Enzymes redox transformations

In DET, the enzymatic and electrode reactions are coupled by direct (mediatorless) electron transfer. In this case, the electron is transferred directly from the electrode to the substrate molecule (or vice versa) via the active site of the enzyme. In such a system, the coupled overall process is the redox transformation of the substrate(s), which can be considered as an enzyme-catalyzed electrode process. According to this mechanism, the electrode surface acts as the enzyme cosubstrate, and the enzymatic and electrode reactions cannot be considered as separate, but as formal stages of the bioelectrocatalytic reaction mechanism. The catalytic effect of the enzyme is the... 

Figure 6.3 Catalase redox transformation diagram. Compounds II, III and IV represent complexes of the enzyme with H202 and iron valence states, Fe5+, Fe4+ and Fe6+, respectively HXOH is a two-electron donor (reducer) X=0, NH, C=0, H(CH2) CH, where n = 1,2, 3 AH is a single-electron donor (reducer) ROOH is hydroperoxide (R is alkyl or acyl radical) and ROH is alcohol.

In enzymatic catalysis, the amazing unity of acid-base and redox mechanisms, in one of BRC H+ and e or H are transferred to prosthetic groups of enzymes, which results in redox transformations of the substrate. 

We began studies with flavin coenzyme analogs in 1972 to probe what structural features in the flavin ring system were requisite for specific aspects of the enzymic catalyses noted above. In particular, evaluations of the 5-carba-5-deazaflavin and the 1-carba-l-deazaflavin system were selected, given the pivotal role of these nitrogens in the redox transformations. 

In spite of their catalytic versatility and their capacity to transform a variety of pollutant compounds, peroxidases are not applied at large scale yet. The challenges that should be solved to use peroxidases for environmental purposes have been recently reviewed [146], Three main protein engineering challenges have been identified (a) the enhancement of operational stability, specifically hydrogen peroxide stability (see Chap. 11) (b) the increase of the enzyme redox potential in order to widen the substrate range (see Chap. 4) (c) the development of heterologous expression and industrial production (see Chap. 12). 

Microbial transformations of foreign substrates, often referred to as precursor fermentation. It may be preferred to perform such transformations with whole cells rather than isolated enzymes when the latter approach would involve the recycling of expensive cofactors. Hence, precursor fermentations are often preferred for conducting redox transformations (see Chapter 6). 

A relationship between ki, pp and van der Waals volumes of peroxides is the only discernible pattern. This strongly suggests that enzyme-peroxide association is the rate-determining step and, as such, is rather irrelevant to the elucidation of the redox transformations. 

Fig. 21. Typical intermediates in hemoprotein enzyme active sites. The iron protoporphyrin IX cofactor (heme) forms dioxygen adducts termed oxy-species. In the course of oxygen activation and catalytic redox transformations, the oxy form can be consecutively converted into hydroperoxo- and oxo-type intermediates, which are usually referred to as compound 0, compound I, and compound II. Reproduced with permission from Ref (183). Copyright Nature Publishing Group.

Figure 39. Electrical communication between an enzyme redox center and a photoexcited species attaining light-induced biocatalyzed transformations (A) direct electrical wiring of the protein by its chemical modification with tethered electron-relay units (B) electrical communication by the immobilization of the protein into a redox-functionalized polymer matrix.

By the second approach, the enzyme is immobilized in a redox polymer assembly (Figure 39B). Electron-transfer quenching of the photosensitizer by the polymer matrix generates an electron pool for the activation of the enzyme. Photoreduction of nitrate to nitrite was accomplished by the physical encapsulation of NitraR in a redox-functionalized 4,4 -bipyridinium acrylamide copolymer [234]. In this photosystem, Ru(bpy)3 + was used as a photosensitizer and EDTA as a sacrificial electron donor. Oxidation of the excited photosensitizer results in electron transfer to the redox polymer, and the redox sites on the polymer mediate further electron transfer to the enzyme redox center, where the biocatalyzed transformation occurs. The rate constant for the MET from the redox polymer functionalities to the enzyme active site is — (9 + 3) x 10 s. Similarly, the enzyme glutathione reductase was electrically wired by interacting the enzyme with a redox polymer composed of polylysine modified with A-methyl-A -carboxyalkyl-4,4 -bipyridinium. The photosensitized reduction of oxidized glutathione (GSSG) (Eq. 21) ... 

While many redox transformations of pesticide compounds can occur abioticaUy, virtually all such reactions in namral systems are facilitated, either directly or indirectly, by biological processes (Wolfe and Macalady, 1992). Some pesticide compounds may be taken up by living organisms and directly oxidized or reduced through the involvement of a variety of redox-active biomolecules (Bollag, 1982). Enzymes that have been found to be responsible for the biological oxidation of pesticide compounds... 

Walsh, C. 1978 Annu. Rev. Biochem. 47, 881-931 Chemical approaches to the study of enzymes catalyzing redox transformations. 

Figure 15. Electrical communication of enzyme redox sites with photoexcited species as a means to drive biocatalyzed photosynthetic transformations (n) electrical communication by a diffusional photogenerated electron mediator (h) direct communication between an electrically wired redox enzyme and a photoexcited species.

Although one of the major goals in designing SOD mimics is to reproduce strict selectivity of the natmal enzyme, the past decade s research has seriously questioned the absolute selectivity of natural SOD enz5unes toward superoxide. The first study to show that appeared in 1999 by Niketic et al. (32), where the authors demonstrated that Mn and Fe SOD enzymes (E. coli), but not CuZnSOD, react with NO imder anaerobic conditions leading to the redox transformation of NO to nitroxyl (NO / HNO) and nitrosonium (NO ) ions which produce enzyme modifications and inactivation, leading to the cleavage of the enzyme polypeptide chain. 

The structure and physicochemical properties of the enzymes which have been used to date to promote electrochemical reactions are briefly outlined. Methods of their immobilization are described. The status of research on redox transformations of proteins and enzymes at the electrode-electrolyte interface is discussed. Current concepts on the ways of conjugation of enzymatic and electrochemical reactions are summarized. Examples of bioelectrocatalysis in some electrochemical reactions are described. Electrocatalysis by enzymes under conditions of direct mediatorless transport of electrons between the electrode and the enzyme active center is considered in detail. Lastly, an analysis of the status of work pertaining to the field of sensors with enzymatic electrodes and to biofuel cells is provided. 

Much progress in the study of electrochemical properties of protein macromolecules has been achieved recently using another method, namely, the study of redox transformations of proteins, the carriers of electrons and enzymes, and their active groups at the electrode-electrolyte interface. This approach is intimately related to the use of enzymes to promote electrochemical reactions and pursues the purpose of elucidation of the mechanism of electron transport and the structural features of enzymes. 

An investigation into the possibility of activating cathodic oxygen reduction by means of cytochrome c oxidase, or, more accurately, sub-mitochondrial particles (SMP) isolated from mitochondria of the bull s heart and enriched with the enzyme has been reported. Investigations were carried out on the pyrographite electrode, using the TMPD-dihydrochloride-WB (perchlorate) couple as mediator. The choice of such a carrier was due to the fact that TMPD is the most active substrate of cytochrome c oxidase. The redox potential of the TMPD-WB couple is 0.72 V and redox transformations on the pyrographite electrode are practically reversible. 

---