Application of Stopped-Flow and Time-Resolved X-Ray Absorption Spectroscopy to the Study of Metalloproteins Molecular Mechanisms

1.1 Metalloproteins as mediators of life processes Revealing detailed molecular mechanisms associated with metalloproteins is highly desired for the advancement of both basic and clinical research. Metalloproteins represent one of the most diverse classes of proteins, containing intrinsic metal ions which provide catalytic, regulatory, or structural roles critical to protein function. The metal ion is usually coordinated by nitrogen, oxygen, or sulfur atoms belonging to amino acids in a polypeptide chain and/or in a macrocyclic ligand incorporated into the protein. The presence of a metal ion rich in electrons allows metalloproteins to perform functions such as redox reactions, phosphorylation, electron transfer, and acid-base catalytic reactions that cannot easily be performed by the limited set of functional groups found in amino acids. Approximately onethird of all proteins possess a bound metal1, and almost half of all enzymes require the presence of a metal atom to function2. The most abundant metal ions in vivo are Mg and Zn, while Fe, Ca, Co, Mn, and Ni are also frequently observed. Metalloproteins play important roles in key biological processes such as photosynthesis, signaling, metabolism, proliferation, and immune response 3-5. For instance, the principal oxygen carrier protein in human, hemoglobin, uses iron (II) ions coordinated to porphyrin rings to bind the dioxygen molecules; calmodulin is a calcium-binding protein mediating a large variety of signal transduction processes in response to calcium binding; cytochromes mediate electron transport and facilitate a variety of redox reactions using iron, which interconverts between Fe2+ (reduced) and Fe3+ (oxidized) states; and many proteolytic enzymes contain zinc ions in their active sites that is highly crucial for peptide bond hydrolysis reaction.