We have measured the binding affinity (K A ) and electron transfer (ET) rate constants (k) for the complex of hemoglobin (Hb) and cytochrome b 5 (b 5 ), using triplet quenching titrations of mixed-metal [ZnM, Fe 3+ (N 3 -)] Hb hybrids and of fully substituted Zn-mesoporphyrin (ZnM)Hb by b 5 (trypsin-solubilized, bovine) (pH values 6.0 and 7.0). The use of the mixed-metal Hb hybrids with Zn in one chain type allows us to selectively monitor the 3 ZnP f Fe 3+ P ET reaction of Fe 3+ b 5 with either the R-chains or the β-chains. The self-consistent analysis of the results for the mixed-metal hybrids and those for the (ZnM)Hb allows us to determine the reactivity and affinity constants for the interactions of b 5 with the individual subunits of T-state Hb. The results confirm that ET occurs within a complex between b 5 and Hb, not through a simple bimolecular collision process. At pH 6.0, the binding affinity constant of the R-chains (K R ≈ 2.0 × 10 4 M -1 ) is ∼4-fold larger than that of the β-chains (K β ) 4.9 × 10 3 M -1 ); the intracomplex ET rate constant of the R-chains (k R ≈ 1500 s -1 ) is ∼2-fold larger than that of the β-chains (k β ≈ 850 s -1 ). The binding affinity and ET rate constant of the R-chains both decrease as the pH is increased from 6.0 to 7.0; the binding affinity of the β-chains is essentially the same at pH 6.0 and 7.0, while the ET reactivity decreases. The kinetic results are consistent with a docking model in which each subunit binds a molecule of b 5 . However, they permit an alternative in which b 5 reacts with the R-chains by binding at a site which spans the R 1 β 2 dimer interface.

We have compared the photoinitiated electron-transfer (ET) reaction between cytochrome b(5) (b(5)) and zinc mesoporphyrin-substituted hemoglobin [(ZnM)Hb] and Hb variants in order to determine whether b(5) binds to the subunit surface of either or both Hb chains, or to sites which span the dimer--dimer interface. Because the dimer--dimer interface would be disrupted for monomers or alpha beta dimers, we studied the reaction of b(5) with alpha ZnM chains and (ZnM)Hb beta W37E, which exists as alpha beta dimers in solution. Triplet quenching titrations of the ZnHb proteins with Fe(3+)b(5) show that the binding affinity and ET rate constants for the alpha-chains are the same when they are incorporated into a Hb tetramer or dimer, or exist as monomers. Likewise, the parameters for beta-chains in tetramers and dimers differ minimally. In parallel, we have modified the surface of the Hb chains by neutralizing the heme propionates through the preparation of zinc deuterioporphyrin dimethyl ester hemoglobin, (ZnD-DME)Hb. The charge neutralization increases the ET rate constants 100-fold for the alpha-chains and 40-fold for the beta-chains (but has has little effect on the affinity of either chain type for b(5), similar to earlier results for myoglobin). Together, these results indicate that b(5) binds to sites at the subunit surface of each chain rather than to sites which span the dimer-dimer interface. The charge-neutralization results further suggest that b(5) binds over a broad area of the subunit face, but reacts only in a minority population of binding geometries.

We have monitored the interactions of the series of trivalent lanthanide cations with the thylakoid membrane surface of spinach chloroplasts using two complementary spectral techniques. Measurements of the fluorescence emission of the extrinsic probe 2-p-toluidinonaphthalene-6-sulfonate (TNS) and the absorbance of the intrinsic chromophore chlorophyll provide two sensitive means of characterizing the dependence of the cationmembrane interaction on the nature of the cation. In these systems, added lanthanide cations adsorb onto the membrane surface to neutralize exposed segments of membrane-embedded protein complexes. The lanthanideinduced charge neutralization increases the proximity of added TNS anion to the membrane surface as evidenced by variations in the TNS fluorescence level and wavelength of maximum emission. Our results reveal a strong dependence of TNS fluorescence parameters on both lanthanide size and total orbital angular momentum L value. Lanthanides with greater charge density (small size and/or low L value) enhance the TNS fluorescence level to a greater extent. A possible origin for the lanthanide-dependent TNS fluorescence levels is suggested in terms of a heterogeneity in the number and type of TNS binding sites. In the absence of the probe TNS, lanthanide-induced changes in the chlorophyll absorption spectrum reflect the shrinkage of chloroplasts accompanying thylakoid membrane stacking. Absorbance increases in the 500-660 nm region, attributed to increases in light scattering arising from the membrane structural reorganization, reveal a dependence on lanthanide identity. The data are consistent with the proposal that larger lanthanides with smaller enthalpies of hydration induce more significant membrane appression. These investigations illustrate a novel utilization of lanthanides in cation binding studies by employing their chemical and physical differences rather than their similarities in luminescence properties