Mapping Energy Transport Networks in Proteins

Abstract

INTRODUCTIONThe response of proteins to chemical reactions or impulsive excitation that occurs within the molecule has fascinated chemists for decades. [1][2][3] In recent years ultrafast X-ray studies have provided ever more detailed information about the evolution of protein structural change following ligand photolysis, 4-5 and time-resolved IR and Raman techniques, e.g., have provided detailed pictures of the nature and rate of energy transport in peptides and proteins, [6][7][8][9][10][11] including recent advances in identifying transport through individual amino acids of several heme proteins. [12][13][14] Computational tools to locate energy transport pathways in proteins have also been advancing. 15 Energy transport pathways in proteins have since some time been identified by molecular dynamics (MD) simulations, [16][17] and more recent efforts have focused on the development of coarse graining approaches, 18-29 some of which have exploited analogies to thermal transport in other molecular materials. [30][31][32][33][34][35] With the identification of pathways in proteins and protein 2 complexes, network analysis has been applied to locate residues that control protein dynamics and possibly allostery, [36][37][38] where chemical reactions at one binding site mediate reactions at distance sites of the protein. In this chapter we review approaches for locating computationally energy transport networks in proteins. We present background into energy and thermal transport in condensed phase and macromolecules that underlies the approaches we discuss before turning to a description of the approaches themselves. We also illustrate the application of the computational methods for locating energy transport networks and simulating energy dynamics in proteins with several examples.One of the themes that we address in this chapter is the difference between energy transport in condensed phase generally and in a folded polymer such as a protein specifically. While the approaches that we present and detail are based on linearresponse theory for transport, we apply them to a system, a protein, where energy transport occurs highly anisotropically.We are mainly concerned with the characterization of such anisotropic transport, not simply the calculation of transport coefficients for a particular object, though that information can also be calculated starting with the approaches we present. The focus here then is on local energy transport coefficients, such as local energy conductivities and diffusivities, as discussed below, and how these local transport properties connect into a network that mediates energy dynamics in the protein. It is the network of such local energy transport coefficients that, once identified and located, can be used to model the global energy dynamics in a protein, as we describe.That energy transport pathways exist, i.e., energy does not simply flow isotropically through a globular protein, is an inherent property of the geometry of a 3 folded protein, [62][63][64][65][66][67][68][69][70] which i...

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