Biomedical subjects
Publications and source records attributed to H B Gray.
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Kinetic studies of the reduction of Pseudomonas aeruginosa ferricytochrome c551 by Fe(EDTA)2- have been made. The reaction was found to follow a second-order rate law: k 4.2 x 10(3) M(-1) s(-1) [25 degrees, micro0.1 M, pH 7.0 (phosphate)]; deltaH+/+ 3.2 kcal/ mol; AS+/+ -30 cal/mol-deg. The electrostatics-corrected self-exchange rate constant (k11 corr) calculated for cytochrome c551 based on the Fe(EDTA)2- cross reaction is 2 M(-1) s(-1), as compared to a value of 6 M(-1) s(-1) for horse heart cytochrome c. The close correspondence of the two k11 corr values is taken as an indication that the two proteins employ very similar electron transfer mechanisms in their reactions with Fe(EDTA)(2-). It is proposed that this mechanism involves reagent contact, but little protein conformational change, at the partially exposed heme edge.
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A method is described for the rapid separation of the complementary strands of homogeneous DNA's. The method takes advantage of a difference in sedimentation coefficients between the complementary strands when these have been dissociated and complexed with poly(U-G). The separation procedure is readily carried out for preparative purposes by zone sedimentation in linear 5--20% (w/v) sucrose gradients containing 2 M NaCl. In analytical band sedimentation experiments, concentrated solutions of CsCl have been used to effect the separation of the complexed complementary strands. Sedimentation coefficients corresponding to corrected values at infinite dilution for the cesium salt of the nucleic acid complexes at 25 degrees C (s 25, w) have been obtained for the complexed complementary strands of coliphage lambdab2b5c DNA and for the uncomplexed single strands of this DNA. The relative values of the parameters affecting s 25, w have been determined. Under these conditions, for which preferential hydration of the sedimenting species is not involved, it was found that the more highly complexed strand has sustained a 24% increase in mass and a 28% decrease in frictional coefficient relative to uncomplexed DNA, while the strand binding the lesser amount of RNA has increased in mass by 11% and changed in frictional coefficient by an insignificant amount relative to the uncomplexed species from that of the uncomplexed DNA do not contribute significantly to the differences in s 25, w between the complexed complementary strands.
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Low temperature absorption, circular dichroism, and magnetic circular dichroism spectral studies of the blue copper proteins Rhus vernicifera stellacyanin, bean plastocyanin, and Pseudomonas aeruginosa azurin have been made. Low energy bands attributable to the d-d transitions 2B2 leads to 2E and 2B2 leads to 2B1 in a flattened tetrahedral (D 2d) copper-(II) center are observed in these proteins at about 5000 and 10,000 cm-1, respectively. The band positions accord well with ligand field calculations based on a tetrahedral structure that is distorted approximately 6 degrees toward a square plane. The ligands in this flattened tetrahedral coordination unit in bean plastocyanin are identified from various spectroscopic experiments as His-38, Cys-85, His-88, and a deprotonated peptide nitrogen (N) a few residues above His-38.
The reactions of horse heart cytochrome c with Fe(ethylenediaminetetraacetate)2-, Co(1,10-phenanthroline)3(3+), Ru(NH3)6(2+), and Fe(CN)6(3-) have been analyzed within the formalism of the Marcus theory of outer-sphere electron transfer, including compensation for electrostatic interactions. Calculated protein self-exchange rate constants based on crossreactions are found to vary over three orders of magnitude, decreasing according to Fe(CN)6(3-) greater than Ru(NH3)6(2+) greater than Fe(EDTA)2-. The reactivity order suggests that the mechanism of electron transfer involves attack by the small molecule reagents near the most nearly exposed region of the heme; this attack is affected by electrostatic interactions with the positively charged protein, by hydrophobic interactions that permit reagent penetration of the protein surface, and by the availability of pi symmetry ligand (or extended metal) orbitals that can overlap with the pi redox orbitals of the heme group.
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