Biochemistry substrate reduction

Li S, LP Wackett (1993) Reductive dehalogenation by cytochrome P450(,j jy[ substrate binding and catalysis. Biochemistry 32 9355-9361. 

Ketley, J. N., Schellenberg, K. A. Substrate stereochemical requirements in the reductive inactivation of uridine diphosphate galactose 4-epimerase by sugar and 5 -uridine monophosphate. Biochemistry 12, 315—320 (1973). 

Figure 15-2 Absorption spectra of NAD+ and NADH. Spectra of NADP+ and NADPH are nearly the same as these. The difference in absorbance between oxidized and reduced forms at 340 nm is the basis for what is probably the single most often used spectral measurement in biochemistry. Reduction of NAD+ or NADP+ or oxidation of NADH or NADPH is measured by changes in absorbance at 340 nm in many methods of enzyme assay. If a pyridine nucleotide is not a reactant for the enzyme being studied, a coupled assay is often possible. For example, the rate of enzymatic formation of ATP in a process can be measured by adding to the reaction mixture the following enzymes and substrates hexokinase + glucose + glucose-6-phosphate dehydrogenase + NADP+. As ATP is formed, it phosphorylates glucose via the action of hexokinase. NADP+ then oxidizes the glucose 6-phosphate that is formed with production of NADPH, whose rate of appearance is monitored at 340 nm.

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

An additional consideration in formulating redox reactions is the possibility of catalysis by substances that mediate the transfer of electrons between the bulk reductant (or oxidant) and the substrate being transformed. Such considerations arise frequently in many areas of chemistry, especially electrochemistry and biochemistry (e.g., 97). In environmental applications, the most common model for mediated electron transfer involves a rapid and reversible redox couple that shuttles electrons from a bulk electron donor to a contaminant that is transformed by reduction. 

The branch pathway for anthocyanin biosynthesis starts with the enzymatic reduction of dihydrofiavonols to their corresponding flavan 3,4-diols (leucoanthocyanidins) by substrate-specific dihydroflavonol 4-reductases (DFR). Flavan 3,4-diols are the immediate precursors for the synthesis of catechins and proanthocyanidins. Catechins are formed by enzymatic reduction of the flavan 3,4-diols in the presence of NADPH to leucoanthocyanidins, which are subsequently converted to anthocyanidins by the 2-oxoglutarate-dependant dioxygenase, anthocyanidin synthase. Further glycosylation, methylation, and/or acylation of the latter lead to the formation of the more stable, colored anthocyanins (Scheme 1.1). The details of the individual steps involved in flavonoid and isoflavonoid biosynthesis, including the biochemistry and molecular biology of the enzymes involved, have recently appeared in two excellent reviews.7,8... 

Wolthers, Kirsten R., Schimerlik, Michael I. (2002) Neuronal nitric oxide synthase substrate and solvent kinetic isotope effects on the steady-state kinetic parameters for the reduction of 2, 6-dichloroindophenol and cytochrome C3+, Biochemistry 41, 196-204. 

Scheme 2 Proposed mechanism for bicarbonate-mediated peroxidation in SOD. After Cu(II) reduction (i), bicarbonate binds to the anion-binding site (ii). In (iii), HOO" reacts with bicarbonate to form peroxycarbonate (iv). In (v), the oxygen radical species formed [O ] may oxidize endogenous or exogenous substrates. This leaves bicarbonate bound to the anion-binding site (vi). (Ref 17. Reproduced by permission of American Society for Biochemistry Molecular Biology)...

Although it was that simple to derive the rule of three functions in biochemistry in general, this modification cannot be calculated as straightforwardly because the reduction of autocatalytic order due to oligomerization depends sensitively on (bio-)chemical network topology. In addition to topological aspects of feedback in the network, the exact kind (manner) of dimerization which interconnects identical metal ions needs to be known, likewise the way by which the substrate interacts with this multimetal center (for example, the Mo dimerization effect is also seen with Mo-dependent nitrogenases but and the other reduc-... 

T.N. Bondar , V.Z. Lankin and V.L. Antonovskii, The reduction of organic hydroperoxides by glutathione peroxidase and glutathione S-transferase the influence of substrate structure, Dokl.Akad.Nauk SSSR 304 (1989) 217-220 (English translation in Doklady Biochemistry). 

Moebitz, H., Boll, M. A Birch-like Mechanism in Enzymatic Benzoyl-CoA Reduction A Kinetic Study of Substrate Analogues Combined with an ab Initio Model. Biochemistry 2002,41,1752-1758. 

Fig. 1.5 Nicotinamide adenine dinucleotide (NAD+) and its phosphorylated analog (NADP+). The difference is indicated in purple. NAD+ and NADP+ undergo reduction to NADH and NADPH by accepting a hydride ion and two electrons and releasing a proton from an oxidized substrate. (Adapted from Fig. 14-13 in Berg JM, Tymoczko JL and Stryer L. Biochemistry, 5th Ed. 2002. W.H. Freeman Co., New York)...

Figure 4-14. Catalytic voltammetry of nitrite reductase is critically dependent on the identity and concentration of the substrate at pH 7. (A) In 1 pM nitrite, reduction of the enzyme successively, turns on, but then attenuates activity. (B) Overlaid derivatives emphasise changes in the waveshape as the nitrite concentration is increased from 1.7 to 265 pM. At higher nitrite concentrations, activity increases upon application of a more negative potential, and this is reflected by the two positive features in the derivative plot. (C) At 1 mM hydroxylamine, reduction of the enzyme does not attenuate the rate of catalysis. (D) Overlaid derivatives show that as the hydroxylamine concentration is raised from 1.1 to 347 mM the waveshape develops two positive features similar to the waveshapes displayed at high rates of nitrite reduction. Reproduced from ref. 70 with permission of the American Society for Biochemistry and Molecular Biology.

Guengerich, EP. and W.W. Johnson (1997). Kinetics of ferric cytochrome P450 reduction by NADPH-cytochrome P450 reductase Rapid reduction in the absence of substrate and variations among cytochrome P450 systems. Biochemistry 36, 14741-14750. 

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