Electron-Transfer Properties and Active-Site Structure of the Type 1 (Blue) Copper Protein Umecyanin

Thermodynamics of the alkaline transition in phytocyanins

The thermodynamics of the alkaline transition which influences the spectral and redox properties of the type 1 copper center in phytocyanins has been determined spectroscopically. The proteins investigated include Rhus vernicifera stellacyanin, cucumber basic protein and its Met89Gln variant, and umecyanin, the stellacyanin from horseradish roots, along with its Gln95Met variant. The changes in reaction enthalpy and entropy within the protein series show partial compensatory behavior. Thus, the reaction free energy change (hence the pK (a) value) is rather variable. This indicates that species-dependent differences in reaction thermodynamics, although containing an important contribution from changes in the hydrogen-bonding network of water molecules in the hydration sphere of the protein (which feature enthalpy-entropy compensation), are to a large extent protein-based. The data for axial ligand variants are consistent with the hypothesis of a copper-binding His as the deprotonating residue responsible for this transition.

A definitive example of a geometric "entatic state" effect: electron-transfer kinetics for a copper(II/I) complex involving A quinquedentate macrocyclic trithiaether-bipyridine ligand

The quinquedentate macrocyclic ligand cyclo-6,6'-[1,9-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]aneS3bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been synthesized and complexed with copper. The CuII/IL redox potential, the stabilities of the oxidized and reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with a series of six counter reagents have been studied in acetonitrile at 25 degrees C, mu = 0.10 M (NaClO4). The Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k11. The latter value has also been determined independently from NMR line-broadening experiments. The cumulative data are consistent with a value of k11 = 1 x 10(5) M(-1) s(-1), ranking this among the fastest-reacting CuII/I systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures show that the geometry of the CuIIL and CuIL complexes are nearly identical, both exhibiting a five-coordinate square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the CuIL complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the transition-state geometry more closely approximating the geometry of the CuIIL ground state. The combination of a nearly constant geometry and a large self-exchange rate constant implies that this CuII/I redox system represents a true geometric "entatic state."

Investigating the Cause of the Alkaline Transition of Phytocyanins

The phytocyanins are a family of plant cupredoxins that have been subdivided into the stellacyanins, plantacyanins, and uclacyanins. All of these proteins possess the typical type 1 His(2)Cys equatorial ligand set at their mononuclear copper sites, but the stellacyanins have an axial Gln ligand in place of the weakly coordinated Met of the plantacyanins, uclacyanins, and most other cupredoxins. The stellacyanins exhibit altered visible, EPR, and paramagnetic (1)H NMR spectra at elevated pH values and also modified reduction potentials. This alkaline transition occurs with a pK(a) of approximately 10 [Dennison, C., Lawler, A. T. (2001) Biochemistry 40, 3158-3166]. In this study we demonstrate that the alkaline transition has a similar influence on the visible, EPR, and paramagnetic NMR spectra of cucumber basic protein (CBP), which is a plantacyanin. The mutation of the axial Gln95 ligand into a Met in umecyanin (UMC), the stellacyanin from horseradish roots, and the axial Met89 into a Gln in CBP have very limited, yet similar, influence on the pK(a) for the alkaline transition as judged from alterations in visible spectra. The complete removal of the axial ligand in the Met89Val variant of CBP results in a slightly larger decrease in the pK(a) for this effect, but similar spectral alterations are still observed at elevated pH. Thus, the axial Gln ligand is not the cause of the alkaline transition in Cu(II) stellacyanins, and alterations in the active site structures of the phytocyanins have a limited effect on this feature. The conserved Lys residue found adjacent to the axial ligand in the sequences of all phytocyanins, and implicated as the trigger for the alkaline transition, has been mutated to an Arg in UMC. The influence of increasing pH on the spectroscopic properties of Lys96Arg UMC is almost identical to those of the wild type protein, and thus, this residue is not responsible for the alkaline transition. However, a positively charged residue in this position seems to be important for the correct folding of UMC. Other possible triggers for the effects seen in the phytocyanins at elevated pH are discussed along with the relevance of the alkaline transition.

Investigations of the alkaline and acid transitions of umecyanin, a stellacyanin from horseradish roots

The effect of pH on Cu(I) and Cu(II) umecyanin (UCu), a phytocyanin obtained from horseradish roots, has been studied by electronic and NMR spectroscopy and using direct electrochemical measurements. A pK(a) value of approximately 9.5-9.8 is observed for the alkaline transition in UCu(II), and this leads to a slightly altered active site structure, as indicated by the changes in the paramagnetic 1H NMR spectrum. Electrochemical studies show that the pK(a) value for this transition in UCu(I) is 9.9. The alkaline transition is caused by the deprotonation of a surface lysine residue, with Lys96 being the most likely candidate. The isotropically shifted resonances in the (1)H NMR spectrum of UCu(II) also shift upon lowering the pH (pK(a) 5.8), and this can be assigned to the protonation of the surface (noncoordinating) His65 residue. This histidine titrates in UCu(I) with a pK(a) of 6.3. The reduction potential of the protein in this range is also dependent on pH, and pK(a) values matching those from NMR, for the two oxidation states of the protein, are obtained. There is no evidence for either of the active site histidines (His44 and His90) titrating in UCu(I) in the pH range studied (down to pH 3.7). Also highlighted in these studies are the remarkable active site similarities between umecyanin and the other phytocyanins which possess an axial Gln ligand.

Effect of pH on the self-exchange reactivity of the plant plastocyanin from parsley

The self-exchange rate constant (25 degrees C) for parsley plastocyanin is 5.0 x 10(4) M-1 s-1 at pH* 7.5 (I = 0.10 M). This value is quite large for a higher plant plastocyanin and can be attributed to a diminished upper acidic patch in this protein. The self-exchange rate constant is almost independent of pH* in the range 7.5-5.6, with a value (25 degrees C) of 5.6 x 10(4) M-1 s-1 at pH* 5.6 (I = 0.10 M). At this pH*, the ligand His87 is protonated in approximately 50% of the reduced protein molecules (pKa* 5.6), and this would be expected to hinder electron transfer between the two oxidation states. However, this effect is counterbalanced by the enhanced association of two parsley plastocyanins at lower pH* due to the partial protonation of the acidic patch.

Redox reactivity of the type 1 copper protein amicyanin from Thiobacillus versutus with its physiological partner cytochrome C550 and inter-protein cross-reaction studies

Reduction potentials Eo' for the T. versutus amicyanin couple, AmCuII/I, were determined at pH values in the range 4.4-9.0 by direct measurement using cyclic voltammetry, and from rate constants for the reactions AmCu1 + [Co(terpy)2]3+ and [Co(terpy)2]2+ + AmCuII, using an Eo' for the [Co(terpy)2]2+/3+ couple of 260 mV. At pH > 7.5 the value obtained is 236 mV, which increases with decreasing pH in keeping with proton inactivation of AmCuI. Together with previously determined Eo' values for the T. versutus cytochrome C550 FeIII/FeII couple, it is concluded that the physiologically relevant reaction AmCuI + cyt C550FeIII (kf) is thermodynamically favourable at pH > 6.25, but that the back reaction cyt C550FeII + AmCuII (kb) is favourable at pH < 6.25. Values of kf (25 degrees C) at pH > 6.25 were determined directly by the stopped-flow method, I = 0.100 M (NaCl). At pH < 6.25 kf values were obtained indirectly from the measured kb and equilibrium constants from delta Eo'. The combined kf variations with pH give an acid dissociation pKa for AmCuIH+ of 6.6. In further studies (25 degrees C) rate constants/M-1 S-1 (pH 6.0-8.6) were determined for the cross-reactions of AmCuI with P. aeruginosa azurin AzCuII, and AmCuI with P. aeruginosa cyt C550FeIII, and are 11.0 x 10(5) and 6.4 x 10(5) M-1 S-1 respectively at pH 8.6. Using the Marcus equations corresponding electron self-exchange rate constants (kese/M-1 S-1) of 1.3 x 10(5) and 0.6 x 10(5) M-1 S-1 were calculated for the exchange of AmCuII with unprotonated AmCuI, in good agreement with the value 1.2 x 10(5) M-1 S-1 determined by NMR at pH 8.6. Information was also obtained as to the effect of pH on these kese values.