HR: 08:30h
AN: B51D-03    [Abstracts]
TI: Chemical potential constraints on the composition and subcellular localization of proteins
AU: * Dick, J M
EM: jedick@berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Helgeson, H C
EM: brogie@socrates.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AB: The distribution and speciation of the metallome in organisms is amenable to study using thermodynamic calculations that take into account the chemical potentials obtaining in living cells. In particular, subcellular spatial gradients of the negative logarithms of the activities of the electron and proton (pe and pH, respectively) strongly influence the speciation of aqueous metals and other inorganic species, as well as aqueous organic and biomacromolecular species. Although pe-pH diagrams are commonly used to describe speciation in inorganic aqueous systems, they have not been applied to assess and quantify the relative stabilities of biomacromolecules in living organisms. Nevertheless, there is much to be gained by doing so. The purpose of the present communication is to demonstrate this by generating pe-pH and other equilibrium activity diagrams for proteins in the system C-H-N-O-S. The relative abundances of amino acid residues in the proteins considered are representative of proteins found in different subcellular locations. For example, the boundaries of the stability fields for extracellular, cytoplasmic, and nuclear proteins can be assessed and portrayed on pe-pH diagrams. By overlaying pe-pH diagrams for proteins with those for metals, one can predict the oxidation states of metals compatible with the proteins found in the different subcellular locations. The standard molal thermodynamic properties of these proteins can be estimated from group additivity algorithms that include provision for protein ionization as a function of solution pH. The temperature and pressure dependence of these properties can be computed with the aid of the revised HKF equations of state. Because quantifying the relative stabilities of proteins is a multidimensional problem, a Gibbs free energy minimization software package was used to carry out a plethora of computer experiments for specified temperatures, pressures, and bulk compositions. Plotting the results of the Gibbs free energy minimization computer experiments on pe-pH diagram results in relative stability fields for extracellular, cytoplasmic, and nuclear proteins at ambient as well as hydrothermal temperatures and pressures. Unlike the lines of equal activity for inorganic species with constant charge, the boundaries of the metastability fields of proteins are curved because the proteins ionization states vary with pH. Reconnaisance calculations indicate that the calculated ranges of pe and pH for these locations compare favorably with reported physiological values at 25 $^\circ$C and 1 bar. The results of this research represent a first step toward quantifying the relative metastabilities of proteins and their interaction with each other in living cells.
DE: 4825 Geochemistry
DE: 4851 Oxidation/reduction reactions
DE: 1045 Low-temperature geochemistry
DE: 1099 General or miscellaneous
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting