PHOTO-TRIGGERED OXIDATION OF RU-MODIFIED CYTOCHROME P450 BM3

The charge separation achieved by the bimolecular ET increases the lifetime of the reactive photosensitizer and provides additional driving force. However, the approach of using [Ru(bpy)3]2+ in solution was unsuccessful with P450 due to the deep burial of the heme active site within the polypeptide matrix of the enzyme. Upon irradiation (eg, laser flash-quench transient absorption studies), both of these quenchers caused Ru-P450 sample degradation as observed by permanent bleaching of P450 Soret (see Appendix C).

Therefore, all the studies described in this chapter use [Ru(NH3)6]Cl3 as an oxidation quencher. In addition, residue 97 is directly adjacent to Trp96, which lies within a hydrogen bond contact with one of the heme propionates. The mass of the conjugate (54,200 Da) corresponds to the apo (heme-free) mass of the unlabeled protein (53,520 Da) and the ruthenium photosensitizer (777 Da), minus the mass of the iodide.

Upon excitation with blue light (e.g. 480 nm), both samples show a broad band of luminescence in the red region of the visible spectrum with λmax = 620 nm. In addition, the structure of RuIIK97C-FeIIIP450 is more similar to that of the substrate-bound (closed) enzyme P450-BM3 (rmsd 0.44 Å for Cα with structure 2UWH22), in which helices F and G (known as the Ru-photosensitizer is well defined only in one monomer of the crystal structure, due to π-stacking of bipyridine ligands with aromatic residues on neighboring crystal units (Figure 2.9).

Glu 244 stacks below one of the bipyridine ligands, within 3.5 Å of the plane of the phenanthroline ligand.

Figure 2.2. [Ru II (bpy) 3 ] 2+ flash-quench and oxidation of the a heme protein active site

Laser flash-quench experiments

In the presence of the exogenous electron transfer quencher, [Ru(NH3)6]Cl3, the luminescence lifetimes decrease. The lifetimes appear to be more monoexponential in the presence of the quencher, and approximate rate constants are obtained using a monoexponential fit. In the absence of exogenous quencher, the TA traces of [Ru(bpy)2(Aphen)]2+ and RuIIK97C-FeIIIP450 after excitation (λex = 480 nm) are essentially identical.

The data at all wavelengths reveal bleaching (ΔAbs < 0) from 390–440 nm, consistent with the well-characterized behavior of Ru-diimine complexes excited by metal-to-ligand charge transfer (MLCT).17 Transient signals return to the base at the same rate as the luminescence decay, and we find no evidence for the formation of additional transient species (Figure 2.13). TA analysis of the free photosensitizer [Ru(bpy) 2 (Aphen)] 2+ in the presence of the quencher allows us to identify the transient features associated with the RuIII photosensitizer species. TA data at all wavelengths examined are characterized by biphasic recovery bleaching; attributed to the first phase of the kinetics.

Quenching of RuIIK97C-FeIIIP450 in the presence of quencher reveals significantly more complex kinetic behavior, indicating the presence of multiple intermediates. These features are distinct from the [RuII(bpy)2(IAphen)]3+ data in both the time scale and wavelength profile and suggest oxidation of the active site heme. The last TA feature is significantly affected by buffer pH, in the pH range of 6-8; the amplitude of the 440 nm feature is larger at high pH (Figure 2.16).

To determine the number of kinetic phases, and therefore the number of possible intermediates formed by quenching, we performed a generalized generalized singular value decomposition (tgSVD) analysis of the TA data (Treatment Tools, Per Christian Hansen,25 see sample script in Appendix D) (Figure 2.17). The tgSVD plot shows the magnitude (y-axis) of the contribution of each rate constant k (x-axis) to the overall fit of the transient absorption data. Clustering of rate constants into five clusters indicates that up to five distinct kinetic phases contribute to the recovery of TA signals to baseline.

The position of each of the clusters also provides a first-order estimate of the rate constant for each kinetic phase. Starting with these rate constants, we performed a global least-squares fit of the TA data recorded at six wavelengths (nm) to the sum of five exponentials with amplitude coefficients ρ1-5 and observed rate constants γ1-5 (equation 1.1) (see sample script for adaptation in Appendix D). Based on our interpretation of the nature and decay rate of the first transient absorption signal (see below), we can determine the first observed rate constant as that for luminescence quenching (obtained by monoexponential fitting of the luminescence decay at 630 nm).

The remaining four rate constants are extracted from the global fit (Figure 2.18) and are listed in Table 2.2. Observed rate constants (γ1-5, s-1) taken from global fitting of single wavelength TA at six wavelengths (390-440 nm).

Figure 2.10. Time resolved 630 nm luminescence decays in the absence of quencher

Discussion

Kinetics Model

This method is used to extract unscaled difference spectra for each transient species (Figure 2.20, see Appendix C for details). Values ​​for k1, k3, k5 and Keq = k6/k−6 must be specified to determine molar difference spectra for the six intermediate species. The balance between k5 and k7 has no effect on the relative difference spectra extracted from the data, so k5 was set equal to k7.

The equilibrium constant Keq was optimized to provide the best agreement between transient difference spectra recorded at three different pH values ​​(Figure 2.20). The difference spectra of *RuIIK97C-FeIIIP450 and RuIIIK97C-FeIIIP450 derived from kinetic analysis show bleaching at 430 nm of the Ru2+ MLCT absorption band. As discussed in Chapter 1, flash oxidation of horseradish peroxidase (HRP) and microperoxidase-8 (MP-8) first proceeds by transient oxidation of the porphyrin ring (FeIII–OH2(P+); internal rearrangement and subsequent deprotonation led to the ferryl, FeIV=O(P), product (CII).

The initial porphyrin radical in HRP and MP-8 is also characterized by a bleaching agent of the heme Soret.15,16 The blue shift in absorption for these porphyrin radical intermediates is also consistent with synthetic models of Fe(III) porphyrin cation radicals. 26. The spectra of P450OX1 and P450OX2 are quite similar, and are also characterized by a bleaching of the Soret absorption band (centered at 420 nm). Arguing by analogy with our results on the oxidation of HRP and MP8, and the similarities of their difference spectra, we propose that P450OX1 and P450OX2 correspond to six-coordinate porphyrin radical cations: RuIIK97C-FeIII(OH2)P+(A)P450 and RuIIK97C-FeIII(OH2)P+(B)P450.

Furthermore, the difference spectrum for this species indicates a red-shifted Soret absorption band analogous to that reported for the FeIV(OH)P center in CPO CII,27 as well as photochemically generated CII in HRP and MP-8. Internal charge transfer in FeIII(OH2)P+(B)P450 is accompanied by rapid loss of a proton (possibly to water), producing FeIV(OH)PP450. The formation of flash-quench-generated CII in HRP was slower (kobs of 4.1 s−1) due to rate-limiting water ligation.

CII formation in P450 proceeds on the millisecond time scale because a water molecule already occupies the sixth coordination site of the ferric heme. The specific rate of FeIII(OH2)P+(A)P450 formation in our conjugate is comparable to that found for reconstituted myoglobin containing a heme attached directly to Ru(diimine)32+.28 This observation suggests that a favorable pathway is the P450 porphyrin to RuIII, possibly involving the Trp96 heme propionate hydrogen bond.29 The conversion of RuIIK97C-FeIII(OH2)P+(A)P450 to RuIIK97C- FeIII(OH2)P+( B)P450 can be a result of changes in polypeptide or solvent conformation in the P450 heme pocket.30. We modeled this process as recovery of both the ferryl species (k7, Ru2+K97C-FeIV(OH)PP450) and its porphyrin radical cation precursor (k5, RuIIK97C- FeIII(OH2)P+(B)P450) (Fig. 4 ), but it is not possible to determine the two rate constants since equilibration between RuIIK97C-FeIII(OH2)P+(B)P450 and RuIIK97C- FeIV(OH)PP450 is faster than the ground state recovery process.

Figure 2.20. Extracted difference spectra of intermediate species. Top: Ru-based intermediates

Concluding Remarks

Acknowledgments

Materials and Methods Chemicals

• Ru photosensitizer Synthesis
• Mutagenesis and expression of P450-BM3 mutants Plasmid
• Ru-P450 conjugation
• Crystallization and structure determination
• Preparation of laser samples

Solid product (5-iodacetamido-1,10-phenanthroline, (IAphen)) was refluxed with Ru(bpy) 2 Cl 2 in methanol for 3 h. The photosensitizer was characterized by nuclear magnetic resonance (NMR), steady-state luminescence, and transient luminescence and absorption. The recombinant P450-BM3 heme domain, consisting of the first 463 residues with an N-terminal 6-histidine tag, was obtained with permission from Andrew Udit (Occidental College, Los Angeles, CA) in the pCWori+ vector, which also contains genes for ampicillin resistance and IPTG induction.

At an OD (600 nm) ~ 1, the temperature was lowered to 30 °C, expression was induced with isopropyl β-D-thiogalactopyranoside (IPTG), and α-aminolevulenic acid was added. P450 enzymes were extracted by sonication in the presence of protease inhibitors (benzamidine hydrochloride and Pefabloc SC hydrochloride). After centrifugation, the supernatant was purified by nickel immobilized metal affinity chromatography on a cluster column.

Dithiothreitol (DTT) was added to protein not intended for immediate use, and samples were immediately frozen in liquid nitrogen and stored at. The reaction solution is gently shaken for ~4 h at 4 °C in the dark, followed by desalting to remove excess photosensitizer and purification of Ru-labeled and unlabeled enzymes by anion exchange chromatography on a MonoQ or HiPrepQ FPLC column . This conjugate was characterized by mass spectrometry, UV-Vis and luminescence (steady-state, time-resolved) spectroscopies and X-ray crystallography.

The conjugate RuIIK97C-FeIIIP450 demonstrates activity in the hydroxylation of lauric acid via the peroxide shunt.31. Crystals formed over a period of 2 days at 4 °C, and were flash-frozen directly from the crystallization solution. Initial model for the RuIIK97C-FeIIIP450 structure was derived from the palmitic acid-bound P450-BM3 structure (pdb ID 2UWH) by molecular replacement using Molrep.

Atomic coordinates and structure factors have been deposited in the Protein Data Bank under the entry 3NPL. Samples were placed in a high-vacuum square quartz cuvette equipped with a small stir bar. For obtaining time-resolved fluorescence and transient absorption data, samples were excited with 8 ns laser pulses at 480 nm.

Direct observation of the iron-porphyrin cation radical as an intermediate in the phototriggered oxidation of iron to Ferryl-Heme bound to Ru(bpy)3 in reconstituted materials.