Enzymes redox alterations

Step 1. The substrate, RH, associates with the active site of the enzyme and perturbs the spin-state equilibrium. Water is ejected from the active site and the electronic configuration shifts to favor the high-spin form in which pentaco-ordinated heme Fe3+ becomes the dominant form-binding substrate. In this coordination state, Fe3+ is puckered out and above the plane in the direction of the sixth ligand site. The change in spin state alters the redox potential of the system so that the substrate-bound enzyme is now more easily reduced. 

Protons are translocated across the membrane by what is described as a proton pnmp . How does the pump operate The change in redox state experienced by the prosthetic gronps of the enzymes in the chain causes conformational changes in the proteins that alter the affinities of some amino acid side-chain gronps for protons. In addition, there is a change in the direction in which these groups face in the membrane. Consequently, oxidation results in an association with a proton on the matrix side of the membrane whereas reduction results in reversal of the direction that the side-chain groups face and an increase in... 

Vance and Miller et al. have shown that the inactivity of enzyme is due to changes in the redox potentials of the enzyme. In order to dismute 02 the redox potential of the enzyme must lie between the E° values for the reactions shown in Equation (3). The E° value of the E. coli MnSOD enzyme is 0.290 V and that for the FeSOD is 0.220 V. The Fe-substituted form of the Mn enzyme has F ° =—0.240 V and Mn-substituted FeSOD has ii° >0.960V. These values are outside the required range and the changes in redox potentials are not due to changes in the metal ligands. Mutations of His-30 and Tyr-34, two conserved residues in the immediate vicinity of the metal binding site, do not alter the redox potential of the enzyme either " ... 

This enzyme [EC 2.7.1.23] catalyzes the reaction of ATP with NAD+ to produce ADP and NADP+. This reaction has the potential for altering the intracellular [NADP+]/ [NADPH] redox potential by converting NAD+ to NADP+, but details on the regulation of this enzyme have not been worked out. 

Table 3.1). Thus, for a redox reaction to be possible, the difference between the redox potential of the enzyme-cofactor system and that of the substrate must be above zero [3]. The catalytic role of the enzyme protein structure in a redox reaction is often to alter the electronic environment of the cofactor, thereby changing its redox potential and hence making the reaction more thermodynamically feasible. (For further in-depth discussion the reader is referred to the excellent text of Bugg [3].)...

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

Molded Dry Chemistry. In general, most enzymes are very fragile and sensitive to pH. solvent, and elevated lemperaiurts. The catalytic activity of most enzymes i> reduced dramatically ils the temperature is increased, Typi cal properties of diagnostic enzymes are given in Table 1. t he presence of ionic salts and other chemicals can considerably influence enzyme stability. To keep or sustain enzymatic activity, the redox centers must remain intact. The bulk of the enzyme, polymeric in composition, is an insulaior. thus. altering ii does not reduce the enzyme s catalytic activity, li... 

Inhibition of DNA repair may result in an adverse event within the cell. As(III) has been shown to inhibit DNA ligase in vitro, but it does not appear to occur by the direct interaction of As(III) with this repair enzyme (Li and Rossman, 1989). It has been suggested that As(III) alters redox levels or affects signal transduction pathways involved with DNA ligase (Hu, Su and Snow, 1998). The nucleotide excision repair system is also impaired by arsenic (Okui and Fujiwara, 1986 Hartwig et al., 1997). 

There may be common themes in the role of protein-coenzyme contacts in these B -dependent enzymatic processes. In particular, these contacts could alter the relative stability of the Co(III)—R, Co(II), and Co(I) states to enhance reactivity. For coenzyme B 12-dependent enzymes, the deoxyadenosyl radical generates a substrate-derived radical, either directly or via a radical chain mechanism through the intermediacy of a protein-side-chain-based radical, such as S of cysteine or O of tyrosine. This protein-bound substrate-derived radical then undergoes rearrangement, possibly assisted by protein contacts. Thus, cofactor-protein contacts are probably very important in the activation of the Co—C bond, in altering the Co redox potentials, and in assisting in the rearrangements. 

In other work, Dandliker et al. have reported the inclusion of iron porphyrins within dendrimers to serve as functional mimics of redox-based proteins (Dandliker et al., 1994, 1995, 1997). These redox-switchable porphyrins show that the Fe3+/Fe2+ redox couple can be altered by the polarity of the surrounding environment. By changing the polarity imposed by the tightly packed branches of the dendritic core, the authors have illustrated that electrochemical behavior can be controlled by slight and subtle through-space environmental factors. These mimics may potentially model a wide variety of redox-driven enzymes and possibly provide mechanistic insights into their function. 

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