From Cellular Function to Industrial Products
29
2.1.3.3 Fermentative Pathways
When the oxidative phosphorylation is inactive (due to the absence of oxygen or lack of some of
the necessary proteins), pyruvate is not oxidized in the TCA cycle since that would lead to an
accumulation of NADH inside the cells. In this situation NADH is oxidized with simultaneous
reduction of pyruvate to acetate, lactic acid, or ethanol. These processes are collectively called
fermentative metabolism.
Fermentative metabolism is not the same in all microorganisms, but there
are many similarities.
Bacteria can regenerate all NAD+ by reduction of pyruvate to lactic acid (reaction (1) in Fig. 2.6A
and B). They can also regenerate all NAD+ by formation of ethanol in the so-called mixed acid
fermentation pathway for which the entry point is the compound Acetyl-CoA. CoA is a cofactor
A
| ----- Formic add
NADH NAD*
Pyruvate -
^
Lactic acid
i
1
M
)
B
|
(3)
Formic add
COj + Hj
Acetyl-CoA
NADH NAD*
Pyruvate
Lactic acid
(3)
,
NAD*
(D
NADH
(2)
CO,
Acetyl-CoA
NADH
Acetyl-P
ADP
ATP
(5)
Acetic acid
Acetaldehyde
(7) | r
NADH
V* NAD*
Ethanol
Acetyl-P
ADP
31 (5)
ATP * *
Acetic acid
Acetaldehyde
<7)iC
**DH
NAD*
Ethanol
NADH NAD*
Acetaldehyde —
Ethano
l
L
NADfP)*
IS i
(9) ^
(10)
+*»
NAD(P)H
Acetyl-CoA ^
— Acetic acid
AMP ATP
Figure 2.6 Different fermentative pathways for reduction of pyruvate. Only the main fermentative
pathways are shown. The enzymes are: (1) lactic acid dehydrogenase; (2) pyruvate dehydrogenase; (3)
pyruvate-formate lyase; (4) phosphate acetyltransferase; (5) acetate kinase; (
6
) and (9) acetaldehyde
dehydrogenase; (7) alcohol dehydrogenase; (
8
) pyruvate decarboxylase; (10) acetyl-CoA synthase.
A. The fermentative (or mixed acid) metabolism of
Escherichia coli.
B. The fermentative metabolism of lactic acid bacteria.
C. The fermentative metabolism in the yeast
Saccharomyces cerevisiae.
Not all reactions occur in the same
compartment, i.e. the pyruvate dehydrogenase catalyzed conversion of pyruvate to acetyl-CoA occurs in the
mitochondrion whereas the other reactions occur in the cytosol.
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