Cytochrome lactate dehydrogenase electrode

In a sensor for lactate a bienzyme system composed of cytochrome 62 for lactate oxidation to pyruvate, and lactate dehydrogenase for conversion of pyruvate back to lactate has been used [321]. Hexacyanoferrate(III) served as electron acceptor for cytochrome b2- The reduced mediator was reoxidized at the electrode, thus giving a measuring signal depending on the analyte concentration. Attempts to determine both substrates of the recycling system have shown that, at tenfold amplification for lactate, the sensitivities for lactate and pyruvate are almost identical. The same recycling scheme has also been used in connection with Fe-EDTA as electron mediator in place of hexacyanoferrate(III) [336]. 

Since mitochondrial cytochrome c was available commercially (horse heart muscle being the most common source) and could readily be purified to a high level, it formed the basic subject for most of the pioneering studies. Many ideas concerning the electrochemical mechanism, in particular, the mode of interaction with the electrode, have developed around the considerable wealth of information that is available [14, 18] on the structure and properties of the protein molecule. The extent to which the metal centre is buried is illustrated well in Fig. 1 which shows the 3D structure [19] of yeast (iso-1) cytochrome c and a view of the exposed active site. The major function of cytochrome c is as electron donor to cytochrome c oxidase (Complex IV), the membrane-bound enzyme that is the terminus of the aerobic respiratory chain and a site for proton translocation. Another physiological oxidant of cytochrome c (in yeasts) is cytochrome c peroxidase, a soluble enzyme whose crystal structure is known (see Sect. 7). The most important reduc-tant of cytochrome c is the cytochrome Cj component of the membrane-bound hcj complex (Complex III), but others (see Sect. 6, Scheme 5) include cytochrome b, sulfite oxidase, and flavocytochrome (lactate dehydrogenase, found in yeasts). 

Lactate/pyruvate Cytochrome b2/lactate dehydrogenase Pt-electrode... 

Fig. 16 Schematic coupling model of the photo-switchable interactions between cytochrome c and the mixed SAM of spiropyran/merocyanine-terminated and 4-pyridine thiol with (a) the reduction of O2 by COx and (c) the oxidation of lactate by lactate dehydrogenase (LDH). (b) When the electrode is in the cationic merocyanine state, repulsive interactions disallow the functioning of the bioelectrocatalytic processes [130].

Lactate is a small biological molecule that functions as metabolite in the mitochondria and a precursor to pyruvate in the citric acid cycle [87]. In 1997, Bardea et al. developed a lactate BFC using NAD -dependent lactate dehydrogenase (LDH) [88]. They introduced a new method for enzyme immobilization that enabled better oxidation of the substrate and allowed the enzyme to have eleetrieal eontact with the electrode. Covalently linked PQQ and native NAD" form a monolayer on gold electrodes to induce affinity interactions with cross-linked NAD -dependent LDH. In 2001, Katz et al. further improved on the concept of this anode, coupling it with a cytochrome c oxidase cathode to produce a self-powered biosensor that is active only in the presence of the anode s substrate, lactate [89]. With this addition, the cell is completely dependent on substrate for voltage and current output, which is ideal for BFCs. 

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