Effects of Concerted Hydrogen Bonding of Distal Histidine on Active Site Structures of Horseradish Peroxidase. Resonance Raman Studies with Asn70 Mutants

Expression of the Caldariomyces fumago chloroperoxidase in Aspergillus niger and characterization of the recombinant enzyme

The Caldariomyces fumago chloroperoxidase was successfully expressed in Aspergillus niger. The recombinant enzyme was produced in the culture medium as an active protein and could be purified by a three-step purification procedure. The catalytic behavior of recombinant chloroperoxidase (rCPO) was studied and compared with that of native CPO. The specific chlorination activity (47 units/nmol) of rCPO and its pH optimum (pH 2.75) were very similar to those of native CPO. rCPO catalyzes the oxidation of various substrates in comparable yields and selectivities to native CPO. Indole was oxidized to 2-oxindole with 99% selectivity and thioanisole to the corresponding R-sulfoxide (enantiomeric excess >98%). Incorporation of (18)O from labeled H(2)18O(2) into the oxidized products was 100% in both cases.

Incorporation of horseradish peroxidase in a Kieselguhr membrane and the application to a mediator-free hydrogen peroxide sensor

Horseradish peroxidase was incorporated in a kieselguhr membrane. The electron-transfer process of the enzyme was examined by cyclic voltammetry. It was observed that the electron-transfer reactivity of horseradish peroxidase was greatly enhanced, and that direct electrochemistry was accordingly feasible. Using the merits of the direct electron-transfer reactivity of horseradish peroxidase and its specific enzymatic catalysis towards hydrogen peroxide, an unmediated hydrogen peroxide biosensor was constructed. The calibration plot of this hydrogen peroxide sensor was linear in the range of 2.0 x 10(-6) mol/L - 6.5 x 10(-4) mol/L. The relative standard deviation was 4.1% for 6 successive determinations at a concentration of 1.0 x 10(-4) mol/L. The detection limit was 1.0 x 10(-6) mol/L.

Harnessing the Power of Diatomics

When a diatomic molecule is broken up into its constituent atoms, these very energetic atoms provide a large driving force for further reaction. Diatomic molecules often do not undergo productive chemistry, however, because the energy needed to break the diatomic bond is also high. In his Perspective, Thorp discusses the work by MacBeth et al., who have synthesized a nonheme iron complex that reacts with O2 to produce two equivalents of a metal-oxo complex. The complex elegantly mimic the ability of some enzymes to influence metal ion coordination spheres.

O2 activation by nonheme iron complexes: A monomeric Fe(III)-Oxo complex derived from O2

Iron species with terminal oxo ligands are implicated as key intermediates in several synthetic and biochemical catalytic cycles. However, there is a dearth of structural information regarding these types of complexes because their instability has precluded isolation under ambient conditions. The isolation and structural characterization of an iron(III) complex with a terminal oxo ligand, derived directly from dioxygen (O2), is reported. A stable structure resulted from placing the oxoiron unit within a synthetic cavity lined with hydrogen-bonding groups. The cavity creates a microenvironment around the iron center that aids in regulating O2 activation and stabilizing the oxoiron unit. These cavities share properties with the active sites of metalloproteins, where function is correlated strongly with site structure.

Resonance raman studies of oxo intermediates in the reaction of pulsed cytochrome bo with hydrogen peroxide

Cytochrome bo from Escherichia coli, a member of the heme-copper terminal oxidase superfamily, physiologically catalyzes reduction of O(2) by quinols and simultaneously translocates protons across the cytoplasmic membrane. The reaction of its ferric pulsed form with hydrogen peroxide was investigated with steady-state resonance Raman spectroscopy using a homemade microcirculating system. Three oxygen-isotope-sensitive Raman bands were observed at 805/X, 783/753, and (767)/730 cm(-)(1) for intermediates derived from H(2)(16)O(2)/H(2)(18)O(2). The experiments using H(2)(16)O(18)O yielded no new bands, indicating that all the bands arose from the Fe=O stretching (nu(Fe)(=)(O)) mode. Among them, the intensity of the 805/X cm(-)(1) pair increased at higher pH, and the species giving rise to this band seemed to correspond to the P intermediate of bovine cytochrome c oxidase (CcO) on the basis of the reported fact that the P intermediate of cytochrome bo appeared prior to the formation of the F species at higher pH. For this intermediate, a Raman band assignable to the C-O stretching mode of a tyrosyl radical was deduced at 1489 cm(-)(1) from difference spectra. This suggests that the P intermediate of cytochrome bo contains an Fe(IV)=O heme and a tyrosyl radical like compound I of prostaglandin H synthase. The 783/753 cm(-)(1) pair, which was dominant at neutral pH and close to the nu(Fe)(=)(O) frequency of the oxoferryl intermediate of CcO, presumably arises from the F intermediate. On the contrary, the (767)/730 cm(-)(1) species has no counterpart in CcO. Its presence may support the branched reaction scheme proposed previously for O(2) reduction by cytochrome bo.

Effects of the location of distal histidine in the reaction of myoglobin with hydrogen peroxide

To clarify how the location of distal histidine affects the activation process of H2O2 by heme proteins, we have characterized reactions with H2O2 for the L29H/H64L and F43H/H64L mutants of sperm whale myoglobin (Mb), designed to locate the histidine farther from the heme iron. Whereas the L29H/H64L double substitution retarded the reaction with H2O2, an 11-fold rate increase versus wild-type Mb was observed for the F43H/H64L mutant. The Vmax values for 1-electron oxidations by the myoglobins correlate well with the varied reactivities with H2O2. The functions of the distal histidine as a general acid-base catalyst were examined based on the reactions with cumene hydroperoxide and cyanide, and only the histidine in F43H/H64L Mb was suggested to facilitate heterolysis of the peroxide bond. The x-ray crystal structures of the mutants confirmed that the distal histidines in F43H/H64L Mb and peroxidase are similar in distance from the heme iron, whereas the distal histidine in L29H/H64L Mb is located too far to enhance heterolysis. Our results indicate that the proper positioning of the distal histidine is essential for the activation of H2O2 by heme enzymes.

The distal cavity structure of carbonyl horseradish peroxidase as probed by the resonance Raman spectra of His 42 Leu and Arg 38 Leu mutants

CO ligation to horseradish peroxidase C (HRPC) was studied by means of site-directed mutagenesis and resonance Raman spectroscopy. The CO complexes of HRPC His 42 --> Leu and Arg 38 --> Leu mutants were characterized at pH values ranging from 3.6 to 9.5. The vibrational frequencies of the Fe-C stretching and Fe-C-O bending modes have been identified by isotopic substitution. Both His 42 --> Leu and Arg 38 --> Leu adducts with CO displayed a single Fe-C stretching band, whereas both recombinant and wild-type HRPC-CO have two bands, corresponding to different conformers. This comparison suggests that CO is H-bonded either to the distal Arg or to the distal His in the two conformers. An acid transition, common to the wild-type protein, was observed for both mutants. This indicates that these distal amino acids do not influence the acid transition. On the contrary, an alkaline transition was only observed for the Arg 38 --> Leu mutant, which suggests that distal His is involved in the alkaline transition of HRPC-CO complex. The spectroscopic information is found to be consistent with the X-ray structure of ferric HRPC. A comparison with the CO complexes of cytochrome c peroxidase and myoglobin is performed, which displays the functional significance of the structural differences between peroxidase classes I and III and between peroxidases and globins, respectively.

Substrate binding and catalysis in heme peroxidases

Peroxidase-catalysed reactions are being analysed at an increasingly advanced level of structural and mechanistic sophistication. A significant development in this respect has been the long-anticipated solution of crystal structures for several plant peroxidases and a fungal peroxidase complexed to benzhydroxamic acid. New insights into peroxide binding and catalysis have been obtained through site-directed mutagenesis, a technique also crucial to recent progress in understanding the diversity of substrate interaction sites associated with peroxidases from different sources.

Structural roles of the highly conserved glu residue in the heme distal site of peroxidases

One of the highly conserved amino acid residues in the heme distal site of various fungal and plant peroxidases, glutamic acid 64 (Glu64) in horseradish peroxidase (HRP), interacts with a distal calcium ion through a hydrogen bond with a water molecule and its peptide carbonyl oxygen on the main-chain forms the hydrogen bond network to the distal His via the adjacent Asn residue, suggesting that the Glu residue is related to the stabilization of the calcium ion and catalytic activity of peroxidase [Nagano, S., Tanaka, M., Ishimori, K., Watanabe, Y., and Morishima, I. (1996) Biochemistry 35, 14251-14258]. To perturb the hydrogen bond with the adjacent Asn, we replaced the Glu with Pro (E64P) or Gly (E64G), which would alter the configuration of the main chain at position 64. Both of the mutants exhibited substantially depressed oxidation activities for hydroquinone and elementary reaction rates in the catalytic cycle. However, the E64S (Glu64 --> Ser) mutant, in which the configuration of the main chain and the hydrogen bond with Asn70 would not be affected but the interactions with the calcium ion are seriously perturbed by removal of the carboxylate, also showed quite low catalytic activity as observed for the E64P and E64G mutants. Spectral features for the E64S mutant are similar to those of the other mutants: the reorientation of the distal His, disruption of the hydrogen bond between the distal His and Asn70, and loss of the calcium ion. Thus, we can conclude that, in addition to forming the hydrogen bond network in the distal site, the Glu residue is a key residue for stable binding of the calcium ion, which maintains the structural integrity of the distal cavity, resulting in high peroxidase activity.

Catalytic activities and structural properties of horseradish peroxidase distal His42 --> Glu or Gln mutant

The distal histidine (His) is highly conserved in peroxidases and has been considered to play a major role as a general acid-base catalyst for peroxidase reaction cycle. Recently, however, the X-ray structure of chloroperoxidase from the marine fungus Caldariomyces fumago has revealed that a glutamic acid is located at the position where most of the peroxidase has a histidine residue, suggesting that the carboxyl group in the glutamic acid (Glu) can also assist cleavage of an O-O bond in peroxides [Sundaramoorthy, M., Terner, J., & Poulos, T. L. (1995) Structure 3, 1367-1377]. In order to investigate catalytic roles of the glutamic acid at the distal cavity, two horseradish peroxidase mutants were prepared, in which the distal His42 has been replaced by Glu (H42E) or Gln (H42Q). The formation rate of compound I in the H42E mutant was significantly greater than that for the H42Q mutant, indicating that the distal Glu can play a role as a general acid-base catalyst. However, the peroxidase activity of the H42E mutant was still lower, compared to that for native enzyme. On the basis of the CD, resonance Raman, and EPR spectra, it was suggested that the basicity of the distal Glu is lower than that of the distal His and the position of the distal Glu is not fixed at the optimal position as a catalytic amino acid residue, although no prominent structural changes around heme environment were detected. The less basicity and improper positioning of the distal Glu would destabilize the heme-H2O2-distal Glu ternary intermediate for the peroxidase reaction. Another characteristic feature in the mutants was the enhancement of the peroxygenase activity. Since the peroxygenase activity was remarkably enhanced in the H42E mutant, the distal Glu is also crucial to facilitate the peroxygenase activity as well as the enlarged distal cavity caused by the amino acid substitution. These observations indicate that the distal amino acid residue is essential for function of peroxidases and subtle conformational changes around the distal cavity would control the catalytic reactions in peroxidase.

Hydrogen bond network in the distal site of peroxidases: spectroscopic properties of Asn70 --> Asp horseradish peroxidase mutant

The distal His in peroxidases forms a hydrogen bond with the adjacent Asn, which is highly conserved among many plant and fungal peroxidases. Our previous work [Nagano, S., Tanaka, M., Ishimori, K., Watanabe, Y., & Morishima, I. (1996) Biochemistry 35, 14251-14258] has revealed that the replacement of Asn70 in horseradish peroxidase C (HRP) by Val (N70V) and Asp (N70D) discourages the oxidation activity for guaiacol, and the elementary reaction rate constants for the mutants was decreased by 10-15-fold. In order to delineate the structure-function relationship of the His-Asn couple in peroxidase activity, heme environmental structures of the HRP mutant, N70D, were investigated by CD, 1H NMR, and IR spectroscopies as well as Fe2+/Fe3+ redox potential measurements. While N70D mutant exhibited quite similar CD spectra and redox potential to those of native enzyme, the paramagnetic NMR spectrum clearly showed that the hydrogen bond between the distal His and Asp70 is not formed in the mutant. The disappearance of the splitting in the 1H NMR signal of heme peripheral 8-methyl group observed in 50% H2O/50% D2O solution of N70D-CN suggests that the hydrogen bond between the distal His and heme-bound cyanide is also disrupted by the mutation, which was supported by the low C-N vibration frequency and large dissociation constant of the heme-bound cyanide in the mutant. Together with the results from various spectroscopies and redox potentials, we can conclude that the improper positioning of the distal His induced the cleavages of the hydrogen bonds around the distal His, resulting in the substantial decrease of the catalytic activity without large structural alterations of the enzyme. The His-Asn hydrogen bond in the distal site of peroxidases, therefore, is essential for the catalytic activity by controlling the precise location of the distal His.