Environmental Engineering Reference
In-Depth Information
the ways in which biological catalysts function in demanding environments and the
way that pairs of redox enzymes function together as they do in biology.
As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts
at ambient temperatures and mild pH, and have significantly higher reaction selectivity
than precious metals. The main disadvantage in applying redox enzymes for electro-
catalysis arises from their large size, which means that the catalytic active site density
is low. Enzymes also have a relatively short lifetime (usually not more than a few
months), making them more suited to disposable applications.
Enzymes such as glucose oxidase, alcohol dehydrogenases, and laccase are
commercially available in reasonably pure form. Other enzymes, such as hydrogenase,
are technically difficult to isolate [Cammack et al., 2001]. On a laboratory
scale, biocatalysts are still expensive (probably more so than noble metals) owing
to the cost of labor for small scale production. However, microorganisms can be
grown on cheap carbon/nitrogen sources, and costs decrease dramatically for large
scale production. Increasingly it is becoming possible to genetically manipulate
readily grown organisms such as E. coli or yeasts to express the enzymes of other
organisms in high yield and with affinity tags genetically attached for one-step puri-
fication. Importantly, biocatalysts are indefinitely renewable.
The protein environment around the buried active site in an enzyme must
provide routes for electron transfer to or from the surface and for mass transport of sub-
strate and product. These features are highlighted on the structure of laccase from the
white rot fungus, Trametes versicolor, in Fig. 17.3 (Plate 17.1) [Bertrand et al., 2002].
Fungal laccases are involved in pigment formation, detoxification, and lignin degra-
dation, using O
2
as an electron acceptor in the oxidation of a range of organic
substrates [Solomon et al., 1996]. The presence of a co-crystallized 2,5-xylidene mol-
ecule in a wide, hydrophobic pocket at the surface of T. versicolor laccase provides
strong evidence that this is the binding site for organic substrates. The pocket is
close to the mononuclear “blue” Cu center, and thus provides a route for sequential
transfer of electrons to the site of O
2
reduction, the trinuclear Cu cluster located
approximately 1.3 nm away. This distance is sufficiently short to allow fast electron
tunneling, and is typical of the spacing between electron transfer relay centers in pro-
teins [Page et al., 1999]. The redox centers are buried in the protein, ensuring that the
environment of the metals is precisely tuned by the amino acids that ligate or surround
them. Two solvent accessible channels leading to the trinuclear center are evident
from the presence of water molecules with well-defined electron density in the crystal
structure, and probably provide routes for O
2
entry [Piontek et al., 2002].
Important inherent characteristics of an enzyme that should be considered are the
substrate affinity, characterized by the Michaelis constant K
M
, the rate of turnover k
cat
,
providing the catalytic efficiency k
cat
/K
M
, and the catalytic potential. Several attempts
to compare enzyme catalysis with that of platinum have been published. Direct compari-
sons are difficult, because enzyme electrodes must be operated in aqueous electrolyte
containing dissolved substrate, whereas precious metal electrodes are often supplied
with a humidified gaseous stream of fuel or oxidant, and produce water as steam. It is
not straightforward to compare true optimal turnover rates per active site, as it is often
unclear how many active sites are being engaged in a film of enzyme on an electrode.
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