Glucose 6-phosphate-NAD

Although glucose 6-phosphate is common to both pathways, the pentose phosphate pathway is markedly different from glycolysis. Oxidation utilizes NADP rather than NAD, and CO2, which is not produced in glycolysis, is a characteristic product. No ATP is generated in the pentose phosphate pathway, whereas ATP is a major product of glycolysis. 

Homogeneous electrochemical enzyme immunoassays for both phenytoin and digoxin have been developed. In both cases the label was glucose-6-phosphate dehydrogenase, which catalyzes the reduction of NAD to NADH. The NADH produced was detected by LCEC at a carbon paste electrode. 

Nicotinamide adenine dinucleotide (NAD) Fructose 1,6-diphosphate Glucose-6-phosphate Isopentenyl pyrophosphate Ribose-6-phosphate-l-pyrophosphate... 

D-Glucose 6-phosphate is converted enzymically into L-wyo-inositol 1-phosphate (20) in a process which requires NAD+. The base-catalysed cyclization of d-xylo-hexos-5-ulose 6-phosphate (21), followed by reduction with borohydride, leads to (20) and epi-inositol 3-phosphate (22) (Scheme 3).59 This has been put forward as a chemical model for the enzymic synthesis. The phosphorylation of inositols with polyphosphoric acid has been described80 and the p-KVs of inositol hexaphosphate have been determined by 31P n.m.r.61... 

NAD (3-nicotinamidoadenine dinucleotide and 1-malic acid) 3-NAD and glucose-6-phosphate]... 

This enzyme [EC 5.5.1.4] catalyzes the conversion of D-glucose 6-phosphate to iL-myo-inositol 1-phosphate. NAD+ is required as a cofactor. 

Figure 9-7 (A) Effect of glucose and glucose 6-phosphate concentrations on reaction rate of yeast hexokinase at equilibrium. Reaction mixtures contain 1-2.2 mM ATP, and 25.6 mM ADP at pH 6.5. From Fromm et al.51 (B) Effect of lactate and pyruvate concentrations on equilibrium reaction rates of rabbit muscle lactate dehydrogenase. Reaction mixtures contained 1.7 mM NAD+, and 30 - 46 pM NADH in Tris-nitrate buffer, pH 7.9, 25°C. From Silverstein and Boyer.53...

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.

C.-H. Wong and G. M. Whitesides, Enzyme-catalyzed organic synthesis NAD(P)H cofactor regeneration by using glucose-6-phosphate and the glucose-5-phosphate dehydrogenase from Leuconosloc mesenteroides, J. Am. Chem. Soc. 1981, 103, 4890-4899. 

The intermediate, NAD- or NADP-, is a radical on the nicotinamide that can react with [(bpy)3Ru]3+. Any enzyme that produces or consumes either NADH or NADPH can be directly monitored by ECL since only the reduced forms NAD(P)H but not the oxidized forms NAD(P)+ can function as a coreactant [31,49], This difference has been exploited in the clinical chemistry assays of ethanol, glucose, bicarbonate, cholesterol, and glucose-6-phosphate dehydrogenase. 

NAD (P) H quinone oxidoreductase (122) + Glucose-6-phosphate dehydrogenase (123) + Epoxide hydrolase, microsomal (124)... 

The core set of reactions of the pathway oxidize glucose 6-phosphate to ribose 5-phosphate and generate NADPH. Thus, as well as generating NADPH, the pathway has a second important role in converting hexoses into pentoses, in particular ribose 5-phosphate. Ribose 5-phosphate or derivatives of it are required for the synthesis of RNA, DNA, NAD+, flavine adenine dinucleotide (FAD), ATP, coenzyme A (CoA) and other important molecules. Thus the two main products of the pathway are NADPH and ribose 5-phosphate. 

The concept of enzymatic stripping has been extended to glucose-6-phosphate and formate (Tab. 1). The amplification factors are determined by the equilibrium constant and by the difference in the permeability of analyte and initiator. Thus the measurement of lactose is not amplified after accumulation of glucose and subsequent addition of NAD, because the molecular weight of the india-tor (NAD ) is much higher than that of the accumulated species. 

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