Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 555

current were A

in presence of camphor, respectively. Under anaerobic conditions addition of 3 mM
ethanolic solution of camphor produced a catalytic response with a cathodic peak
potential of

375 mV and 44 µ A in the absence of camphor and 350 mV and 50 µ

A. This indicated a typical
fast reversible electrochemistry of heme Fe 3 /2 redox species coupled to a subsequent
process, such as hydroxylation of camphor or H 2 O 2 production by uncoupled turnover.
The authors therefore claimed that even in degassed buffer the oxygen in the aerobic
ethanolic camphor solution was suffi cient for the reaction to take place.
Cytochrome CYP101 immobilized with polyion can successfully catalyze the oxi-
dation of styrene [224-225]. GC-MS product analysis (styrene oxide) showed a turno-
ver number of 9.3 h 1 , which is larger than the turnover number when CYP101 was in
solution, which is 0.35 h 1 . Catalysis of styrene as well as benzaldehyde oxidation has
also been observed by protein polyion/CYP101-modifi ed carbon cloth (CC) electrodes
[226]. The formal potential was the same as that reported with CYP101 multilayer-
modifi ed Au electrode [224], but the turnover number for the catalysis of styrene was
slightly lower at 7.2 h 1 . Conversion of styrene-to-styrene oxide has also been studied
with a biosensor based on CYP1A2 [227]. Cytochrome CYP1A2 is the main enzyme
that metabolizes caffeine but it is also relatively active in converting styrene, although
not as active as CYP2E1 and CYP2B6. In addition to CYP101 and CYP1A2, CYP2E1
has also been studied with phospholipid-modifi ed GC and Au electrodes. Reversible
electrochemical response was observed both with DDAB and PDDA-modifi ed GC
electrodes.
Besides catalyzing styrene and benzaldehyde, CYP enzymes play an important role
in the metabolism of endogenous compounds as well as in pharmacokinetics and toxi-
cokinetics. Joseph [228] developed a biosensor with human CYP3A4 as a novel drug-
screening tool. It was constructed by assembling enzyme fi lms on Au electrodes by
alternate adsorption of a layer of CYP3A4 on top of a layer of PDDA. The biosensor
was applied to detect verapamil, midazolam, quinidine, and progesterone.

430 mV and a cathodic peak current of 14 µ

17.3.1.3 Biosensors based on direct electron transfer of myoglobin
Myoglobin is a kind of heme protein containing a single polypeptide chain with an
iron heme as its prosthetic group. Although Mb does not function in biological elec-
tron-transfer chains, they are designated honorary enzymes due to their peroxidase and
cytochrome P450 catalytic activities [229]. The main physiological function of Mb is
to store dioxygen and increase the diffusion rate of dioxygen in the cell. The ferrous
heme iron in Mb can bind many diatomic molecules, such as oxygen, carbon monox-
ide, NO, H 2 O 2 , and so on. So the biosensors based on the direct electron transfer of
Mb are often used to detect these molecules.
The biomembrane-like fi lms can provide a favorable microenvironment for proteins
and enhance direct electron-transfer rate between proteins and electrodes, so many
biopolymers such as methyl cellulose (MC) and dihexadecylphospate (DHP) have
been used to immobilize Mb and make biosensors [230-232]. The electrochemical
catalytic reduction of oxygen by the Mb-MC/EPG was examined by CVs. Mb-MC