Sandeep Modi

Kinetic studies on the oxidation of phenols by the horseradish peroxidase compound II

Oxidation of substituted phenols by horseradish peroxidase compound II were studied using stopped-flow technique. Dissociation constants (K(D)) of HRP-II-phenol complexes were deduced from the kinetic data. Magnitudes of K(D) fall in a relatively narrow range of 3-11 mM. These are comparable to 3-10 mM reported for the binding of substituted phenols to native HRP, suggesting that the mode of binding of phenols to native HRP and HRP compound II may be similar. pH dependence of the apparent second order rate constants (k(app)) of the reactions of all the phenols were determined. The k(app) values of reactions other than the reaction of tyrosine, were observed to increase in the acidic region but decreased in the alkaline region. The increase was attributed to the deprotonation of distal carboxylic acid residue on enzyme with pK(a) values of 4.2-5.2. For tyrosine, however, the apparent second-order rate constant was observed to further increase non linearly on increasing the pH in the alkaline region. Results were interpreted quantitatively on the basis that protonated form of the enzyme reacted with the protonated form of the phenol with different individual rate constants.

1H, 15N and 13C NMR resonance assignment, secondary structure and global fold of the FMN-binding domain of human cytochrome P450 reductase.

The FMN-binding domain of human NADPH-cytochrome P450 reductase,corresponding to exons 3-;7, has been expressed at high level in anactive form and labelled with 13C and 15N. Mostof the backbone and aliphatic side-chain 1H, 15Nand 13C resonances have been assigned using heteronucleardouble- and triple-resonance methods, together with a semiautomaticassignment strategy. The secondary structure as estimated from the chemicalshift index and NOE connectivities consists of six α-helices and fiveβ-strands. The global fold was deduced from the long-range NOEsunambiguously assigned in a 4D 13C-resolved HMQC-NOESY-HMQCspectrum. The fold is of the alternating α/β type, with the fiveβ-strands arranged into a parallel β-sheet. The secondarystructure and global fold are very similar to those of the bacterialflavodoxins, but the FMN-binding domain has an extra short helix in place ofa loop, and an extra helix at the N-terminus (leading to the membrane anchordomain in the intact P450 reductase). The experimental constraints werecombined with homology modelling to obtain a structure of the FMN-bindingdomain satisfying the observed NOE constraints. Chemical shift comparisonsshowed that the effects of FMN binding and of FMN reduction are largelylocalised at the binding site.

Interaction of aromatic donor molecules with horseradish peroxidase: identification of the binding site and role of heme iron in the binding and activity

The interaction of aromatic substrates with horseradish peroxidase (HRP) was studied. Chemical modification of HRP was performed using diethylpyrocarbonate (DEPC) and for the first time the amino acid involved in binding with these substrates has been identified. The kinetic parameters for this interaction have been calculated and the role of heme iron in the oxidation of aromatic substrates by HRP has been discussed.

Kinetic studies on oxidation of aromatic donor molecules by horseradish peroxidase and lactoperoxidase

Based on kinetic evidence, it has been shown for the first time that the mode of binding of aromatic donor molecules is similar in horseradish peroxidase and lactoperoxidase; also that the nature of the heme plays an important role in the reaction with hydrogen peroxide, and has no effect on the reaction of the intermediate compound II with aromatic substrates.