Redox enzymes enzymatic activity

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

Therefore, for preparative applications of redox enzymes, effective and simple methods for the continuous recycling of the active cofactors have to be available. In addition, such systems must be stable over long time periods and the separation of the product must be simple to render technical processes economically feasible. Until now, this problem has generally been solved by the application of a second enzymatic reaction (enzyme-coupled regeneration, Fig. 2). 

Step 3 is very fast, especially at pH 5, when the enzymatic activity of GO is the highest. The second-order rate constant k i equals 1.6 x 106 M-1 s-1 (23,24) but decreases more than 100-fold at pH>8 (24). Similar pH effect has been reported for GO from Penicillium vitale (25). The oxidation of FADH2 occurs in two le steps. At pH 5.3 the corresponding directly obtained redox potentials for the Aspergillus niger enzyme equal 0.048 and -0.050 V vs. NHE (26). The rate constant k3 is a very important characteristic of GO because a good mediator must oxidize the reduced enzyme not slower than dioxygen. 

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... 

More recently, a new series of water dispersed anionic polymers, the AQ 29D, 38D and 55D polymers were released by Eastman Kodak. Since that time, these polymers were used as electrode modifier (12, 13), as covering membrane (14) and as support for enzyme immobilization (15, 16). AQ polymers are high molecular weights (14,000 to 16,000 Da) sulfonated polyester type polymers (17, 18). Their possible structures have been recently presented (18). The AQ polymer serie shows many interesting characteristics useful for the fabrication of biosensors. They are water dispersed polymers and thus compatible with enzymatic activity. They have sulfonated pendant groups similar to Nafion and they can act as a membrane barrier for anionic interferring substances and they offer the possibility to immobilize redox mediators by ion exchange. 

Suzuki and Banfield (1999) classify methods of microbial uranium accumulation as either metabolism dependent or metabolism independent. The former consists of precipitation or complexa-tion with metabohcally produced ligands, processes induced by active cellular pumping of metals, or enzyme-mediated changes in redox state. Examples include precipitation of uranyl phosphates due the activity of enzymes such as phosphatases, formation of chelating agents in response to metal stress, and precipitation of uraninite through enzymatic uranium reduction. 

Activation of drugs to give toxic products is common. Apart from non-enzymatic activation (e.g., via autoxidation), activation by enzymatic one-electron oxidation or reduction frequently occurs. Several non-specific oxidases and reductases are encountered in mammalian tissues. Enzyme systems that have been studied in detail are peroxidases and microsomal oxidases and reductases. Xanthine oxidase also has received some attention. In many insta .ces the end products of the reaction are critically dependent upon the presence of oxygen in the system. This is because oxygen is an excellent electron acceptor, i.e., it can oxidize donor radicals, forming superoxide in the process. In this way a redox cycle is set up in which the xenobiotic substrate is recovered. The toxic effects of the xenobiotic often can be attributed to the oxidative stress arising from such a cycle. However, it seems that for some substrates, oxidative stress of this kind can be less damaging than anaerobic reduction. Anaerobic reduction can lead to formation of further reduced products with additional toxicity. 

SODs are differentiated mainly by the redox-active metal in the active site copper, manganese, or iron. The iron and manganese SODs are structurally similar (5-11) and are structurally distinct from the Cu,Zn SOD (12). The dramatic features of these enzymes are that they catalytically dismutate superoxide at rates that are not only diffusion controlled but have been shown to be electrostatically facilitated (13). In these systems, modifications of amino acid residues near the active site have been shown to alter the enzymatic activity, indicating that superoxide is electrostatically drawn into the active site channel (14). In addition, in contrast to the spontaneous dismutation rate of 02 and the dismutation rates of 02 by many metal complexes, all of which are pH dependent, the enzymatic dismutation rate is largely pH independent over the pH range (5-10). 

Iron and sulfur can be extracted from F. the resulting apoferredoxin is reactivated by iron(II) salts and sulfides. The synthesis of the iron-free protein has been achieved by the Merrifield technique. On account of their properties as redox systems (Fe +e" Fe ") the F. effect electron transport between enzyme systems but do not exhibit any enzymatic activity. They transport electrons in the respiratory chain, in photosynthesis, and in nitrogen fixation. The iron-sulfur protein P439 of the Fc4S4-type (Mr 11600) plays a role in photosynthesis. Conclusions can be drawn about the evolutionary histories of plants from the similarities and differences in the amino acid sequences. For the evolutionary history of F. in photosynthesis, see Lit.K F. with FejSj- and FejSg-clusters also occur in bacteria Lit. TrendsBiochem.Sci. 13,30-33(1988). FEMSMicrobiol. Rev. 54,155-176 (1988) Trends Biochem. Sci. 13,369 f. (1988). 

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