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Geeraerts Z, Celis AI, Mayfield JA, Lorenz M, Rodgers KR, DuBois JL, Lukat-Rodgers GS. Distinguishing Active Site Characteristics of Chlorite Dismutases with Their Cyanide Complexes. Biochemistry 2018; 57:1501-1516. [PMID: 29406727 PMCID: PMC5849076 DOI: 10.1021/acs.biochem.7b01278] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
O2-evolving chlorite dismutases (Clds) efficiently convert chlorite (ClO2-) to O2 and Cl-. Dechloromonas aromatica Cld ( DaCld) is a highly active chlorite-decomposing homopentameric enzyme, typical of Clds found in perchlorate- and chlorate-respiring bacteria. The Gram-negative, human pathogen Klebsiella pneumoniae contains a homodimeric Cld ( KpCld) that also decomposes ClO2-, albeit with an activity 10-fold lower and a turnover number lower than those of DaCld. The interactions between the distal pocket and heme ligand of the DaCld and KpCld active sites have been probed via kinetic, thermodynamic, and spectroscopic behaviors of their cyanide complexes for insight into active site characteristics that are deterministic for chlorite decomposition. At 4.7 × 10-9 M, the KD for the KpCld-CN- complex is 2 orders of magnitude smaller than that of DaCld-CN- and indicates an affinity for CN- that is greater than that of most heme proteins. The difference in CN- affinity between Kp- and DaClds is predominantly due to differences in koff. The kinetics of binding of cyanide to DaCld, DaCld(R183Q), and KpCld between pH 4 and 8.5 corroborate the importance of distal Arg183 and a p Ka of ∼7 in stabilizing complexes of anionic ligands, including the substrate. The Fe-C stretching and FeCN bending modes of the DaCld-CN- (νFe-C, 441 cm-1; δFeCN, 396 cm-1) and KpCld-CN- (νFe-C, 441 cm-1; δFeCN, 356 cm-1) complexes reveal differences in their FeCN angle, which suggest different distal pocket interactions with their bound cyanide. Conformational differences in their catalytic sites are also reported by the single ferrous KpCld carbonyl complex, which is in contrast to the two conformers observed for DaCld-CO.
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Affiliation(s)
- Zachary Geeraerts
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102, USA
| | - Arianna I. Celis
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, USA
| | - Jeffery A. Mayfield
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA
| | - Megan Lorenz
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102, USA
| | - Kenton R. Rodgers
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102, USA
| | - Jennifer L. DuBois
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, USA
| | - Gudrun S. Lukat-Rodgers
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102, USA
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pH dependence of cyanide and imidazole binding to the heme domains of Sinorhizobium meliloti and Bradyrhizobium japonicum FixL. J Inorg Biochem 2015; 153:88-102. [PMID: 26499393 DOI: 10.1016/j.jinorgbio.2015.10.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2015] [Revised: 09/25/2015] [Accepted: 10/05/2015] [Indexed: 01/07/2023]
Abstract
Equilibrium and kinetic properties of cyanide and imidazole binding to the heme domains of Sinorhizobium meliloti and Bradyrhizobium japonicum FixL (SmFixLH and BjFixLH) have been investigated between pH5 and 11. KD determinations were made at integral pH values, with the strongest binding at pH9 for both ligands. KD for the cyanide complexes of BjFixLH and SmFixLH is 0.15±0.09 and 0.50±0.20μM, respectively, and 0.70±0.01mM for imido-BjFixLH. The association rate constants are pH dependent with maximum values of 443±8 and 252±61M(-1)s(-1) for cyano complexes of BjFixLH and SmFixLH and (5.0±0.3)×10(4) and (7.0±1.4)×10(4)M(-1)s(-1) for the imidazole complexes. The dissociation rate constants are essentially independent of pH above pH5; (1.2±0.3)×10(-4) and (1.7±0.3)×10(-4)s(-1) for the cyano complexes of BjFixLH and SmFixLH, and (73±19) and (77±14) s(-1) for the imidazole complexes. Two ionizable groups in FixLH affect the rate of ligand binding. The more acidic group, identified as the heme 6 propionic acid, has a pKa of 7.6±0.2 in BjFixLH and 6.8±0.2 in SmFixLH. The second ionization is due to formation of hydroxy-FixLH with pKa values of 9.64±0.05 for BjFixLH and 9.61±0.05 for SmFixLH. Imidazole binding is limited by the rate of heme pocket opening with maximum observed values of 680 and 1270s(-1) for BjFixLH and SmFixLH, respectively.
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Shimizu T, Huang D, Yan F, Stranava M, Bartosova M, Fojtíková V, Martínková M. Gaseous O2, NO, and CO in signal transduction: structure and function relationships of heme-based gas sensors and heme-redox sensors. Chem Rev 2015; 115:6491-533. [PMID: 26021768 DOI: 10.1021/acs.chemrev.5b00018] [Citation(s) in RCA: 131] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- Toru Shimizu
- †Department of Cell Biology and Genetics and Key Laboratory of Molecular Biology in High Cancer Incidence Coastal Chaoshan Area of Guangdong Higher Education Institutes, Shantou University Medical College, Shantou, Guangdong 515041, China
- ‡Department of Biochemistry, Faculty of Science, Charles University in Prague, Prague 2 128 43, Czech Republic
- §Research Center for Compact Chemical System, National Institute of Advanced Industrial Science and Technology (AIST), Sendai 983-8551, Japan
| | - Dongyang Huang
- †Department of Cell Biology and Genetics and Key Laboratory of Molecular Biology in High Cancer Incidence Coastal Chaoshan Area of Guangdong Higher Education Institutes, Shantou University Medical College, Shantou, Guangdong 515041, China
| | - Fang Yan
- †Department of Cell Biology and Genetics and Key Laboratory of Molecular Biology in High Cancer Incidence Coastal Chaoshan Area of Guangdong Higher Education Institutes, Shantou University Medical College, Shantou, Guangdong 515041, China
| | - Martin Stranava
- ‡Department of Biochemistry, Faculty of Science, Charles University in Prague, Prague 2 128 43, Czech Republic
| | - Martina Bartosova
- ‡Department of Biochemistry, Faculty of Science, Charles University in Prague, Prague 2 128 43, Czech Republic
| | - Veronika Fojtíková
- ‡Department of Biochemistry, Faculty of Science, Charles University in Prague, Prague 2 128 43, Czech Republic
| | - Markéta Martínková
- ‡Department of Biochemistry, Faculty of Science, Charles University in Prague, Prague 2 128 43, Czech Republic
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Shimizu T. The Heme-Based Oxygen-Sensor Phosphodiesterase Ec DOS (DosP): Structure-Function Relationships. BIOSENSORS 2013; 3:211-37. [PMID: 25586128 PMCID: PMC4263535 DOI: 10.3390/bios3020211] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/24/2013] [Revised: 05/24/2013] [Accepted: 06/13/2013] [Indexed: 02/05/2023]
Abstract
Escherichia coli Direct Oxygen Sensor (Ec DOS, also known as Ec DosP) is a heme-based O2-sensing phosphodiesterase from Escherichia coli that catalyzes the conversion of cyclic-di-GMP to linear di-GMP. Cyclic-di-GMP is an important second messenger in bacteria, highlighting the importance of understanding structure-function relationships of Ec DOS. Ec DOS is composed of an N-terminal heme-bound O2-sensing PAS domain and a C-terminal phosphodiesterase catalytic domain. Notably, its activity is markedly enhanced by O2 binding to the heme Fe(II) complex in the PAS sensor domain. X-ray crystal structures and spectroscopic and catalytic characterization of the wild-type and mutant proteins have provided important structural and functional clues to understanding the molecular mechanism of intramolecular catalytic regulation by O2 binding. This review summarizes the intriguing findings that have obtained for Ec DOS.
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Affiliation(s)
- Toru Shimizu
- Department of Cell Biology, Shantou University Medical College, Shantou 515041, China.
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Abstract
Formation of protein-ligand complexes causes various changes in both the receptor and the ligand. This review focuses on changes in pK and protonation states of ionizable groups that accompany protein-ligand binding. Physical origins of these effects are outlined, followed by a brief overview of the computational methods to predict them and the associated corrections to receptor-ligand binding affinities. Statistical prevalence, magnitude and spatial distribution of the pK and protonation state changes in protein-ligand binding are discussed in detail, based on both experimental and theoretical studies. While there is no doubt that these changes occur, they do not occur all the time; the estimated prevalence varies, both between individual complexes and by method. The changes occur not only in the immediate vicinity of the interface but also sometimes far away. When receptor-ligand binding is associated with protonation state change at particular pH, the binding becomes pH dependent: we review the interplay between sub-cellular characteristic pH and optimum pH of receptor-ligand binding. It is pointed out that there is a tendency for protonation state changes upon binding to be minimal at physiologically relevant pH for each complex (no net proton uptake/release), suggesting that native receptor-ligand interactions have evolved to reduce the energy cost associated with ionization changes. As a result, previously reported statistical prevalence of these changes - typically computed at the same pH for all complexes - may be higher than what may be expected at optimum pH specific to each complex. We also discuss whether proper account of protonation state changes appears to improve practical docking and scoring outcomes relevant to structure-based drug design. An overview of some of the existing challenges in the field is provided in conclusion.
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Affiliation(s)
- Alexey V Onufriev
- Department of Computer Science and Physics, 2050 Torgersen Hall, Virginia Tech, Blacksburg, VA 24061, USA.
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Su Y, Majtan T, Freeman KM, Linck R, Ponter S, Kraus JP, Burstyn JN. Comparative study of enzyme activity and heme reactivity in Drosophila melanogaster and Homo sapiens cystathionine β-synthases. Biochemistry 2013; 52:741-51. [PMID: 23002992 PMCID: PMC3751582 DOI: 10.1021/bi300615c] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Cystathionine β-synthase (CBS) is the first and rate-limiting enzyme in the transsulfuration pathway, which is critical for the synthesis of cysteine from methionine in eukaryotes. CBS uses coenzyme pyridoxal 5'-phosphate (PLP) for catalysis, and S-adenosylmethionine regulates the activity of human CBS, but not yeast CBS. Human and fruit fly CBS contain heme; however, the role for heme is not clear. This paper reports biochemical and spectroscopic characterization of CBS from fruit fly Drosophila melanogaster (DmCBS) and the CO/NO gas binding reactions of DmCBS and human CBS. Like CBS enzymes from lower organisms (e.g., yeast), DmCBS is intrinsically highly active and is not regulated by AdoMet. The DmCBS heme coordination environment, the reactivity, and the accompanying effects on enzyme activity are similar to those of human CBS. The DmCBS heme bears histidine and cysteine axial ligands, and the enzyme becomes inactive when the cysteine ligand is replaced. The Fe(II) heme in DmCBS is less stable than that in human CBS, undergoing more facile reoxidation and ligand exchange. In both CBS proteins, the overall stability of the protein is correlated with the heme oxidation state. Human and DmCBS Fe(II) hemes react relatively slowly with CO and NO, and the rate of the CO binding reaction is faster at low pH than at high pH. Together, the results suggest that heme incorporation and AdoMet regulation in CBS are not correlated, possibly providing two independent means for regulating the enzyme.
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Affiliation(s)
- Yang Su
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin, 53706 USA
| | - Tomas Majtan
- Department of Pediatrics, University of Colorado, Denver, Aurora, Colorado 80045
- Department of Genomics & Biotechnology, Institute of Molecular Biology SAS, Dubravska cesta 21, Bratislava, 84551, Slovakia
| | - Katherine M. Freeman
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin, 53706 USA
| | - Rachel Linck
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin, 53706 USA
| | - Sarah Ponter
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin, 53706 USA
| | - Jan P. Kraus
- Department of Pediatrics, University of Colorado, Denver, Aurora, Colorado 80045
| | - Judith N. Burstyn
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin, 53706 USA
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Wang L, Witham S, Zhang Z, Li L, Hodsdon ME, Alexov E. In silico investigation of pH-dependence of prolactin and human growth hormone binding to human prolactin receptor. COMMUNICATIONS IN COMPUTATIONAL PHYSICS 2013; 13:207-222. [PMID: 24683423 PMCID: PMC3966486 DOI: 10.4208/cicp.170911.131011s] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Experimental data shows that the binding of human prolactin (hPRL) to human prolactin receptor (hPRLr-ECD) is strongly pH-dependent, while the binding of the same receptor to human growth hormone (hGH) is pH-independent. Here we carry in silico analysis of the molecular effects causing such a difference and reveal the role of individual amino acids. It is shown that the computational modeling correctly predicts experimentally determined pKa's of histidine residues in an unbound state in the majority of the cases and the pH-dependence of the binding free energy. Structural analysis carried in conjunction with calculated pH-dependence of the binding revealed that the main reason for pH-dependence of the binding of hPRL-hPRLr-ECD is a number of salt- bridges across the interface of the complex, while no salt-bridges are formed in the hGH-hPRlr-ECD. Specifically, most of the salt-bridges involve histidine residues and this is the reason for the pH-dependence across a physiological range of pH. The analysis not only revealed the molecular mechanism of the pH-dependence of the hPRL-hPRLr-ECD, but also provided critical insight into the underlying physic-chemical mechanism.
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Affiliation(s)
- Lin Wang
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson, SC 29634
| | - Shawn Witham
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson, SC 29634
| | - Zhe Zhang
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson, SC 29634
| | - Lin Li
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson, SC 29634
| | - Michael E. Hodsdon
- Department of Laboratory Medicine and the Department of Pharmacology, Yale School of Medicine, New Haven, Connecticut 06520
| | - Emil Alexov
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson, SC 29634
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Chen J, Zhao L, Bai H, Shi G. Electrochemical detection of dioxygen and hydrogen peroxide by hemin immobilized on chemically converted graphene. J Electroanal Chem (Lausanne) 2011. [DOI: 10.1016/j.jelechem.2011.03.005] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
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Abstract
The role of electrostatics in protein-protein interactions and binding is reviewed in this paper. A brief outline of the computational modeling, in the framework of continuum electrostatics, is presented and the basic electrostatic effects occurring upon the formation of the complex are discussed. The effect of the salt concentration and pH of the water phase on protein-protein binding free energy is demonstrated which indicates that the increase of the salt concentration tends to weaken the binding, an observation that is attributed to the optimization of the charge-charge interactions across the interface. It is pointed out that the pH-optimum (pH of optimal binding affinity) varies among the protein-protein complexes, and perhaps is a result of their adaptation to particular subcellular compartments. The similarities and differences between hetero- and homo-complexes are outlined and discussed with respect to the binding mode and charge complementarity.
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Affiliation(s)
- Zhe Zhang
- Computational Biophysics and Bioinformatics, Department of Physics, Clemson University, Clemson,SC 29634, USA
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Mitra RC, Zhang Z, Alexov E. In silico modeling of pH-optimum of protein-protein binding. Proteins 2010; 79:925-36. [PMID: 21287623 DOI: 10.1002/prot.22931] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2010] [Revised: 10/12/2010] [Accepted: 10/29/2010] [Indexed: 01/05/2023]
Abstract
Protein-protein association is a pH-dependent process and thus the binding affinity depends on the local pH. In vivo the association occurs in a particular cellular compartment, where the individual monomers are supposed to meet and form a complex. Since the monomers and the complex exist in the same micro environment, it is plausible that they coevolved toward its properties, in particular, toward the characteristic subcellular pH. Here we show that the pH at which the monomers are most stable (pH-optimum) or the pH at which stability is almost pH-independent (pH-flat) of monomers are correlated with the pH-optimum of maximal affinity (pH-optimum of binding) or pH interval at which affinity is almost pH-independent (pH-flat of binding) of the complexes made of the corresponding monomers. The analysis of interfacial properties of protein complexes demonstrates that pH-dependent properties can be roughly estimated using the interface charge alone. In addition, we introduce a parameter beta, proportional to the square root of the absolute product of the net charges of monomers, and show that protein complexes characterized with small or very large beta tend to have neutral pH-optimum. Further more, protein complexes made of monomers carrying the same polarity net charge at neutral pH have either very low or very high pH-optimum of binding. These findings are used to propose empirical rule for predicting pH-optimum of binding provided that the amino acid compositions of the corresponding monomers are available.
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Affiliation(s)
- Rooplekha C Mitra
- Physics Department, Computational Biophysics and Bioinformatics, Clemson University, Clemson, South Carolina 29634, USA
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Streit BR, Blanc B, Lukat-Rodgers GS, Rodgers KR, DuBois JL. How active-site protonation state influences the reactivity and ligation of the heme in chlorite dismutase. J Am Chem Soc 2010; 132:5711-24. [PMID: 20356038 PMCID: PMC3050645 DOI: 10.1021/ja9082182] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Chlorite dismutase catalyzes O(2) release from chlorite with exquisite efficiency and specificity. The spectroscopic properties, ligand binding affinities, and steady-state kinetics of chlorite dismutase from Dechloromonas aromatica were examined over pH 3-11.5 to gain insight into how the protonation state of the heme environment influences dioxygen formation. An acid-base transition was observed by UV/visible and resonance Raman (rR) spectroscopy with a pK(a) of 8.7, 2-3 pH units below analogous transitions observed in typical His-ligated peroxidases. This transition marks the conversion of a five-coordinate high-spin Fe(III) to a mixed high/low-spin ferric hydroxide, as confirmed by rR spectroscopy. The two Fe-OH stretching frequencies are quite low, consistent with a weak Fe-OH bond, despite the nearly neutral imidazole side chain of the proximal histidine ligand. The hydroxide is proposed to interact strongly with a distal H-bond donor, thereby weakening the Fe-OH bond. The rR spectra of Cld-CO as a function of pH reveal two forms of the complex, one in which there is minimal interaction of distal residues with the carbonyl oxygen and another, acidic form in which the oxygen is under the influence of positive charge. Recent crystallographic data reveal arginine 183 as the lone H-bond-donating residue in the distal pocket. It is likely that this Arg is the strong, positively charged H-bond donor implicated by vibrational data to interact with exogenous axial heme ligands. The same Arg in its neutral (pK(a) approximately 6.5) form also appears to act as the active-site base in binding reactions of protonated ligands, such as HCN, to ferric Cld. The steady-state profile for the rate of chlorite decomposition is characterized by these same pK(a) values. The five-coordinate high-spin acidic Cld is more active than the alkaline hydroxide-bound form. The acid form decomposes chlorite most efficiently when the distal Arg is protonated/cationic (maximum k(cat) = 2.0(+/-0.6) x 10(5) s(-1), k(cat)/K(M) = 3.2(+/-0.4) x 10(7) M(-1) s(-1), pH 5.2, 4 degrees C) and to a somewhat lesser extent when it acts as a H-bond donor to the axial hydroxide ligand under alkaline conditions.
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Affiliation(s)
- Bennett R. Streit
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
| | - Béatrice Blanc
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
| | - Gudrun S. Lukat-Rodgers
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050
| | - Kenton R. Rodgers
- Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050
| | - Jennifer L. DuBois
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
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Tanaka A, Shimizu T. Ligand binding to the Fe(III)-protoporphyrin IX complex of phosphodiesterase from Escherichia coli (Ec DOS) markedly enhances catalysis of cyclic di-GMP: roles of Met95, Arg97, and Phe113 of the putative heme distal side in catalytic regulation and ligand binding. Biochemistry 2009; 47:13438-46. [PMID: 19053256 DOI: 10.1021/bi8012017] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Phosphodiesterase (Ec DOS) from Escherichia coli is a gas-sensor enzyme in which binding of gas molecules, such as O(2), CO, and NO, to the Fe(II)-protoporphyrin IX complex in the sensor domain stimulates phosphodiesterase activity toward cyclic-di-GMP. In this study, we report that external axial ligands, such as cyanide or imidazole, bind to Fe(III)-protoporphyrin IX in the sensor domain and induce a 10- to 11-fold increase (from 8.1 up to 86 min(-1)) in catalysis, which is more substantial than that (6.3 to 7.2-fold) observed for other gas-stimulated Fe(II) heme-bound enzymes. Catalytic activity (50 min(-1)) of the heme-free mutant, H77A, was comparable to that of the ligand-stimulated enzymes. Accordingly, we propose that the heme at the sensor domain inhibits catalysis and that ligand binding to the heme iron complex releases this catalytic suppression. Furthermore, mutations of Met95, Arg97, and Phe113 at the putative heme distal side suppressed the ligand effects on catalysis. The rate constants (19,000 x 10(-5) microM(-1)min(-1)) for cyanide binding to the M95A and M95L mutants of the full-length enzyme were 633-fold higher than that to wild-type Ec DOS (30 x 10(-5) microM(-1)min(-1)). The absorption spectrum of the F113Y mutant suggests that the Tyr O(-) group directly coordinates to the Fe(III) complex and that the cyanide binding rate to the mutant is very slow, compared with those of the wild-type and other mutant proteins. We observed a similar trend in the binding behavior of imidazole to full-length mutant enzymes. Therefore, while Met95 and Phe113 are not direct axial ligands for the Fe(III) complex, catalytic, spectroscopic, and ligand binding evidence suggests that these residues are located in the vicinity of the heme.
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Affiliation(s)
- Atsunari Tanaka
- Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
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