Characterization of topa quinone cofactor

Defining the role of tyrosine and rational tuning of oxidase activity by genetic incorporation of unnatural tyrosine analogs

While a conserved tyrosine (Tyr) is found in oxidases, the roles of phenol ring pKa and reduction potential in O2 reduction have not been defined despite many years of research on numerous oxidases and their models. These issues represent major challenges in our understanding of O2 reduction mechanism in bioenergetics. Through genetic incorporation of unnatural amino acid analogs of Tyr, with progressively decreasing pKa of the phenol ring and increasing reduction potential, in the active site of a functional model of oxidase in myoglobin, a linear dependence of both the O2 reduction activity and the fraction of H2O formation with the pKa of the phenol ring has been established. By using these unnatural amino acids as spectroscopic probe, we have provided conclusive evidence for the location of a Tyr radical generated during reaction with H2O2, by the distinctive hyperfine splitting patterns of the halogenated tyrosines and one of its deuterated derivatives incorporated at the 33 position of the protein. These results demonstrate for the first time that enhancing the proton donation ability of the Tyr enhances the oxidase activity, allowing the Tyr analogs to augment enzymatic activity beyond that of natural Tyr.

Significant Improvement of Oxidase Activity through the Genetic Incorporation of a Redox-active Unnatural Amino Acid

Incorporation of 3-methoxytyrosine boosts the oxidase activity of the myoglobin model of oxidase, stressing the importance of the redox potential tuning of tyrosine.

While nature employs various covalent and non-covalent strategies to modulate tyrosine (Y) redox potential and pK_a in order to optimize enzyme activities, such approaches have not been systematically applied for the design of functional metalloproteins. Through the genetic incorporation of 3-methoxytyrosine (OMeY) into myoglobin, we replicated important features of cytochrome c oxidase (CcO) in this small soluble protein, which exhibits selective O₂ reduction activity while generating a small amount of reactive oxygen species (ROS). These results demonstrate that the electron donating ability of a tyrosine residue in the active site is important for CcO function. Moreover, we elucidated the structural basis for the genetic incorporation of OMeY into proteins by solving the X-ray structure of OMeY specific aminoacyl-tRNA synthetase complexed with OMeY.

Kinetics of redox interaction between substituted quinones and ascorbate under aerobic conditions

One-electron reduction of quinones (Q) by ascorbate (AscH ); (1) AscH + Q --> Q*- + Asc*- + H+, followed by the oxidation of semiquinone (Q*-) by molecular oxygen; (2) Q*- + O2 --> Q + O2*-, results in the catalytic oxidation of ascorbate (with Q as a catalyst) and formation of active forms of oxygen. Along with enzymatic redox cycling of Q. this process may be related to Q cytotoxicity and underlie an antitumor activity of some Qs. In this work, the kinetics of oxygen consumption accompanied the interaction of ascorbate with 55 Qs including substituted 1,4- and 1,2-benzoquinones, naphthoquinones and other quinoid compounds were studied in 50 mM sodium phosphate buffer, pH 7.40, at 37 degrees C by using the Clark electrode technique. The capability of Q to catalyze ascorbate oxidation was characterized by the effective value of kEFF calculated from the initial rate of oxygen consumption (R(OX)) by the equation R(OX) = kEFF[Q][AscH-] as well as by a temporary change in R(OX). The correlation of kEFF with one-electron reduction potential, E(Q/Q*-), showed a sigma-like plot, the same for different kinds of Qs. Only the Qs which reduction potential E(Q/Q*-) ranged from nearly -250 to + 50 mV displayed a pronounced catalytic activity, kEFF increased with shifting E(Q/Q*-) to positive values. The following linear correlation between kEFF (in M (-1) s(-1)) and E(Q/Q*-) (in mV) might be suggested for these Qs: lg(kEFF)= 3.91 + 0.0143E(Q/Q*-). In contrast, Qs with E(Q/Q*-) < - 250 mV and E(Q/Q*-) > + 50 mV showed no measurable catalytic activity. The Qs studied displayed a wide variety in the kinetic regularities of oxygen consumption. When E(Q/Q*-) was more negative than - 100 mV, Q displayed a simple ('standard') kinetic behavior--R(OX) was proportional to [AscH-][Q] independently of concentration of individual reagents, [AscH-] and [Q]; R(OX) did not decrease with time if [AscH-] was held constant: Q recycling was almost reversible. Meanwhile, Qs with E(Q/Q*-) > - 100 mV demonstrated a dramatic deviation from the 'standard' behavior that was manifested by the fast decrease in R(OX) with time and non-linear dependence of even starting values of R(OX) on [Q] and [AscH-]. These deviations were caused basically by the participation of Q*- in side reactions different from (2). The above findings were confirmed by kinetic computer simulations. Some biological implications of Q-AscH- interaction were discussed.

Mechanistic study on the roles of a bifidogenetic growth stimulator based on physicochemical characterization

2-Amino-3-carboxy-1,4-naphthoquinone, discovered as a novel bifidogenetic growth stimulator (BGS), has been characterized by determination of redox and acid-base equilibria, partition properties, and UV-vis and electron spin resonance spectral properties. BGS is proposed to function as an electron transfer mediator from NADH to O2. BGS is reduced by NADH-reduced diaphorase (or related enzymes) and the reduced BGS is reoxidized by autoxidation and a peroxidase-catalyzed reaction. The proposed reaction would spare pyruvate as an important metabolic intermediate, and minimize the cytotoxic effects of H2O2 generated by the autoxidation. Kinetic studies were performed in model enzymatic systems using 2-methyl-1,4-naphthoquinone (VK3) as a reference compound with a very weak growth-stimulating effect. The results support our proposal and reveal the superiority of BGS to VK3 as an electron transfer mediator in the proposed reactions.

Kinetics of redox interaction between substituted 1,4-benzoquinones and ascorbate under aerobic conditions: critical phenomena

Redox cycling is believed to be the most general molecular mechanism of quinone (Q) cytotoxicity. Along with redox cycling induced by a reductase, a similar process is known to occur via electron transfer from ascorbate (AscH-) to Q with formation of a semiquinone radical (Q.-): (1) Q + AscH- (k1)--> Q.- + Asc.- + H+ (2) Q.- + O2 --> Q + O2.-. The net effect of reactions (1) and (2) provides for the catalytic oxidation of AscH-, with Q serving as a catalyst. In this work, the kinetics of oxygen consumption accompanying this process were studied with several substituted 1,4-benzoquinones (BQ) at 37 degrees C in phosphate buffer, pH 7.40, using the Clark electrode technique. The value of k1 determined from the initial rate of oxygen consumption was typically found to increase when the one-electron reduction potential E(Q/Q.-) shifted to more positive values. With Q, for which E(Q/Q.-) is less than -100 mV, the rate of oxygen uptake (R(OX)) was found to be directly correlated with the [Q][AscH-] value independent of the concentration of individual reagents, remaining constant for a long period. With mono- and dialkyl-substituted 1,4-BQs, for which E(Q/Q.-) is higher than -100 mV, significant deviations from the above simple kinetic regularities were observed. In particular, R(OX) decreased dramatically with time and critical phenomena (the existence of certain concentrations of Q and/or AscH- above or below which the catalytic oxidation of AscH- ceased completely after a non-stationary period of short duration) were observed. These abnormalities can be explained on the basis of the kinetic scheme which contains, in addition to reactions (1) and (2), several side reactions including that between Q.- and AscH-. Implications of critical phenomena discovered in this study for the problems of Q toxicity and vitamin C avitaminosis are discussed.