Oxidoreductases enzyme catalysis

Heyes DJ, Hunter CN. Making light work of enzyme catalysis protochlorophyllide oxidoreductase. Trends Biochem Sci 2005 30 642-9. 

Catalase (H202 H202 oxidoreductase EC 1.11.1.6) was one of the first enzymes to be isolated in a high state of purity, and its crystallization (29) from beef liver extracts ranked among the early triumphs of biochemistry. Extensive physicochemical studies which followed (30-41) led to an impressive elucidation of its properties, but as yet not to a definition of the apoprotein function in the enzyme catalysis. 

Since the beginning of enzyme catalysis in microemulsions in the late 1970s, several biocatalytic transformations of various hydrophilic and hydrophobic substrates have been demonstrated. Examples include reverse hydrolytic reactions such as peptide synthesis [44], synthesis of esters through esterification and transesterification reactions [42,45-48], resolution of racemic amino acids [49], oxidation and reduction of steroids and terpenes [50,51], electron-transfer reactions, [52], production of hydrogen [53], and synthesis of phenolic and aromatic amine polymers [54]. Isolated enzymes including various hydrolytic enzymes (proteases, lipases, esterases, glucosidases), oxidoreductases, as well as multienzyme systems [52], were anployed. 

The EC mechanism is central to the concept of homogeneous redox catalysis, which is used to promote a redox reaction between an electrogenerated mediator and soluble reactant, under conditions where heterogeneous electron transfer to the reactant is restricted on kinetic grounds. Many redox processes between soluble mediators and oxidoreductase enzymes have also been shown to reduce to the simple EC mechanism under limiting conditions. ... 

As mentioned in part 2.1.3 hydrolytic enzymes are the most frequently used enzymes in organic chemistry. There are several reasons for this. Firstly, they are easy to ttse because they do not need cofactors like the oxidoreductases. Secondly, there are a large amormt of hydrolytic enzymes available because of their industrial interest. For instance detergent enzymes comprise proteases, celltrlases, amylases and lipases. Even if hydrolytic enzymes catalyse a chemically simple reaction, many important featirres of catalysis are still contained such as chemo-, regio- and stereoselectivity and specificity. 

Other non-heme enzymes that use dioxygen are 4-methoxy-benzoate O-demethylase, extradiol catechol dioxygenases, the oxidoreductase isopenicillin N synthase, and a-keto acid-dependent enzymes (28). Moreover, the BH4-dependent glyceryl-ether monooxygenase (GEM) also appears to be dependent on nonheme iron for catalysis (see also Section I.E). 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

One important aspect of the catalytic principle of these oxidoreductases—i.e., enzyme-NAD as the vehicle for hydrogen transfer—is the obligatory presence of enzyme-NAD for catalytic action only enzyme-NAD can serve as hydrogen acceptor to initiate catalysis. In contrast, enzyme-NADH is inactive and can not accept hydrogen from the substrate. Consequently, the ratio of enzyme-NAD to enzyme-NADH is responsible for the net catalytic activity of a particular preparation. Kalckar and co-workers (38) were the first to recognize that preincuba-... 

For a long time Fe/S clusters in the enzyme complexes of the respiratory chain of oxidative phosphorylation have been suggested to be directly involved in energy transduction, e.g., in the generation of a proton-motive force. A specific example is the putative cubane, center N2, in NADH Q oxidoreductase [6], One could formally write the process as a catalysis of the reaction H+in -> H+out. 

Metalloenzymes contain a bound metal ion as part of their structure. This ion can either partidpate directly in the catalysis, or stabilize the active conformation of the enzyme. In Lewis acid catalysis (typically with zinc, vanadium, and magnesium), the M"+ ion is used instead of H+. Many oxidoreductases use metal centers such as V, Mo, Co, and Fe in much the same way as homogeneous catalysis uses ligand-metal complexes. Figure 5.7 shows a simplified mechanism for the halide oxidation readion catalyzed by vanadium chloroperoxidase. The vanadium atom ads as a Lewis add, activating the bound peroxide [30]. 

Since the release of HCN is a common defense mechanism for plants, the number of available HNLs is large. Depending on the plant family they are isolated from, they can have very different structures some resemble hydrolases or carbox-ypeptidase, while others evolved from oxidoreductases. Although many of the HNLs are not structurally related they all utilize acid-base catalysis. No co-factors need to be added to the reactions nor do any of the HNL metallo-enzymes require metal salts. A further advantage is that many different enzymes are available, R- or S-selective [10]. For virtually every application it is possible to find a stereoselective HNL (Table 5.1). In addition they tend to be stable and can be used in organic solvents or two-phase systems, in particular in emulsions. 

The second step in the synthesis of bile acids, according to Hylemon et al. (1991), is the conversion of 7a-hydroxycholesterol to 7a-hydroxy-4-cholesten-3-one by NAD+-dependent 3/3-hydroxy-A5-C27-steroid oxidoreductase. This enzyme is located in the endoplasmic reticulum of liver, and its catalysis of the 3/3-hydroxy group also results in isomerization of the double bond from A5 to A4. 

The deprotonation and addition of a base to thiazolium salts are combined to produce an acyl carbanion equivalent (an active aldehyde) [363, 364], which is known to play an essential role in catalysis of the thiamine diphosphate (ThDP) coenzyme [365, 366]. The active aldehyde in ThDP dependent enzymes has the ability to mediate an efScient electron transfer to various physiological electron acceptors, such as lipoamide in pyruvate dehydrogenase multienzyme complex [367], flavin adenine dinucleotide (FAD) in pyruvate oxidase [368] and Fc4S4 cluster in pyruvate ferredoxin oxidoreductase [369]. 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

Uric acid (Fig. 1) in the human body is the end product of purine metabolism. It is produced by the enzymatic conversion of hypoxanthine to xanthine and then to uric acid. The enzyme involved here is xanthine oxidoreductase. This enzyme exists in two forms xanthine dehydrogenase and xanthine oxidase. The latter is able to produce oxidizing species during enzymatic catalysis [4]. In most organisms uric acid is enzymatically degraded by an enzyme called urate... 

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