Structure-Function Relationships in Hexacoordinate Heme Proteins: Mechanism of Cytoglobin Interactions With Exogenous Ligands
Cytoglobin (Cygb) and neuroglobin (Ngb) are among the newest members of vertebrate globin family characterized by a classical 3-over-3 α-helical fold and a heme prosthetic group capable of reversibly binding small ligands such as O2, CO and NO. The physiological functions of Cygb and Ngb remain to be determined; however, current data suggest that both proteins have a significant role in cytoprotection in hypoxic and genotoxic conditions. Cytoglobin and Ngb are distinct from their better-known counterparts, hemoglobin (Hb) and myoglobin (Mb), in several structural features. First, in the absence of an external ligand, the sixth coordination site of the heme iron in Cygb and Ngb is occupied by a distal histidine residue, leading to a complex ligand rebinding mechanism dependent on the rate of distal His dissociation from the heme iron. Although hexacoordination was observed before in plant and bacterial hemoglobins, the physiological role of this feature remains unknown. Second, both Ngb and Cygb are capable of forming an intraprotein disulfide bond, which has been shown to regulate ligand binding affinity, leading to a hypothesis that intracellular function of these proteins is redox-dependent. Lastly, Cygb contains ∼20 amino acid long extensions on both N- and C- termini, a unique feature among vertebrate globins with unknown physiological function. The work presented in the dissertation reveals that hexacoordinate heme reactivity is distinct from that of pentacoordinate heme and is strongly influenced by the distal histidine residue and the disulfide bond. In the case of human Cygb, experimental and computational approaches demonstrated that the disulfide bond regulates the flexibility of the N terminus and the accessibility of the 1,8-ANS binding site. Furthermore, molecular dynamics of the hexa- and pentacoordinate human Ngb were probed computationally to elucidate structural requirements that govern signal transmission between CD loop and the distal pocket. Lastly, Ngb and Cygb were reconstituted with a fluorescent analog of the native heme group to produce hexacoordinate variants with favorable photophysical properties that can be used to characterize protein-protein interactions.
Tangar, Antonija, "Structure-Function Relationships in Hexacoordinate Heme Proteins: Mechanism of Cytoglobin Interactions With Exogenous Ligands" (2018). ProQuest ETD Collection for FIU. AAI13806009.