that have architectures different from that of globins. Many isoleucines, filtered by the same distance criteria, also occupy positions where a histidine may react. For in vitro applications, screening of pH conditions and permissive exogenous ligands that seat the heme properly can be systematically explored. Thus, it is reasonable to expect that many natural heme proteins can be modified to undergo the reaction, or artificial proteins can be designed to present a histidine with appropriate geometry to the heme.