Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 500

isozymes (E 2 ), while the other enzyme (E 0 2 ) is fully active. Sufficient activity remains through

isozyme E 0 2

to ensure adequate synthesis of P 2 .
An alternative approach is concerted feedback inhibition. Here, a single enzyme with two
allosteric binding sites (for P 1 and P 2 ) controls entry into the pathway. A high level of either
P 1 or P 2 is not sufficient by itself to inhibit enzyme E 2 , while a high level of both P 1 and P 2 will
result in full inhibition.
A third possibility is sequential feedback inhibition, by which an intermediate at the branch
point can accumulate and act as the inhibitor of metabolic flux into the pathway. High levels
of P 1 and P 2 inhibit enzymes E 4 and E 5 , respectively. If either E 4 or E 5 is blocked, M 3 will
accumulate, but not as rapidly as when both E 4 or E 5 are blocked. Thus, intermediate
flux levels are allowed if either P 1 or P 2 is high, but the pathway is inactivated if both P 1
and P 2 are high.
Other effects are possible in more complex pathways. A single allosteric enzyme may have
effector sites for several end products of a pathway; each effector causes only partial inhibi-
tion. Full inhibition is a cumulative effect, and such control is called cumulative feedback inhi-
bition or cooperative feedback inhibition. In other cases, effectors from related pathways may also
act as activators. Typically, this situation occurs when the product of one pathway was the
substrate for another pathway.
One may wonder what are the differences between feedback inhibition and repression.
Inhibition occurs at the enzyme level and is rapid; repression occurs at the genetic level
and is slower and more difficult to reverse. In bacteria where growth rates are high,
unwanted enzymes are diluted out by growth. Would such a strategy work for higher cells
in differentiated structures? Clearly not, since growth rates would be nearly zero. In higher
cells (animals and plants), the control of enzyme levels is done primarily through the control
of protein degradation rather than at the level of synthesis. Most of our discussion has
centered on prokaryotes; the extension of these concepts to higher organisms must be
done carefully.
Another caution is that the control strategy that one organism adopts for a particular
pathway may differ greatly from that adopted by even a closely related organism with an
identical pathway. Even if an industrial organism is closely related to a well-studied
organism, it is prudent to check whether the same regulatory strategy has been adopted
by both organisms. Knowing the cellular regulatory strategy facilitates choosing optimal
fermenter operating strategy, as well as guiding strain improvement programs.
We have touched on some aspects of cellular metabolic regulation. A related form of regu-
lation that we are just now beginning to appreciate has to do with the cell surface.

Example 10-2. Sequential feedback control of branched pathways.
1. Derive a rate expression for the production of P 1 and consumption of M 1 for sequential
feedback as shown in Fig. 10.15 c. Assume PSSH applies.
2. Discuss what happens if the concentration of P 1 is high in the cell, and if both P 1 and P 2 are
high in the cell.
3. If experimental data were collected for the variations of concentrations of M 1 ,P 1 , and P 2
with time, what reaction rate expression(s) you would recommend based on this analysis
to analyze the measured data?