Biomedical Engineering Reference
In-Depth Information
In addition, calculations of heats of combustion (kilocalorie [Kcal]/g-carbon) of various
substrates and associated cell yields (g-biomass/g-carbon) indicate that MTBE and TBA
support the growth of only 20-30% of the yields measured when using high energy substrates.
Recent calculations by M¨ller et al. (
2007
) accounting for half-maximum substrate concentra-
tions (K
s
), maintenance and decay coefficients, and the likely metabolic pathway for MTBE,
indicate that cell yields should be 0.87 g-cells/g-MTBE utilized, significantly higher than the
values actually measured.
Aerobic MTBE oxidation is thought to proceed via an unstable hemiacetal (Figure
10.2
) that
can be further oxidized to
tertiary
-butyl formate (TBF) or that decomposes abiotically to TBA
(Steffan et al.,
1997
; Fran¸ois et al.,
2002
; Salanitro,
2002
; Fiorenza and Rifai,
2003
; Fayolle
et al.,
2003
;M
¨
ller et al.,
2007
). TBA can be further degraded to form 2-hydroxyisobutyrate
(HIBA). TBA and HIBA metabolism, as well as initial ether-cleavage to TBA, represent the
main enzymatic impediments to the ready metabolism of MTBE. Thus, these steps are likely
responsible for the low cell yields in aerobic systems, as well as the difficulty that has been
experienced in demonstrating metabolism of MTBE beyond TBA in both anaerobic enrichment
cultures and in environmental samples adjusted to low redox potentials. Although the actual
reason for this biochemical enigma has not been determined, two mechanisms have been
proposed: (1) microbial dehydrogenases and oxygenases have weak affinities for ethers and
alcohols with tertiary-carbon structures (MTBE and TBA), and (2) low-level activation of HIBA
by a coenzyme A system or slow oxidative attack by an unknown enzyme or free-radical
mechanism (M¨ller et al.,
2007
) may be needed for the transformation of HIBA to isopropanol
or acetone.
H
2
O
O
2
CH
3
O
CH
3
CH
3
O
CH
3
OH
CH
3
O
CH
3
O
H
3
C
C
CH
2
H
3
C
C
CH
3
H
3
C
C
CH
tert
-butoxy methanol
2[H]
Methyl
tert-
butyl ether (MTBE)
tert
-butyl formate (TBF)
OH-
CH
3
OH
CH
3
O
H
C
H
3
C
C
CO
2
O
-
Formate
O
2
H
2
O
CH
3
CH
2
OH
H
3
C
C
OH
CH
3
CH
3
O
CO
2
H
3
C
C
CH
3
H
3
C
CH
H
3
C
C
C
2-methyl-2-hydroxy-1-propanol
OH
O
OH
OH
Acetone
Isopropanol
2-hydroxyisobutyric acid
(HIBA)
CO
2
Figure 10.2. Proposed aerobic MTBE biodegradation pathway in Mycobacterium austroafricanum
IFP 2012 (modified from Fayolle et al.,
2003
).
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