The commercial production of citrate by Aspergillus niger is, to some
extent, an aerobic process; oxygen is required to carry out the net
oxidation necessary to convert glucose to citrate. Most of the carbons
of glucose, though, end up as citrate, and, in that respect, citrate
production resembles a fermentation.
The conversion of glucose to citrate is called a fermentation, but
this is using fermentation in an extended sense. In addition to the
strict definition of a fermentation as an anaerobic process, Wikipedia
also states that
"Fermentation is also used much more broadly to refer to the bulk
growth of microorganisms on a growth medium. No distinction is made
between aerobic and anaerobic metabolism when the word is used in this
sense."
http://en.wikipedia.org/wiki/Fermentation
There is an article that is explicit on the role of oxygen; it states:
"The metabolic pathway is known, as are the fermentation conditions
that result in high yields (approximately 200 g/L of citric acid from
240 g/L of glucose or sucrose) in submerged culture. The critical
parameters for citric acid production by A. niger were defined
empirically and include: high carbohydrate concentration, low but
finite manganese concentrations (~10ppb), maintenance of high
dissolved oxygen, constant agitation,and low pH (Schreferl 1986; Zhang
and Ro?hr,2002a,b)."
The above is from section 3.1.1 of
http://www.pnl.gov/biobased/docs/organic_acids_ch_12.pdf
Most of the glucose is converted to citrate, yet this differs from
true anaerobic fermentations in that some oxidation by an outside
agent is necessary because there is a net oxidation even though 6
carbons of glucose result in 6 carbons of citrate.
Figure 12.1 later in section 3.1.1 shows the overall conversion of
glucose to citric acid, but this is labeled as simplified. The figure
does not explicitly show how the conversion differs from an anaerobic
fermentation, such as the glucose -> 2 lactate fermentation. In the
lactate fermentation, there is no net oxidation; part of the glucose
molecule is oxidized and part is reduced. Missing from the simplified
scheme in figure 12.1 is the net production of NADH that somehow needs
to oxidized back to the starting NAD+. In essence, figure 12.1 really
shows just carbon atom balance and is not concerned with the oxidation
changes.
The need for oxidation is as follows: Figure 12.1 shows glucose going
to 2 pyruvate as an initial change; this is one step short of going to
lactate. In the lactate fermentation, the last step is the conversion
of pyruvate to lactate, which oxidizes the reduced NADH generated in
glycolysis back to the original NAD+. This oxidation of NADH does not
happen in the glucose to citrate conversion; in this situation, the
pyruvate is used to make citrate, and the NADH needs to be oxidized
back to NAD+ using oxygen.
In the glucose to citrate conversion, the two pyruvates go on to make
one citrate - one pyruvate is further oxidized to acetyl-CoA
(generating another NADH that needs to be oxidized by oxygen) and one
pyruvate is carboxylated to oxaloacetate. These then are combined to
form citrate.
The use of oxygen is mentioned later in section 3.1.1 (starting on the
bottom of page 318):
"The alternative oxidase (AOX) is an inducible component of the
alternative respiratory pathway in fungi. In A. niger, active AOX is
necessary for citric acid production and is another example of a
component of A. niger that is sensitive to the presence of manganese
ions (Kubicek et al., 1980; Zehentgruber et al., 1980; Kirimura et
al., 2000). In addition, AOX is apparently inactivated at low
dissolved oxygen concentrations (Zehentgruber et al., 1980). This is
one component of the observed physiological requirement for
undisrupted oxygen supply to maintain citric acid production. Although
the protein appears to be constitutively expressed in A. niger,
expression is increased on transition to the citric acid production
phase (Kirimura et al., 1987). The AOX has the desirable effect of
regenerating the intracellular pool of NAD |
