Tryptophan-to-heme electron transfer in ferrous myoglobins

It was recently demonstrated that in ferric myoglobins (Mb) the fluorescence quenching of the photoexcited tryptophan 14 (*Trp¹⁴) residue is in part due to an electron transfer to the heme porphyrin (porph), turning it to the ferrous state. However, the invariance of *Trp decay times in ferric and ferrous Mbs raises the question as to whether electron transfer may also be operative in the latter. Using UV pump/visible probe transient absorption, we show that this is indeed the case for deoxy-Mb. We observe that the reduction generates (with a yield of about 30%) a low-valence Fe–porphyrin π Fe^II(porph^●−)] -anion radical, which we observe for the first time to our knowledge under physiological conditions. We suggest that the pathway for the electron transfer proceeds via the leucine 69 (Leu⁶⁹) and valine 68 (Val⁶⁸) residues. The results on ferric Mbs and the present ones highlight the generality of Trp–porphyrin electron transfer in heme proteins.Electron transfer plays a fundamental role in many biological systems (1–3) ranging from photosynthetic proteins (4) to iron–sulfur (5), copper (6), and heme (7, 8) proteins. It was demonstrated that electron transfer can be used to produce from heme proteins in situ drugs with antimalarial activity (9) and it might have a role in protein folding (2). In general, electron transfer in proteins can occur over long distances (>10 Å) by hopping through different residues, thus reducing the time that would be needed for a single step tunneling from the donor to the acceptor (10–12). Aromatic amino acids and Tryptophan (Trp) in particular can act as a relay in such processes (13–19). Trp also acts as a phototriggered electron donor, e.g., in DNA repair by photolyase (16–18) and in cryptochromes (20, 21). When no obvious electron acceptors are present, excited Trp or (*Trp) still displays shorter lifetimes than its nanosecond decay times in solution (22, 23). This is due to its strong tendency to act as an electron donor, undergoing electron transfer toward the protein’s backbone as in the case of apo-myoglobin mutants (24), small cyclic peptides (25), and human γ–d-crystallin (26). It is interesting to note that in wild-type horse heart (WT-HH) apo-myoglobin the fluorescence lifetime of the two *Trp residues was reported to be comparable to that in water (27), demonstrating the absence of deactivation mechanisms, either by energy or by electron transfer.The protein visible absorption spectrum is dominated by their cofactors, e.g., heme or flavins, whereas the UV absorption in the region between 250 nm and 300 nm is mainly due to the three aromatic amino acids, Trp, tyrosine (Tyr), and phenylalanine (Phe) (28), with Trp having the highest molar extinction coefficient. The high sensitivity of Trp to the local environment and the possibility to correlate it with its fluorescence response (28) have led to its widespread use as a local natural probe of protein structure and dynamics in time-resolved fluorescence resonance energy transfer (FRET) studies, and it has emerged as the “spectroscopic ruler” in such studies (28–30). FRET is mediated by dipole–dipole coupling between a donor *Trp and an acceptor molecule, and its rate is inversely proportional to the sixth power of the distance between them and to the relative orientation of their dipoles.Myoglobin (Mb) is a small heme protein composed of ∼150 residues (31) arranged in eight α-helices (from A to H) (SI Appendix, Fig. S7), whose biological function is to store molecular oxygen in muscles of vertebrates (32). This is accomplished by its prosthetic group: a Fe–Protoporphyrin IX complex bound to the protein structure via the proximal histidine (His⁹³) (SI Appendix, Fig. S7). Both ferric and ferrous hemes tend to bind small diatomic molecules (e.g., O₂, CO, NO, and CN) at the Fe site. Mb has two Trp residues that are situated in the α-helix A: Trp⁷ toward the solvent and Trp¹⁴ within the protein and closer to the heme (SI Appendix, Fig. S7) (33). Previous time-resolved fluorescence studies on various Mb complexes have reported decay times (SI Appendix, Table S1) of ∼120 ps and ∼20 ps, for *Trp⁷ and *Trp¹⁴, respectively (34–38). These decay times appear invariant with respect to the ligand and the oxidation state of the iron ion in the heme. They were attributed to *Trp-to-porphyrin energy transfer via FRET over different donor–acceptor distances (37, 38) the Trp⁷-Heme and Trp¹⁴-Heme center-to-center distances are 21.2 Å and 15.1 Å, respectively (33, 39) (SI Appendix, Fig. S7)]. We recently showed, using ultrafast 2D-UV and visible transient absorption (TA) spectroscopy, that in the ferric myoglobins (MbCN and MbH₂O) the relaxation pathway of *Trp¹⁴ involves not only a *Trp-to-heme FRET but also an electron transfer from the *Trp to the heme (40) in a ratio of approximately 60–40%. One can expect that due to its ferric character, the heme is a strong electron acceptor in these cases, and indeed our study showed the formation of an Fe^II heme.However, the invariance of *Trp decay times in ferric and ferrous Mbs (SI Appendix, Table S1) suggests that similar electron transfer processes may also occur in ferrous Mbs. In this event, questions arise as to (i) whether a formally Fe^I heme is formed, which has to date been observed only in cryo-radiolysis experiments (41, 42), or (ii) whether the electron localizes on the porphyrin ring or even on the ligand that binds to the Fe ion. Theoretical investigations have suggested that an iron porphyrin anion radical can be formed (43–45).To address these questions, here we present a UV-pump/visible-probe TA study of ferrous Mbs. In the latter case with apical diatomic ligands, e.g., MbNO and MbCO, heme photoexcitation leads to dissociation of the ligand, followed by its recombination to the heme, which can be both geminate (the ligand stays inside the protein scaffold) and nongeminate (the ligand migrates out of the protein scaffold) (46–48). For the NO ligand, recombination timescales are typically ∼10 ps, ∼30 ps, and ∼200 ps (46, 47), whereas for CO they span up to the millisecond range (46, 49–51). The presence of recombination timescales in the order of *Trp decay times leads to additional signal contributions, which complicate the analysis of the data. These problems are avoided using deoxy-Mb, which has a penta-coordinated heme bound only to the His⁹³. Upon heme photoexcitation, the system recovers to the ground state within a few picoseconds (46, 52). This allows investigating the *Trp–heme interaction without any overlapping contributions.We show here that just as in the ferric Mbs (40), also in deoxy-Mb does *Trp¹⁴ partly decay to the heme by electron transfer, competing with the FRET pathway. We find that the transferred electron is localized on the porphyrin ring, contrary to the ferric case where it resides on the metal center. This is due to the highly negative reduction potential of the Fe^II/Fe^I couple (53, 54), which is close to the porphyrin reduction potential (55). To our knowledge, this is the first report of a low-valent myoglobin, under physiological conditions.The experimental setup, the sample preparation, and the data analysis are described in SI Appendix.