My group is conducting research on the enzyme system called photosystem II that oxidizes H2O to O2. One project focuses on the metallobiochemistry of this complex: four Mn atoms, a Ca2+ and a Cl- are essential for the redox activity that produces O2. We are in the process of analyzing the consequences of depleting Ca2+ and Cl- from the complex, as well as exploring the properties of samples in which these ions have been replaced by surrogates. Experiments are now under way to characterize the role of Cl- in regulating the redox activity of the Mn cluster. We are also characterizing the properties of the tetranuclear Mn complex using techniques that allow us to prepare partially reduced derivatives that can be examined by X-ray absorption spectroscopy in collaboration with the Penner-Hahn group. We are in the process of conducting comparative investigations on modified clusters containing Mn atoms that have been reduced by either one or two electrons.
A second project is devoted to determining how a key protein of the complex, called Manganese Stabilizing Protein, regulates Mn ligation and redox cycling. For these investigations, we have developed a system for overexpression of the protein in E. coli. We exploit this system for producing site-directed mutants of the protein, and then analyze the effects of the mutations by replacing the native protein with various mutants. We have identified domains on the protein that are essential for its biological activity, and are now focusing on specific amino acids, or on very short amino acid sequences, in experiments to precisely define the residues required for activity. A second project involves characterizing the solution structure of Manganese Stabilizing Protein. We have shown that in solution, the protein fails to exhibit the organized tertiary structure expected of most proteins. This 'natively unfolded' behavior is now under investigation, using many of the same mutants that we employ in the research on the function of this interesting protein.
H. Popelkova, A.J. Wyman, C.F. Yocum. "Amino acid sequences and solution structures of manganese stabilizing protein that affect reconstitution of photosystem II activity" Photosynthesis Research, 2003, 77, 21-34.
H. Popelkova, M.M. Im, C.F. Yocum. "Binding of manganese stabilizing protein to photosystem II: Identification of essential N-terminal threonine residues and domains that prevent nonspecific binding" Biochemistry, 2003, 42, 6193-6200.
Popelkova, H., Im, M., and Yocum, C.F. "N-terminal truncations of manganese stabilizing protein identify two amino acid sequences required for binding of the eukaryotic protein to photosystem II, and reveal the absence of one binding-related sequence in cyanobacteria." Biochemistry, 2002, 41, 10038-10045.
H. Popelkova, M.M. Im, N. Lydakis-Simantiris, S. Betts, C.F. Yocum. "N-terminus of the photosystem II manganese stabilizing protein: Effects of sequence elongation and truncation." Biochemistry 41, 2002, 2702-2711.
K. Vander Meulen, A. Hobson, C.F. Yocum. "Calcium extraction alters the structure of the photosystem II oxygen evolving complex." Biochemistry 41, 2002, 958-966.