Anion- and pH-linked conformational transition in horseradish peroxidase

Catalytic properties of a short manganese peroxidase from Irpex lacteus F17 and the role of Glu166 in the Mn2+-independent activity

Il-MnP1 (GenBank: AGO86670.2) has been confirmed by sequence analysis as a short manganese peroxidase (MnP) from Irpex lacteus F17 (CCTCC AF 2014020). To investigate the catalytic properties, the oxidation of typical aromatic substrates and the pathways of guaiacol oxidation by Il-MnP1, both in the presence and absence of Mn²⁺ at either pH 4.0 or pH 7.4, were analyzed. Results showed that Il-MnP1 exhibited higher oxidative activity in the presence of Mn²⁺ than in the absence of Mn²⁺ toward the majority of the selected substrates at pH 4.0. Additionally, the similar product compositions suggested that the oxidation of guaiacol mainly belongs to a series of polymeric reactions of radicals initiated by Il-MnP1, whether they were in the presence and absence of Mn²⁺ at either pH 4.0 or 7.4. Furthermore, two variants (E166G, E166Q) were found using site-directed mutagenesis, to improve the Mn²⁺-independent oxidative activity significantly. The catalytic efficiency (K_cat/K_m) of E166G and E166Q in 2, 6-dimethoxyphenol oxidation was higher than Il-MnP1 by 170 and 34 times, respectively. The study revealed certain differences in catalytic properties between Mn²⁺ dependent and independent oxidation by Il-MnP1. More importantly, a residue (E166) was related to the Mn²⁺-independent activity of a short MnP.

How active-site protonation state influences the reactivity and ligation of the heme in chlorite dismutase

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.

Effects of mutations in the helix G region of horseradish peroxidase

Horseradish peroxidase (HRP) has long attracted intense research interest and is used in many biotechnological fields, including diagnostics, biosensors and biocatalysis. Enhancement of HRP catalytic activity and/or stability would further increase its usefulness. Based on prior art, we substituted solvent-exposed lysine and glutamic acid residues near the proximal helix G (Lys 232, 241; Glu 238, 239) and between helices F and F' (Lys 174). Three single mutants (K232N, K232F, K241N) demonstrated increased stabilities against heat (up to 2-fold) and solvents (up to 4-fold). Stability gains are likely due to improved hydrogen bonding and space-fill characteristics introduced by the relevant substitution. Two double mutants showed stability gains but most double mutations were non-additive and non-synergistic. Substitutions of Lys 174 or Glu 238 were destabilising. Unexpectedly, notable alterations in steady-state Vm/E values occurred with reducing substrate ABTS (2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)), despite the distance of the mutated positions from the active site.

Thermodynamic analysis of the binding of aromatic hydroxamic acid analogues to ferric horseradish peroxidase

Peroxidases typically bind their reducing substrates weakly, with K(d) values in the millimolar range. The binding of benzhydroxamic acid (BHA) to ferric horseradish peroxidase isoenzyme C (HRPC) [K(d) = 2.4 microM; Schonbaum, G. R. (1973) J. Biol. Chem. 248, 502-511] is a notable exception and has provided a useful tool for probing the environment of the peroxidase aromatic-donor-binding site and the distal heme cavity. Knowledge of the underlying thermodynamic driving forces is key to understanding the roles of the various H-bonding and hydrophobic interactions in substrate binding. The isothermal titration calorimetry results of this study on the binding of aromatic hydroxamic acid analogues to ferric HRPC under nonturnover conditions (no H(2)O(2) present) confirm the significance of H-bonding interactions in the distal heme cavity in complex stabilization. For example, the binding of BHA to HRPC is enthalpically driven at pH 7.0, with the H-bond to the distal Arg38 providing the largest contribution (6.74 kcal/mol) to the binding energy. The overall relatively weak binding of the hydroxamic acid analogues to HRPC is due to large entropic barriers (-11.3 to -37.9 eu) around neutral pH, with the distal Arg38 acting as an "entropic gate keeper". Dramatic enthalpy-entropy compensation is observed for BHA and 2-naphthohydroxamic acid binding to HRPC at pH 4.0. The enthalpic loss and entropic gain are likely due to increased flexibility of Arg38 in the complexes at low pH and greater access by water to the active site. Since the Soret absorption band of HRPC is a sensitive probe of the binding of hydroxamic acids and their analogues, it was used to investigate the binding of six donor substrates over the pH range of 4-12. The negligible pH dependence of the K(d) values corrected for substrate ionization suggests that enthalpy-entropy compensation is operative over a wide pH range. Examination of the thermodynamics of binding of ring-substituted hyrazides to HRPC reveals that the binding affinities of aromatic donors are highly sensitive to the position and nature of the ring substituent.