Biochemical Reaction Networks
131
Figure 5.3 Batch ferm entation o f
L. cremoris
on a m edium containing glucose and a com plex nitrogen
source.
A. M easurements of the biom ass concentration ( • ), the specific acid production rate ( ■ ), and the specific
growth rate (■*■).
B.
Plot o f the specific acid production rate versus the specific grow th rate.
B>0.3 h 1
f xATP = 15 m m oles A T P (g DWy'^ATP = 18 m m oles ATP (g D W h ) 1
B<0.3 h 1
TxATP = 50 m m oles A T P (g D W ) 'm ATP = 7 m m oles A TP (g D W h)’1
In the first grow th phase, TxATP is very low and
mAIP
is high. H ere the m edium is very rich; i.e., all building
blocks are supplied from the m edium , and the cell therefore uses A TP m ainly for polym erization reactions.
In the second grow th phase, TxATP is higher and
mATP
is low. H ere one (or m ore) com pound(s) in the com plex
nitrogen source has been exhausted, and the cells have to synthesize this (or these) com pound(s). The ATP
requirement for grow th is therefore higher. W ith the higher A T P requirem ent for cellular grow th, the cell is
apparently able to get som e savings by reducing the A T P consum ption in m aintenance processes; i.e., it is
able to adjust its behavior to the m ore hostile environm ent.
It has not been possible to identify the (probably N -containing) com pound(s) w hich cause(s) the change in
metabolism at about 7 hours, but analysis o f the m edium throughout the ferm entation show ed that m ainly
peptides are m etabolized during the first grow th phase, w hereas both peptides and free am ino acids are
metabolized in the second grow th phase.
L. cremoris
has the ability to take up peptides, and it norm ally
grows better on a m edium containing peptides than on a m edium that contains only free am ino acids. The
first shift in m etabolism m ay be explained by exhaustion o f sm all peptides, w hich m ay support a very rapid
growth.
Based on average m acrom olecular com position o f
L. cremoris,
Benthin (1992) calculated that the
theoretical A T P requirem ent for cellular synthesis w ith grow th on a com plex m edium is 26 m m ole o f ATP
per gram dry w eight. It is observed that during rapid grow th this value is higher than the experim entally
found TxATP, and this can be explained only by the ability o f the cell to extract free energy from the com plex
nitrogen source. A t the end o f the ferm entation the experim entally found TxATP is alm ost tw ice as high. Here
the m aintenance processes becom e relatively expensive com pared to the grow th process, as was discussed
in Note 5.2.
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