Ph.D. University of Michigan, 1994

Phone: 718 951 6636

Grant Support: American Heart Association,
$198,000

Nature of Research

Heme proteins are vital components of aerobic respiration and heme protein disfunction leads to oxidative damage.  Heme proteins provide conduits for dioxygen transport & storage (the globins), sites for electron transport (cytochromes), sites for dioxygen & substrate activation (peroxidases, catalases, P-450s) as well as dioxygen reduction (cytochrome c oxidase).  Despite over eighty years of heme protein research since the cytochromes were first named, the fundamental role of the various porphyrin architectures observed in biology  (hemes a, b, c and d) on the biochemical function of heme proteins remains poorly defined. 

Our approach to defining the role of the heme structures in biology is to compare their affinity for protein scaffolds in both the Fe(III) and Fe(II) oxidation states and their resulting electrochemistry.  We have previously compared heme b to heme a in a designed heme protein maquette to demonstrate the distinct roles of the 2-hydroethylfarnesyl and 8-formyl groups on heme a function in aerobic respiration.  Additionally, these studies provided a thermodynamic rationale for the observed biosynthetic pathway from heme b to heme a.  In a study funded by the American Heart Association, we are currently investigating the role of the thioether linkages between the protein and the heme in natural cytochromes c. These covalent linkages are proposed to be an evolutionary vestige, a guide to protein folding, a mechanism to increase the density of hemes in proteins and a mechanism to increase the effective concentration of the axial ligands. The cysteine residues involved in forming the thioether linkages have been mutated to alanines to generate cytochrome c-to-b conversions proteins.  We will perform detailed coordination equilibria analyses on these cyt c-to-b conversion proteins to elucidate the thermodynamics of ferric and ferrous heme affinity.  These binding constant determinations will be coupled with the related electrochemical measurements taken using UV-visible spectroelectrochemistry. The results of these coordination chemistry studies will provide thermodynamic data with which to differentiate between the various roles proposed for thioether linkages in c-type cytochromes.

References

Zhuang, J.; Reddi, A.R.; Wang, Z.; Khodaverdian, B.; Hegg, E.L.; Gibney, B.R. Evaluating the Roles of the Heme a Sidechains in Cytochrome c Oxidase Using Designed Heme Proteins, Biochemistry, 2006, 45, 12530-12538.

Zhuang, J.; Amoroso, J.H.; Kinloch, R.; Dawson, J.H.; Baldwin, M.J.; Gibney, B.R. Evaluation of Electron-withdrawing Group Effects on Heme Binding in Designed Proteins: Implications for Heme a in Cytochrome c Oxidase, Inorg. Chem., 2006, 45, 4685-4694