1,721,112 research outputs found
Modified Microperoxidases Exhibit Different Reactivity Towards Phenolic Substrates
No full textThe reactivity of several microperoxidase derivatives with different
distal-site environments has been studied. The distal-site environments
of these heme peptides include a positively charged
one, an uncharged environment, two bulky and doubly or triply
positively charged ones, and one containing aromatic apolar residues.
The reactivity in the catalytic oxidation of two representative
phenols, carrying opposite charges, by hydrogen peroxide
has been investigated. This allows the determination of the binding
constants and of the electron-transfer rate from the phenol
to the catalyst in the substrate/microperoxidase complex. The
electron-transfer rates scarcely depend on the redox and charge
properties of the phenol, but depend strongly on the microperoxidase.
Information on the disposition of the substrate in the adducts
with the microperoxidases has been obtained through determination
of the paramagnetic contribution to the 1H NMR relaxation
rates of the protons of the bound substrates. The data
show that the electron-transfer rate drops when the substrate
binds too far away from the iron and that the phenols bind to
microperoxidases at similar distances to those observed with peroxidases.
While the reaction rate of microperoxidases with peroxide
is significantly smaller than that of the enzymes, the efficiency
in the one-electron oxidation of phenolic substrates is almost
comparable. Interestingly, the oxyferryl form of the triply positively
charged microperoxidases shows a reactivity larger than that
exhibited by horseradish peroxidas
Reactivity study on microperoxidase-8
Full text linkThe catalytic activity of the microperoxidase-8/
H2O2 system toward tyramine and 3-(4-hydroxyphenyl)
propionic acid has been determined in acetate buffer,
pH 5.0. Operating with a strong excess of hydrogen
peroxide, the rate-determining step of the reaction was
substrate oxidation. Owing to the fast microperoxidase-
8 degradation, only the very initial phase of the reactions
were analyzed. The reaction rates follow a substrate
saturation behavior, with turnover numbers [kcat=
26±1 s)1 for 3-(4-hydroxyphenyl)propionic acid and
kcat=22±1 s)1 for tyramine] that were similar for the
two substrates. In contrast, the KM values indicated a
reduced affinity for the catalyst active species by the
positively charged phenol, probably due to repulsive
interaction with the protonated N-terminal microperoxidase-
8 amino group. The reactivity of the catalyst
active species was studied upon incubation of microperoxidase-
8 with a small excess hydrogen peroxide, followed
by reaction with the phenolic substrates. The
kinetic analysis showed that more than two active species
are accumulated. The species responsible for the
faster reactions was present in solution as a minor
fraction. The active intermediate which accumulated in a
larger amount (intermediate III) has a reduced substrate
oxidation activity. Comparison of this activity with the
kinetic constants obtained under turnover experiments
shows that intermediate III is not involved in the microperoxidase-
8 catalytic cycle. The active species of the
catalytic process are intermediates I and II, which in the
absence of substrate rapidly convert to intermediate III
Mechanistic Insight into the Catechol Oxidase Activity by a Biomimetic Dinuclear Copper Complex
Full text linkThe biomimetic catalytic oxidation of 3,5-ditert-
butylcatechol by the dicopper(II) complex of the
ligand a,a¢-bis{bis[1-(1¢-methyl-2¢-benzimidazolyl)
methyl]amino}-m-xylene in the presence of dioxygen has
been investigated as a function of temperature and pH in
a mixed aqueous/organic solvent. The catalytic cycle
occurs in two steps, the first step being faster than the
second step. In the first step, one molecule of catechol is
oxidized by the dicopper(II) complex, and the copper(II)
centers are reduced. From the pH dependence, it is deduced
that the active species of the process is the
monohydroxo form of the dinuclear complex. In the
second step, the second molecule of catechol is oxidized
by the dicopper(I)-dioxygen complex formed upon
oxygenation of the reduced complex. In both cases,
catechol oxidation is an inner-sphere electron transfer
process involving binding of the catechol to the active
species. The binary catechol-dicopper(II) complex
formed in the first step could be characterized at very
low temperature (90 C), where substrate oxidation is
blocked. On the contrary, the ternary complex of dicopper(
I)-O2-catechol relevant to the second step does
not accumulate in solution and could not be characterized,
even at low temperature. The investigation of the
biphasic kinetics of the catalytic reaction over a range of
temperatures allowed the thermodynamic (DH and DS)
and activation parameters (DH „ and DS „ ) connected
with the key steps of the catecholase process to be
obtained
Potential applications of peroxidases in the fine chemical industries
No full textA description of selected types of reactions catalyzed by heme peroxidases is given. In particular, the discussion is focused mainly on those of potential interest for fine chemical synthesis. The division into subsections has been done from the point of view of the enzyme action, i.e., giving emphasis to themechanism of the enzymatic reaction, and from that of the substrate, i.e., analyzing the type of transformation promoted by the enzyme. These two approaches have several points in common
Recognition and Sensing of Nucleoside Monophosphates by a Dicopper(II) Cryptate
Full text linkThe dicopper complex of a bis-tren cryptand in which the spacer consists of two furane subunits
connected in 2,2' by a -CH2- fragment selectively recognizes guanosine monophosphate with respect
to other nucleoside monophospates (NMPs) in a MeOH/water solution at pH 7. Recognition is efficiently
signaled through the displacement of the indicator 6-carboxyfluorescein bound to the receptor, monitoring
its yellow fluorescent emission. Titration experiments evidenced the occurrence of several simultaneous
equilibria involving 1:1 and 2:1 receptor/NMP and receptor/indicator complexes. It was demonstrated that
the added NMP displaces the indicator from the 2:1 receptor/indicator complex, forming the 1:1 receptor/
analyte inclusion complex. Recognition selectivity is thus ascribed to the nature of nucleotide donor atoms
involved in the coordination and their ability to encompass the CuII-CuII distance within the cryptate
A Cu-bis(imidazole) Substrate Intermediate Is the Catalytically Competent Center for Catechol Oxidase Activity of Copper Amyloid-β
No full textInteraction of copper ions with Aβ peptides alters the redox activity of the metal ion and can be associated with neurodegeneration. Many studies deal with the characterization of the copper binding mode responsible for the reactivity. Oxidation experiments of dopamine and related catechols by copper(II) complexes with the N-terminal amyloid-β peptides Aβ16 and Aβ9, and the Aβ16[H6A] and Aβ16[H13A] mutant forms, both in their free amine and N-acetylated forms show that efficient reactivity requires the oxygenation of a CuI-bis(imidazole) complex with a bound substrate. Therefore, the active intermediate for catechol oxidation differs from the proposed "in-between state" for the catalytic oxidation of ascorbate. During the catechol oxidation process, hydrogen peroxide and superoxide anion are formed but give only a minor contribution to the reaction
Enantio-differentiating catalytic oxidation by a biomimetic trinuclear copper complex containing L-histidine residues
No full textThe trinuclear complex [Cu3PHI]6+, derived from a ligand
containing two chiral L-histidine residues, performs the
catalytic oxidation of L- and D-Dopa with remarkable
enantio-differentiation; this depends on the anchoring effect
provided by the copper center which is not participating in
the catalytic reaction and recognizes the chirality of the
substrate
Mechanistic insight into the activity of tyrosinase from variable temperature studies in aqueous-organic solvent
No full textThe activity of mushroom tyrosinase on a representative series of phenolic and diphenolic substrates structurally related to tyrosine has been investigated in the mixed solvent of 34.4% methanol-glycerol (7/1 v/v) and 65.6% (v/v) aqueous 50 mM Hepes buffer pH 6.8 at various temperatures. The kinetic activation parameters ruling the enzymatic reactions and the thermodynamic parameters associated with the substrate binding process to the enzyme active species have been deduced from the temperature variation of the kcat and KM parameters. The activation free energy is dominated by the enthalpic term, which occurs in the relatively narrow range of 61 kJ mol-1 independent of substrate and reaction type (monophenolase or diphenolase). The activation entropies are small and generally negative and contribute no more than 10% to the activation free energy. The substrate binding parameters are characterized by large and negative enthalpy and entropy contributions, which are typically dictated by polar protein-substrate interactions. The substrate 4-hydroxyphenyl propionic acid exhibits a strikingly anomalous temperature dependence of the enzymatic oxidation rate, with deltaH 150 kJ mol-1 and deltaS 280 J K-1 mol-1, due to the fact that it can competitively bind to the enzyme through the phenol group, like the other substrates, or the carboxylate group, like carboxylic acid inhibitors. A kinetic model that includes the dual nature of substrate/inhibitor of this compound enables to account for the anomalous behavior
Copper-β-amyloid peptides exhibit neither monooxygenase nor superoxide dismutase activities
Full text linkContrary to earlier claims, the CuII complex with the soluble Aβ16 peptide, and also that with Aβ28, exhibit no phenol monooxygenase (tyrosinase-like) activity; the complexes neither exhibit superoxide dismutase activity
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