Biomedical Engineering Reference
In-Depth Information
Organic growth
substrate
CO 2
Intermediate products
(ex: propane, butane)
Metabolism
Monooxygenase
enzyme
Cometabolism
Cl
Cl
Cl
Cl
CC
CC
CO 2
+
Cl -
O 2
Cl
H
Cl
O
TCE epoxide
H
TCE
Figure 8.1. Conceptual model of the cometabolism of TCE by microbes expressing monooxygen-
ase enzymes.
trichoroethene (TCE), by forming a TCE epoxide. The unstable epoxide breaks down and forms
a range of transformation products described by Little et al. ( 1988 ) and Fox et al. ( 1990 ) and
others (see review of Arp et al., 2001 ). In an effective cometabolic system, the end products are
carbon dioxide, water and chloride ion.
Cometabolic transformations do not provide energy or carbon for organism growth, so a
primary substrate must be supplied to stimulate growth of the cometabolizing microorganisms.
In oxidative cometabolism, the microbes usually require the presence of the growth substrate to
induce the monooxygenase enzymes, although there are constitutive systems (i.e., the enzyme is
continuously produced without induction or repression) that do not require the presence of a
growth substrate (Shields and Reagin, 1992 ).
Cometabolic biotransformation is a complex process at the whole cell and enzyme level.
Potential challenges include substrate inhibition, transformation product toxicity and energy
limitations (see detailed reviews of these processes in McCarty [ 1997 ], Arp et al. [ 2001 ] and
Alvarez-Cohen and Speitel [ 2001 ]). Substrate inhibition is a common concern, because the
growth substrate and the CAH must compete for the same enzyme site. This competition leads
to inhibition of the rates of utilization of the growth substrate in the presence of the CAH, and
inhibition of the transformation of the CAH in the presence of the growth substrate.
During transformation reactions, products may form that pose toxic threats to cells or
enzymes, thereby inactivating them. This phenomenon, termed transformation product toxic-
ity, may be assigned one of several parameters to account for cell/enzyme death in mathemati-
cal models. Transformation capacity (T c ) represents one such parameter, defined as the
quantity of a compound that a specific mass of microorganisms can degrade before they are
inactivated by toxicity from transformation products. Units of transformation capacity are
typically mass of degrading substrate per mass of cells (Alvarez-Cohen and Speitel, 2001 ).
When cultures are bioaugmented for their catalytic transformation potential, having a high
transformation capacity is an important parameter (Duba et al., 1996 ; Steffan et al., 1999 ).
Finally, energy is required to sustain the organisms responsible for the desired cometabolic
reactions. Oxygenase enzymes consume molecular oxygen and reductants such as NAD(P)H
(reduced form of nicotinamide adenine dinucleotide phosphate [NADP + ]) during oxidation of
the energy generating and cometabolic substrate (see review of Alvarez-Cohen and Speitel,
[ 2001 ]). In the subsequent metabolism steps the reductant is regenerated. Thus, the rate and
extent of the cometabolic transformation is limited in the absence of growth substrate. Some
aerobic cometabolic cultures can regenerate reductant using alternate energy substrates, such
as formate, or internal storage polymers such as poly- b -hydroxybutyrate (PHB). Duba et al.
 
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