Biomedical Engineering Reference
In-Depth Information
Bacteria can regenerate all NAD
þ
by reduction of pyruvate to lactic acid in
Fig. 10.28
a
and b. They can also regenerate all NAD
þ
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 with a free
SH group that can be acetylated to CH
3
CO-S- as illustrated in
Fig. 10.23
, either directly from acetate or by capturing two of the carbon atoms of pyruvate
with the last carbon atom liberated as carbon dioxide or formic acid, HCOOH. Lactic acid
bacteria (
Fig. 10.28
b) have both pathways for conversion of one pyruvate to acetyl-CoA,
whereas E. coli only has the pyruvate formate lyase catalyzed reaction. In yeast,
the fermentative pathway does not proceed via acetyl-CoA but instead by decarboxylation
of pyruvate to acetaldehyde. From acetate, cytosolic acetyl-CoA may be synthesized, and
this serves as precursor for fatty acid biosynthesis, whereas the mitocondrial acetyl-CoA
(a)
NADH NAD
+
Lactic acid
Pyruvate
Formic acid
H
2
+ CO
2
NADH
Acetyl-CoA
NAD
+
NADH NAD
+
ATP ADP
Ethanol
Acetaldehyde
Acetic acid
Acetyl-P
(b)
NADH NAD
+
Lactic acid
Pyruvate
NAD
+
NADH
CO
2
Formic acid
H
2
+ CO
2
NADH
Acetyl-CoA
NAD
+
NADH NAD
+
ATP ADP
Acetaldehyde
Ethanol
Acetic acid
Acetyl-P
(c)
NADH NAD
+
CO
2
Pyruvate
Ethanol
Acetaldehyde
NAD
+
NADH
CO
2
NAD(P)
+
NAD(P)H
AMP ATP
Acetyl-CoA
Acetic acid
FIGURE 10.28
Different major fermentative pathways for reduction of pyruvate. (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 S. cerevisiae. Not all reactions occur in the same compartment, i.e. the pyruvate dehydro-
genase catalyzed conversion of pyruvate to acetyl-CoA occurs in the mitochondrion whereas the other reactions
occur in the cytosol.
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