Relationship between oxygen affinity and autoxidation of myoglobin

Redox Engineering of Myoglobin by Cofactor Substitution to Enhance Cyclopropanation Reactivity

Design of metal cofactor ligands is essential for controlling the reactivity of metalloenzymes. We investigated a carbene transfer reaction catalyzed by myoglobins containing iron porphyrin cofactors with one and two trifluoromethyl groups at peripheral sites (FePorCF3 and FePor(CF3)2, respectively), native heme and iron porphycene (FePc). These four myoglobins show a wide range of Fe(II)/Fe(III) redox potentials in the protein of +147 mV, +87 mV, +42 mV and -198 mV vs. NHE, respectively. Myoglobin reconstituted with FePor(CF3)2 has a more positive potential, which enhances the reactivity of a carbene intermediate with alkenes, and demonstrates superior cyclopropanation of inert alkenes, such as aliphatic and internal alkenes. In contrast, engineered myoglobin reconstituted with FePc has a more negative redox potential, which accelerates the formation of the intermediate, but has low reactivity for inert alkenes. Mechanistic studies indicate that myoglobin with FePor(CF3)2 generates an undetectable active intermediate with a radical character. In contrast, this reaction catalyzed by myoglobin with FePc includes a detectable iron-carbene species with electrophilic character. This finding highlights the importance of redox-focused design of the iron porphyrinoid cofactor in hemoproteins to tune the reactivity of the carbene transfer reaction.

An Engineered Glutamate in Biosynthetic Models of Heme-Copper Oxidases Drives Complete Product Selectivity by Tuning the Hydrogen-Bonding Network

Efficiently carrying out the oxygen reduction reaction (ORR) is critical for many applications in biology and chemistry, such as bioenergetics and fuel cells, respectively. In biology, this reaction is carried out by large, transmembrane oxidases such as heme-copper oxidases (HCOs) and cytochrome bd oxidases. Common to these oxidases is the presence of a glutamate residue next to the active site, but its precise role in regulating the oxidase activity remains unclear. To gain insight into its role, we herein report that incorporation of glutamate next to a designed heme-copper center in two biosynthetic models of HCOs improves O2 binding affinity, facilitates protonation of reaction intermediates, and eliminates release of reactive oxygen species. High-resolution crystal structures of the models revealed extended, water-mediated hydrogen-bonding networks involving the glutamate. Electron paramagnetic resonance of the cryoreduced oxy-ferrous centers at cryogenic temperature followed by thermal annealing allowed observation of the key hydroperoxo intermediate that can be attributed to the hydrogen-bonding network. By demonstrating these important roles of glutamate in oxygen reduction biochemistry, this work offers deeper insights into its role in native oxidases, which may guide the design of more efficient artificial ORR enzymes or catalysts for applications such as fuel cells.

Effect of the Electron Density of the Heme Fe Atom on the Nature of Fe-O2 Bonding in Oxy Myoglobin

Mössbauer spectroscopy has been used to characterize oxygenated myoglobins (oxy Mbs) reconstituted with native and chemically modified ⁵⁷Fe-enriched heme cofactors with different electron densities of the heme Fe atom (ρ_Fe) and to elucidate the effect of a change in the ρ_Fe on the nature of the bond between heme Fe and oxygen (O₂), i.e., the Fe-O₂ bond, in the protein. Quadrupole splitting (ΔE_Q) was found to decrease with decreasing ρ_Fe, and the observed ρ_Fe-dependent ΔE_Q confirmed an increase in the contribution of the ferric-superoxide (Fe³⁺-O₂^-) form to the resonance hybrid of the Fe-O₂ fragment with decreasing ρ_Fe. These observations explicitly accounted for the lowering of O₂ affinity of the protein due to an increase in the O₂ dissociation rate and a decrease in the autoxidation reaction rate of oxy Mb through decreasing H⁺ affinity of the bound ligand with decreasing ρ_Fe. Therefore, the present study demonstrated the mechanism underlying the electronic control of O₂ affinity and the autoxidation of the protein through the heme electronic structure. Carbon monoxide (CO) adducts of reconstituted Mbs (CO-Mbs) were similarly characterized, and we found that the resonance between the two canonical forms of the Fe-CO fragment was also affected by a change in ρ_Fe. Thus, the nature of the Fe-ligand bond in the protein was found to be affected by the ρ_Fe.

Structural and functional characterization of complexes between heme and dimeric parallel G-quadruplex DNAs

Heme has been receiving considerable interest as a prosthetic group of ribozymes and deoxyribozymes, because heme-bound nucleic acids exhibit peroxidase-like catalytic activities (Travascio, P., Li, Y., and Sen, D. (1998) Chem. Biol, 5, 505-517). The interaction of heme with dimeric G-quadruplexes formed from d(TAGGGTTAGGGT) and d(TAGGGTTAGGGA) has been characterized to gain a deeper understanding of the molecular recognition of G-quadruplex DNAs by heme. We found that heme binds selectively to the 3'-terminal G-quartet of a dimeric parallel G-quadruplex of d(TAGGGTTAGGGT), whereas binding of heme to a dimeric antiparallel G-quadruplex of d(TAGGGTTAGGGA) does not occur, suggesting that an orderly arrangement of the constituent guanine deoxyribose rings, with respect to the G-quartet plane, is crucial for the binding of heme to the DNA. The preferential binding of heme to the 3'-terminal G-quartet of parallel G-quadruplex DNAs allowed a systematic modification of the heme environment in the complex through the DNA sequence. The activity of the complexes was found to increase with increasing number of adenine bases adjacent to the heme in the complexes, possibly due to improvement of the accessibility of aromatic substrate, i.e., 10-acetyl-3,7-dihydroxyphenoxazine, to the heme, and an increase in the frequency of appearance of a specific orientation of the adenine bases, with respect to the heme, optimized for its activity as an acid-base catalyst to enhance the peroxidase activity of the complex.

Intra-cardiac Side-Firing Light Catheter for Monitoring Cellular Metabolism using Transmural Absorbance Spectroscopy of Perfused Mammalian Hearts

Absorbance spectroscopy of cardiac muscle provides non-destructive assessment of cytosolic and mitochondrial oxygenation via myoglobin and cytochrome absorbance respectively. In addition, numerous aspects of the mitochondrial metabolic status such as membrane potential and substrate entry can also be estimated. To perform cardiac wall transmission optical spectroscopy, a commercially available side-firing optical fiber catheter is placed in the left ventricle of the isolated perfused heart as a light source. Light passing through the heart wall is collected with an external optical fiber to perform optical spectroscopy of the heart in near real- time. The transmission approach avoids numerous surface scattering interference occurring in widely used reflection approaches. Changes in transmural absorbance spectra were deconvolved using a library of chromophore reference spectra, providing quantitative measures of all the known cardiac chromophores simultaneously. This spectral deconvolution approach eliminated intrinsic errors that may result from using common dual wavelength methods applied to overlapping absorbance spectra, as well as provided a quantitative evaluation of the goodness of fit. A custom program was designed for data acquisition and analysis, which permitted the investigator to monitor the metabolic state of the preparation during the experiment. These relatively simple additions to the standard heart perfusion system provide a unique insight into the metabolic state of the heart wall in addition to conventional measures of contraction, perfusion, and substrate/oxygen extraction.

Structures and Catalytic Activities of Complexes between Heme and All Parallel-Stranded Monomeric G-Quadruplex DNAs

Heme in its ferrous and ferric states [heme(Fe²⁺) and heme(Fe³⁺), respectively] binds selectively to the 3'-terminal G-quartet of all parallel-stranded monomeric G-quadruplex DNAs formed from inosine(I)-containing sequences, i.e., d(TAGGGTGGGTTGGGTGIG) DNA(18mer) and d(TAGGGTGGGTTGGGTGIGA) DNA(18mer/A), through a π-π stacking interaction between the porphyrin moiety of the heme and the G-quartet, to form 1:1 complexes [heme-DNA(18mer) and heme-DNA(18mer/A) complexes, respectively]. These complexes exhibited enhanced peroxidase activities, compared with that of heme(Fe³⁺) alone, and the activity of the heme(Fe³⁺)-DNA(18mer/A) complex was greater than that of the heme(Fe³⁺)-DNA(18mer) one, indicating that the 3'-terminal A of the DNA sequence acts as an acid-base catalyst that promotes the catalytic reaction. In the complexes, a water molecule (H₂O) at the interface between the heme and G-quartet is coordinated to the heme Fe atom as an axial ligand and possibly acts as an electron-donating ligand that promotes heterolytic peroxide bond cleavage of hydrogen peroxide bound to the heme Fe atom, trans to the H₂O, for the generation of an active species. The intermolecular nuclear Overhauser effects observed among heme, DNA, and Fe-bound H₂O indicated that the H₂O rotates about the H₂O-Fe coordination bond with respect to both the heme and DNA in the complex. Thus, the H₂O in the complex is unique in terms of not only its electronic properties but also its dynamic ones. These findings provide novel insights into the design of heme-deoxyribozymes and -ribozymes.

Characterization of Catalytic Activities and Heme Coordination Structures of Heme-DNA Complexes Composed of Some Chemically Modified Hemes and an All Parallel-Stranded Tetrameric G-Quadruplex DNA Formed from d(TTAGGG)

Heme binds selectively to the 3'-terminal G-quartet (G6 G-quartet) of an all parallel-stranded tetrameric G-quadruplex DNA, [d(TTAGGG)]₄, to form a heme-DNA complex. Complexes between [d(TTAGGG)]₄ and a series of chemically modified hemes possessing a heme Fe atom with a variety of electron densities were characterized in terms of their peroxidase activities to evaluate the effect of a change in the electron density of the heme Fe atom (ρ_Fe) on their activities. The peroxidase activity of a complex decreased with a decreasing ρ_Fe, supporting the idea that the activity of the complex is elicited through a reaction mechanism similar to that of a peroxidase. In the ferrous heme-DNA complex, carbon monoxide (CO) can bind to the heme Fe atom on the side of the heme opposite the G6 G-quartet, and a water molecule (H₂O) is coordinated to the Fe atom as another axial ligand, trans to the CO. The stretching frequencies of Fe-bound CO (ν_CO) and the Fe-C bond (ν_Fe-C) of CO adducts of the heme-DNA complexes were determined to investigate the structural and electronic natures of the axial ligands coordinated to the heme Fe atom. Comparison of the ν_CO and ν_Fe-C values of the heme-DNA complexes with those of myoglobin (Mb) revealed that the donor strength of the axial ligation trans to the CO in a complex is considerably weaker than that of the proximal histidine in Mb, as expected from the coordination of H₂O trans to the CO in the complex.

Comparison of the oxidative reactivity of recombinant fetal and adult human hemoglobin: implications for the design of hemoglobin-based oxygen carriers

Hemoglobin (Hb)-based oxygen carriers (HBOCs) have been engineered to replace or augment the oxygen carrying capacity of erythrocytes. However, clinical results have generally been disappointing, in part due to the intrinsic oxidative toxicity of Hb. The most common HBOC starting material is adult human or bovine Hb. However, it has been suggested that fetal Hb may offer advantages due to decreased oxidative reactivity. Large-scale manufacturing of HBOC will likely and ultimately require recombinant sources of human proteins. We, therefore, directly compared the functional properties and oxidative reactivity of recombinant fetal (rHbF) and recombinant adult (rHbA) Hb. rHbA and rHbF produced similar yields of purified functional protein. No differences were seen in the two proteins in: autoxidation rate; the rate of hydrogen peroxide reaction; NO scavenging dioxygenase activity; and the NO producing nitrite reductase activity. The rHbF protein was: less damaged by low levels of hydrogen peroxide; less damaging when added to human umbilical vein endothelial cells (HUVEC) in the ferric form; and had a slower rate of intrinsic heme loss. The rHbA protein was: more readily reducible by plasma antioxidants such as ascorbate in both the reactive ferryl and ferric states; less readily damaged by lipid peroxides; and less damaging to phosphatidylcholine liposomes. In conclusion in terms of oxidative reactivity, there are advantages and disadvantages to the use of rHbA or rHbF as the basis for an effective HBOC.

Intracardiac light catheter for rapid scanning transmural absorbance spectroscopy of perfused myocardium: measurement of myoglobin oxygenation and mitochondria redox state

Absorbance spectroscopy of intrinsic cardiac chromophores provides nondestructive assessment of cytosolic oxygenation and mitochondria redox state. Isolated perfused heart spectroscopy is usually conducted by collecting reflected light from the heart surface, which represents a combination of surface scattering events and light that traversed portions of the myocardium. Reflectance spectroscopy with complex surface scattering effects in the beating heart leads to difficulty in quantitating chromophore absorbance. In this study, surface scattering was minimized and transmural path length optimized by placing a light source within the left ventricular chamber while monitoring transmurally transmitted light at the epicardial surface. The custom-designed intrachamber light catheter was a flexible coaxial cable (2.42-Fr) terminated with an encapsulated side-firing LED of 1.8 × 0.8 mm, altogether similar in size to a Millar pressure catheter. The LED catheter had minimal impact on aortic flow and heart rate in Langendorff perfusion and did not impact stability of the left ventricule of the working heart. Changes in transmural absorbance spectra were deconvoluted using a library of chromophore reference spectra to quantify the relative contribution of specific chromophores to the changes in measured absorbance. This broad-band spectral deconvolution approach eliminated errors that may result from simple dual-wavelength absorbance intensity. The myoglobin oxygenation level was only 82.2 ± 3.0%, whereas cytochrome c and cytochrome a + a₃ were 13.3 ± 1.4% and 12.6 ± 2.2% reduced, respectively, in the Langendorff-perfused heart. The intracardiac illumination strategy permits transmural optical absorbance spectroscopy in perfused hearts, which provides a noninvasive real-time monitor of cytosolic oxygenation and mitochondria redox state.NEW & NOTEWORTHY Here, a novel nondestructive real-time approach for monitoring intrinsic indicators of cardiac metabolism and oxygenation is described using a catheter-based transillumination of the left ventricular free wall together with complete spectral analysis of transmitted light. This approach is a significant improvement in the quality of cardiac optical absorbance spectroscopic metabolic analyses.

Influence of heme environment structure on dioxygen affinity for the dual function Amphitrite ornata hemoglobin/dehaloperoxidase. Insights into the evolutional structure-function adaptations

Sea worm, Amphitrite ornata, has evolved its globin (an O(2) carrier) also to serves as a dehaloperoxidase (DHP) to detoxify haloaromatic pollutants generated by competing species. A previous mutagenesis study by our groups on both DHP and sperm whale myoglobin (SW Mb) revealed some structural factors that influence the dehaloperoxidase activities (significantly lower for Mb) of both proteins. Using an isocyanide/O(2) partition constant measurement method in this study, we have examined the effects of these structural factors on the O(2) equilibrium constants (KO2) of DHP, SW Mb, and their mutants. A clear trend of decreasing O(2) affinity and increasing catalytic activity along with the increase in the distal His N(ε)-heme iron distance is observed. An H93K/T95H Mb double mutant mimicking the DHP proximal His positioning exhibited markedly enhanced O(2) affinity, confirming the essential effect of proximal His rotation on the globin function of DHP. For DHP, the L100F, T56G and M86E variants showed the effects of distal volume, distal His flexibility and proximal electronic push, respectively, on the O(2) affinity. This study provides insights into how DHP has evolved its heme environment to gain significantly enhanced peroxidase capability without compromising its primary function as an O(2) carrier.

Incorporation of modified and artificial cofactors into naturally occurring protein scaffolds

As a possible modification of cofactor-containing proteins, cofactor-substitution typically leads to drastic changes of protein function. In particular heme, a porphyrin iron complex, is a representative, replaceable cofactor for this methodology and numerous cofactor-modified hemoproteins (reconstituted hemoproteins) have been prepared with the goal of elucidating their operational mechanism and/or engineering the protein function. In a series of hemoproteins, myoglobin, an oxygen storage hemoprotein, is one of the most rewarding scaffolds to generate a modified protein with an artificial cofactor. In this chapter, we describe practical procedures for the preparation of apomyoglobin and incorporation of zinc porphyrin as an artificial cofactor. Furthermore, we discuss the methodology to characterize the obtained cofactor-substituted proteins and the design of several artificial cofactors.

Constructing a man-made c-type cytochrome maquette in vivo: electron transfer, oxygen transport and conversion to a photoactive light harvesting maquette

The successful use of man-made proteins to advance synthetic biology requires both the fabrication of functional artificial proteins in a living environment, and the ability of these proteins to interact productively with other proteins and substrates in that environment. Proteins made by the maquette method integrate sophisticated oxidoreductase function into evolutionarily naive, non-computationally designed protein constructs with sequences that are entirely unrelated to any natural protein. Nevertheless, we show here that we can efficiently interface with the natural cellular machinery that covalently incorporates heme into natural cytochromes c to produce in vivo an artificial c-type cytochrome maquette. Furthermore, this c-type cytochrome maquette is designed with a displaceable histidine heme ligand that opens to allow functional oxygen binding, the primary event in more sophisticated functions ranging from oxygen storage and transport to catalytic hydroxylation. To exploit the range of functions that comes from the freedom to bind a variety of redox cofactors within a single maquette framework, this c-type cytochrome maquette is designed with a second, non-heme C, tetrapyrrole binding site, enabling the construction of an elementary electron transport chain, and when the heme C iron is replaced with zinc to create a Zn porphyrin, a light-activatable artificial redox protein. The work we describe here represents a major advance in de novo protein design, offering a robust platform for new c-type heme based oxidoreductase designs and an equally important proof-of-principle that cofactor-equipped man-made proteins can be expressed in living cells, paving the way for constructing functionally useful man-made proteins in vivo.

The globins of cyanobacteria and algae

Approximately, 20 years ago, a haemoglobin gene was identified within the genome of the cyanobacterium Nostoc commune. Haemoglobins have now been confirmed in multiple species of photosynthetic microbes beyond N. commune, and the diversity of these proteins has recently come under increased scrutiny. This chapter summarizes the state of knowledge concerning the phylogeny, physiology and chemistry of globins in cyanobacteria and green algae. Sequence information is by far the best developed and the most rapidly expanding aspect of the field. Structural and ligand-binding properties have been described for just a few proteins. Physiological data are available for even fewer. Although activities such as nitric oxide dioxygenation and oxygen scavenging are strong candidates for cellular function, dedicated studies will be required to complete the story on this intriguing and ancient group of proteins.