Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

430 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Table 20.5
mediator
The ideal characteristics and properties of an electron
1. Exhibits reversible kinetics
2. Reacts readily with the reduced form of an enzyme
3. Has a low oxidation potential and is pH independent
4. Is stable in both redox forms
5. Easily retained at the surface of an electrode
6. Unreactive toward oxygen
7. Chemically unreactive with the immobilized biological material
mediator. Mediators are usually low molecular redox couples that shuttle the electrons from
the enzyme’s active site to the surface of the transducer.
H 2 O 2 + 2H + + 2e − −→ 2H 2 O (20.1)
Mediator red −→ Mediator ox + e − (20.2)
Equations (20.3)–(20.6) illustrate a peroxidase-mediated reaction on a biosensor surface.
Equation (20.3) shows the resting state peroxidase (in its reduced form) reacting with
hydrogen peroxide and two hydrogen ions to form an intermediary oxidized-peroxidase
compound and two water molecules. Equation (20.4) shows the reaction of the intermediary
oxidized-peroxidase compound with the reduced form of a mediator to produce the resting
reduced-state peroxidase and an oxidized mediator. Equation (20.5) shows the oxidized
mediator reacting with two electrons, thus reverting to the reduced form of the mediator.
It is these electrons that the transducer detects and converts into an electrical signal. The
final Eq. (20.6) shows the overall reaction, whereby one hydrogen peroxide molecule in
the presence of two electrons and two hydrogen ions is converted to two water molecules
(Ryan et al., 2006).
H 2 O 2 + 2H + + PO red −→ PO ox + 2H 2 O (20.3)
PO ox + Mediator red −→ PO red + Mediator ox (20.4)
Mediator ox + 2e − −→ Mediator red (20.5)
H 2 O 2 + 2e − + 2H + −→ 2H 2 O (20.6)
It is often possible to use a multienzyme electrochemical biosensor system to detect the
presence and determine the concentration of a particular compound in a material. This
method has two or more enzymes in proximity with each other and in contact with a
mediator or directly in contact with the transducer. As seen in Fig. 20.9, enzyme 1 (glucose
oxidase) generates H 2 O 2 as a byproduct by the catalysis of glucose by glucose oxidase. The
H 2 O 2 generated is catalyzed by a peroxidase enzyme and the redox cycling of the mediator.
The cycling of the peroxidase enzyme in the presence of H 2 O 2 generates electrons that are