Mycobacterial hemerythrin-like proteins (HLPs) are important contributors to the survival of these pathogens in mammalian macrophages. In contrast to the carboxylate-bridged nonheme diiron cluster of hemerythrins which contain only carboxylate and imidazolate ligands, HLPs diiron clusters include a single tyrosine sidechain coordinated to one of the two iron(III) center. In 2022, we showed how this asymmetric coordination sphere at the diferric center results in unusual spectroscopic signatures and unique reactivity toward nitric oxide (NO) which we assigned to an initial radical attack by NO at the μ-oxo group that bridges the two Fe(III) center (Albert and Moënne-Loccoz 2022). More recently, we showed that a distant HLP ortholog from Azotobacter vinelandii retains all protein sidechains coordinating the diiron clusters in mycobacterial HLPs, but requires a single Thr47Phe outer-sphere point mutation to stabilize a μ-oxo bridge and gain reactivity toward NO (Albert et al,. 2025). In this oral presentation, I will review these published results and discuss ways a Phe sidechain might control the protonation state of a bridging solvent molecule at the diferric cluster, the reduction potential of the Fe(III) centers, and the NO reactivity in HLPs. I will also discuss our latest efforts to characterize intermediate species within the NO reactions of HLPs using a photolabile caged NO compound at cryogenic temperatures - an approach we recently validated with the NO dioxygenation reaction of oxymyoglobin (Albert et al., 2024).