Fast Kinetic Studies of Protein Folding and Function

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DESCRIPTION (provided by applicant): This project applies techniques for fast time-resolved magnetic circular dichroism (TRMCD), natural circular dichroism (TRCD, and ordinary absorption spectroscopies to the study of function in heme proteins and folding in heme proteins and small peptides. The novel optical methods employed use near-null ellipsometry and polarimetry to study rapid kinetic processes (nanosecond to seconds) in biomolecules that contain magneto-optically active chromophores, such as heme and the aromatic amino acids, and naturally chiral chromophores, such as the amide groups of proteins and peptides. These techniques will be used to identify and study the earliest (submillisecond) events in the folding reactions of heme proteins such as cytochrome c. A major goal is the determination of a parameter that is fundamental to understanding the nature of protein folding: the speed with which the different unfolded conformations interconvert with one another. If this is slow compared to folding itself, then understanding protein folding will require more complicated theories (e.g., energy landscape) than the transition state theory used for typical chemical reactions. Such understanding may ultimately prove helpful in developing therapies for the many diseases associated with protein misfolding, such as cystic fibrosis, type 2 diabetes, and Alzheimer's, Parkinson's, and Creutzfeldt- Jakob disease. A major goal of the functional studies is to understand how the four subunits that make up the hemoglobin molecule cooperate with each other to transport oxygen more efficiently. A recent hypothesis about this cooperativity (Ackers symmetry rule), based originally on thermodynamic measurements, is tested by kinetic measurements in this project. A novel model for hemoglobin allostery, emerging from this linkage of thermodynamics and kinetics, holds promise for simplifying and systematizing our understanding of hemoglobin's dynamics and its control in the body by allosteric effectors such as organic phosphates. In addition, TRMCD studies of the aromatic amino acid residue tryptophan [337, positioned at a site critical for cooperativity, are intended to further clarify how hemoglobin's subunits work together as an efficient "molecular machine" for transporting oxygen from the lungs to the tissues.