Mononuclear nonheme iron (mnhFe) enzymes perform an array of chemically diverse reactions. A large class of mnhFe enzymes contains a reduced Fe(II) ion that activates dioxygen, forming a highly reactive FeIII-O2– species. Once formed, this FeIII-O2– intermediate can promote a large number of different reactions leading to an enormous diversity in chemical reactivity. Despite the surprising similarities in active-site (and sometimes substrate) structures, each enzyme promotes a highly specific reaction and yields a highly specific product. The factors leading to such high reaction specificity from these similar active-site structures are not fully understood. For example, the enzymes cysteine dioxygenase (CDO) and isopenicillin-N-synthase (IPNS) possess similar active-sites yet can selectively promote two vastly different reactions on structurally similar substrates: sulfur oxygenation (CDO) vs C-H atom abstraction (IPNS). In this study we will disclose our efforts towards understanding these different reactions using metallopeptide based mimics of CDO and IPNS. Fe(II) is coordinated within virtually identical coordination environments in these metallopeptides, yet, upon exposure to dioxygen different reactions are promoted; the CDO mimic will promote an O-atom transfer from O2 to an internal thiol-containing substrate while the IPNS mimic promotes H-atom abstraction events. It will be shown that the differences in reactivity are solely attributable to the different coordination geometries of the coordinated thiolate substrate relative to a putative Fe(III)-O2– intermediate. The influence the different coordination geometry of the thiolate substrate has on the electronic structure and subsequently the reactivity of the metallopeptide mimics will be discussed.