Ch2oh

D-Gluconate

D-Glucose + O2

glucose oxidase

> D-Glucono-S-lactone+ H2O2

FIGURE 7-10 Sugars as reducing agents. (a) Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling's reaction. The cuprous ion (Cu+) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemi-acetal (ring) form, C-1 of glucose cannot be oxidized by Cu2 + . However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion. The reaction with Cu2+ is not as simple as the equation here implies; in addition to d-gluconate, a number of shorter-chain acids are produced by the fragmentation of glucose. (b) Blood glucose concentration is commonly determined by measuring the amount of H2O2 produced in the reaction catalyzed by glucose oxidase. In the reaction mixture, a second enzyme, peroxidase, catalyzes reaction of the H2O2 with a colorless compound to produce a colored compound, the amount of which is then measured spectrophotometrically.

betes mellitus. Now, more sensitive methods for measuring blood glucose employ an enzyme, glucose oxidase (Fig. 7-10b). ■

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