Proteins electrical fields

A surprising finding is that for 16 of the 19 Trps, protein contributes a red shift to the steady state Stokes shift, a result that is statistically very improbable. The extreme bias toward red shifts for the protein contributions suggests that protein electric fields relative to the modest ground-state dipole of the Trp residue may be important in the evolution of the protein folds. 

Callis PR, Petrenko A, Muino PL, Tusell JR (2007) Ab initio prediction of tryptophan fluorescence quenching by protein electric field enabled electron transfer. J Phys Chem B 111(35) 10335-10339... 

Classical electrostatic modeling based on the Coulomb equation demonstrated that the model system chosen could account for as much as 85% of the effect of the protein electric field on the reactants. Several preliminary computations were, moreover, required to establish the correct H-bond pattern of the catalytic water molecule (WAT in Fig. 2.6). Actually, in the crystal structure of Cdc42-GAP complex [60] the resolution of 2.10 A did not enable determination of the positions of the hydrogen atoms. Thus, in principle, the catalytic water molecule could establish several different H-bond patterns with the amino acids of the protein-active site. Both classical and quantum mechanical calculations showed that WAT, in its minimum-energy conformation,... 

Another mode of protein-electric field interaction has been proposed by Blank (18-21). Blank considers that the effects of an electric field on membrane protein mainly arise from its effect on the electric double layer at the water-membrane interface. In other words, an electric field can change the concentration of ions near a membrane protein, which results in a stimulation or a reduction in its activity. The surface compartmental model has been used to interpret the ac stimulated adenosine triphosphate (ATP) splitting of Na, K-ATPase (adenosine triphosphatase) and ribonucleic acid (RNA) synthesis in various types of cells (19-21). 

Laberge, M., Vanderkooi, J.M., Sharp, K.A. Effect of a protein electric field on the CO stretch fi equency. Finite difference Poisson-Boltzmann calculations on carbonmonoxycy-tochromes c. J. Phys. Chem. 100, 10793-10801 (1996)... 

Klapper, I., Hagstrom, R., Fine, R., Sharp, K., Honig, B. Focusing of electric fields in the active site of cu,zn superoxide dismutase. Proteins Struct. Pune. Genet. 1 (1986) 47-79. 

Klapper 1, R Hagstrom, RFine, K Sharp and B Honig 1986. Focusing of Electric Fields in tire Actir e Sit of CuZn Superoxide Dismutase Effects of Ionic Strength and Amino-Acid Substitution. Proteins Structure, Function and Genetics 1 47-59. 

Electrophoresis is used primarily to analyze mix tures of peptides and proteins rather than individual ammo acids but analogous principles apply Because they incorporate different numbers of ammo acids and because their side chains are different two pep tides will have slightly different acid-base properties and slightly different net charges at a particular pH Thus their mobilities m an electric field will be differ ent and electrophoresis can be used to separate them The medium used to separate peptides and proteins is typically a polyacrylamide gel leading to the term gel electrophoresis for this technique... 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Electroultrafiltration has been demonstrated on clay suspensions, electrophoretic paints, protein solutions, oil—water emulsions, and a variety of other materials. Flux improvement is proportional to the appHed electric field E up to some field strength E where particle movement away from the membrane is equal to the Hquid flow toward the membrane. There is no gel-polarization layer and (in theory) flux equals the theoretical permeate flux. It... 

Because protein samples are actually ampholytes, when samples are loaded onto the gel and a current is appHed, the compounds migrate through the gel until they come to their isoelectric point where they reach a steady state. This technique measures an intrinsic physicochemical parameter of the protein, the pi, and therefore does not depend on the mode of sample appHcation. The highest sample load of any electrophoretic technique may be used, however, sample load affects the final position of a component band if the load is extremely high, ie, high enough to titrate the gradient ampholytes or distort the local electric field. 

Charged macromolecules, such as proteins or polymers, are often separated elec-trophoretically. The rate of migration through an electric field increases with net charge and field strength. Molecular size of analytes and viscosity of separation media both have inverse relationships with rate of migration. These variables must all be taken into account in order to optimize the conditions for an efficient electrophoretic separation. 

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