Nucleotides, catalytic oxidation

The ALDs are a subset of the superfamily of medium-chain dehydrogenases/reductases (MDR). They are widely distributed, cytosolic, zinc-containing enzymes that utilize the pyridine nucleotide [NAD(P)+] as the catalytic cofactor to reversibly catalyze the oxidation of alcohols to aldehydes in a variety of substrates. Both endobiotic and xenobiotic alcohols can serve as substrates. Examples include (72) ethanol, retinol, other aliphatic alcohols, lipid peroxidation products, and hydroxysteroids (73). 

The catalytic significance of this observation is not known since no deviation from a two-electron Nemst plot is observed with NADH as reductant and no kinetic studies have been done to compare the rate of the NAD -facilitated comproportionation reaction with the rate of catalytic turnover. No comparable studies on the effect of NADP on the oxidation-reduction potential of ferredoxin-NADP reductase have been, to our knowledge, published. Inasmuch as the physiological role for this enzyme is reduction of the pyridine nucleotide rather than its oxidation, the potential of the enzyme should be significantly lower than that of the pyridine nucleotide couple. Indeed, a value of —445 mV has been determined for this flavoenzyme with the driving force for its reduction being due to a decrease of 90 mV in the one-electron potential of the ferredoxin reductant. This increase... 

A mechanism has been proposed for NADH-cytochrome bs reductase based on these elegant studies (Fig. 17) (308, 355). The hydrogen which is stereospeciflcally and directly transferred has been printed in boldface. Catalysis proceeds in a clockwise direction, and after the first catalytic cycle the dissociation of oxidized pyridine nucleotide and the association of reduced pyridine nucleotide are shown as a single step since both are very rapid processes and this emphasizes that the thiol is not (kinetically speaking) exposed during catalysis. The interaction of the thiol and the... 

The metabolic function of the flavin coenzymes is as electron carriers in a wide variety of oxidation and reduction reactions central to aU metabolic processes, including the mitochondrial electron transport chain. Unlike the nicotinamide nucleotide coenzymes (Section 8.4.1), which act as cosubstrates, leaving the catalytic site of the enzyme at the end of the reaction, the flavin coenzymes remain bound to the enzyme throughout the catalytic cycle. 

Nucleotide phosphates. Nucleotide H-phosphonates are oxidized to the phosphates in dichloromethane by bis(trimethylsilyl) peroxide in the presence of N.O-bis(triraethylsilyl)acetamide and a catalytic amount of Me,SiOTf. 

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