Femtosecond Coherence Spectroscopy & Ultrafast Kinetic Study of Heme Proteins

Northeastern University, Boston MA

Investigators

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Abstract

This project aims to extend our understanding of the structure, function, and dynamics of heme proteins such as cytochrome P450, mammalian peroxidases, nitric oxide synthase, soluble guanylate cyclase, nitrophorin, and cytochrome c. These proteins are involved in broad array of catalytic, signaling, and electron transport processes and are capable of an amazingly broad range of functions, even when the heme axial ligands are identical. This indicates that the protein architecture, and its influence on the heme structure, plays an important functional role. By using coherence spectroscopy, a femtosecond optical pump-probe technique, the soft out-of-plane (OOP) low-frequency vibrational modes of the heme can be excited and analyzed even in an aqueous environment. These vibrational modes have not been documented previously because they are difficult to access using traditional spectroscopic methods. They fall in the region of ambient thermal excitations (<200cm-1 ~300K) and are therefore most likely to be utilized as reaction coordinates by proteins. The observed coherence spectral intensities of these soft modes depend upon the magnitude of the heme structural distortions that are induced by the protein architecture. These OOP heme motions are functionally significant, as demonstrated by the importance of the heme doming mode in the diatomic ligand binding reaction. The rich spectrum of the low-frequency heme motions is just beginning to be appreciated, as a wider variety of proteins and model compounds is examined. This project aims to explore the functional role of both static distortions and thermally excited low-frequency vibrations in heme proteins. Distortions (such as heme ruffling and saddling) that alter the electronic orbital interactions between the iron atom and its surrounding molecular framework are hypothesized to affect the redox potential of the metal center. Vibrational motions along these same, thermally accessible, OOP coordinates are excellent candidates to mediate and control electron transfer. Coherence spectroscopy is uniquely positioned to probe these modes in aqueous solution. For example, we will examine electron transfer partners, such as Pdx and CYP101, in order to monitor changes in the low frequency spectrum that occur when the protein complex is formed. The low frequency modes of Fe-S proteins will also be examined. Kinetic probes on ultrafast timescales, stretching over 10 decades in time, will be used to study the rapid time-scale, non-equilibrium processes, that take place immediately following the electronic rearrangements associated with biochemical reactions. For example, the two geminate phases for oxygen rebinding to the heme in myoglobin exhibit very different Arrhenius prefactors, suggesting that the entropic barrier for recombination is time dependent. Such non-equilibrium processes will be studied to learn if they allow heme proteins to enhance discrimination between different classes of diatomic ligands.