Date of Award
Doctor of Philosophy
Biochemistry, Biophysics and Molecular Biology
Mark S. Hargrove
The heme prosthetic group in hemoglobins is most often attached to the globin through coordination of either one or two histidine side chains. Those proteins with one histidine coordinating the heme iron are called “pentacoordinate” hemoglobins, a group represented by red blood cell hemoglobin and most other oxygen transporters. Those with two histidines are called “hexacoordinate hemoglobins”, which have broad representation among eukaryotes. Coordination of the second histidine in hexacoordinate Hbs is reversible, allowing for binding of exogenous ligands like oxygen, carbon monoxide, and nitric oxide. Research over the past several years has produced a fairly detailed picture of the structure and biochemistry of hexacoordinate hemoglobins from several species including neuroglobin and cytoglobin in animals, and the nonsymbiotic hemoglobins in plants. However, a clear understanding of the physiological functions of these proteins remains an elusive goal. The aim of this research was to understand physiological functions based on the molecular structure of plant hemoglobins. All plants contain hemoglobins that fall into distinct phylogenetic classes. The subset of plants that carry out symbiotic nitrogen fixation express hemoglobins that scavenge and transport oxygen to bacterial symbiotes within root nodules. These “symbiotic” oxygen transport hemoglobins are distinct in structure and function from the non-oxygen transport (“nonsymbiotic”) Hbs found in all plants. Hemoglobins found in two closely related plants present a paradox concerning hemoglobin structure and function. Parasponia andersonii is a nitrogen fixing plant that expresses a symbiotic hemoglobin (ParaHb) characteristic of oxygen transport hemoglobins in having a pentacoordinate ferrous heme iron, moderate oxygen affinity, and a relatively rapid oxygen dissociation rate constant. A close relative that does not fix nitrogen, Trema tomentosa, expresses hemoglobin (TremaHb) sharing 93% amino acid identity to ParaHb, but its phylogeny predicts a typical nonsymbiotic hemoglobin with a hexacoordinate heme iron, high oxygen affinity, and slow oxygen dissociation rate constant. We characterized heme coordination and oxygen binding in TremaHb and ParaHb, along with their crystal structures, to investigate whether or not two hemoglobins with such high sequence similarity are actually so different in functional behavior. Our results demonstrate distinct mechanisms for convergent evolution of oxygen transport in different phylogenetic classes of plant hemoglobins.
Kakar, Smita, "Structure and Reactivity of Hexacoordinate Hemoglobins" (2010). Graduate Theses and Dissertations. 11235.