Redox cofactors

Figure 18.5 Plausible sequence of steps responsible for rapid and selective reduction of O2 to H2O by mixed-valence CcO. The square frames signify the catalytic site (Fig. 18.4c) imidazole ligation of Cub is omitted for clarity in some or aU intermediates, Cub may additionally be ligated by an exogenous ligand, such as H2O (in Cu ) or OH (in Cu ) such ligation is not established, and hence is omitted in all but compound Pm and the putative hydroperoxo intermediate. The dashed frames signify the noncatalytic redox cofactors. Typically used phenomenological names of the spectroscopically observed intermediates (compounds A, E, H, etc.) are also indicated.

Berks, B. (1996). A common export pathway for proteins binding complex redox cofactors Mol. Microbiol. 22, 393-404. 

By means of this reaction, the use of the costly and unstable natural redox cofactor reduced nicotinamide adenine dinucleotide phosphate (NADPH) was circumvented and the reactions were carried out in a straightforward procedure in a chemical laboratory (Scheme 10.2, Table 10.1). 

The oxidation/reduction of redox cofactors in biological systems is often coupled to proton binding/release either at the cofactor itself or at local amino acid residues, which provides the basic mechanochem-ical part of a proton pump such as that foimd in cytochrome c oxidase (95). Despite a thermodynamic cycle that provides that coupling of protonation of amino acids to the reduction process will result in a 60 mV/pH decrease unit in the reduction potential per proton boimd between the pAa values in the Fe(III) and Fe(II) states, the essential pumping of protons in the respiratory complexes has yet to be localized within their three-dimensional structures. 

In the cell, redox reactions are catalyzed by enzymes, which work together with soluble or bound redox cofactors. 

One excellent example of a Emax-type allosteric enzyme is Escherichia coli phosphoglycerate dehydrogenase (PGDH), a tetramer of identical subunits that catalyzes the formation of D-3-phosphohydroxypyruvate from D-3-phosphoglycerate in a reaction that uses NAD+ as a redox cofactor. This regulatory enzyme is allosteri-cally controlled by serine. All available information suggests that the effects on the for substrate are minor... 

Copper-containing amine oxidases (non-blue copper proteins) catalyze the oxidative deamination of primary amines to the corresponding aldehydes with the release of ammonia and concomitant reduction of oxygen to hydrogen peroxide. They typically use a quinone redox cofactor [topaquinone (TPQ)], which is bound covalently in the active site, and are thought to form a Cu(I)-TPQ semi-quinone radical intermediate during the redox reaction [13]. 

Amine oxidases catalyze the oxidative deamination of both xenobiotic and biogenic amines, and thus have many critical biological functions. Two distinct classes differ in the nature of their prosthetic groups [1]. The flavin-(FAD flavin adenine dinucleotide)-dependent amine oxidases include monoamine oxidases (MAO A and B) and polyamine oxidases. Amine oxidases not containing FAD, the so-called semicarbazide-sensitive amine oxidases (SSAO), include both plasma amine oxidases and tissue amine oxidases. These contain quinonoid structures as redox cofactors that are derived from posttranslationally modified tyrosine or tryptophan side chains, topaoquinone frequently playing this role [2]. 

In studies of analogs of the redox cofactor pyrroloquinoline quinone (PQQ), synthetic efforts have focused initially on isosteric, isomeric structures that reflect on important mechanisms of electron-transfer catalysis mediated by PQQ. These studies provide insight into the choice of PQQ as an electron-transfer catalyst in nature, and bear directly on pharmaceutical applications of this vitamin-like nutritional factor. 

Nature s design of PQQ was not frivolous. Even subtle changes in the structure of PQQ can result in an alternative redox cofactor with little utility in any cell. 

List four molecules that have this function. You should name the additional pathway where each of these is found and briefly describe what it does in this second pathway. Do not use a redox cofactor as one of your choices. 

Chloroplasts with redox cofactor 10 mL of chloroplast suspension... 

Compare the relative rates of proton uptake for each experimental condition and explain any differences. Did the addition of light-sensitive redox cofactor affect the rate of pH shift What is the effect of the addition of... 

Explain the effect, if any, of the redox cofactor on the rate of proton uptake. 

NADP+ differs from NAD+ only by phosphorylation of the C-2 OH group on the adenosyl moiety. The redox potentials differ only by about 5 mV. Why do you suppose it is necessary for the cell to employ two such similar redox cofactors Thiamine-pyrophosphate-dependent enzymes catalyze the reactions shown below. Write a chemical mechanism that shows the catalytic role of the coenzyme, (a) O O... 

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