The rate-limiting step of glycolysis
All of the pages in this series are in urgent need of updating.
The biochemical principles have not changed, of course, but textbooks have: some of
those that were current when I first prepared these pages in 2000 have appeared in new editions, and
others have ceased to be widely used. New books have appeared that are not discussed. Unfortunately
I do not have easy access to any of the commonly used textbooks, as I work in a research (not teaching)
environment in a country where English is not the everyday working language. I could buy them, of course,
but that would represent rather a large investment for the sake of a few web pages.
Accordingly
I should be grateful if someone would collaborate with me in the revision. If you have access to
all of the textbooks published in English in the past ten years (say 1996 or later)
that are commonly used for teaching biochemistry, and if you would like to
help, please contact me at acornish@ibsm.cnrs-mrs.fr.
Examples of the problem
Lehninger, Nelson and Cox (p. 427) express the general idea that underlies
much of the discussion in textbooks:
In every metabolic pathway there is at least one reaction that, in the cell, is far from equilibrium because of the
relatively low activity of the enzyme that catalyzes it (Fig. 14-16). The rate of this reaction is not limited by
substrate availability, but only by the activity of this enzyme. The reaction is therefore said to be
enzyme-limited, and because its rate limits the rate of the whole reaction sequence, the step is called
the rate-limiting step in the pathway. In general, these rate-limiting steps are very exergonic reactions and are
therefore essentially irreversible under cellular conditions.
Afterwards they single out phosphofructokinase as the rate-limiting enzyme in glycolysis, a
popular choice with other authors, such as Campbell (p. 348):
The phosphorylation of fructose 6-phosphate is highly exergonic and irreversible, and phosphofructokinase,
the enzyme that catalyzes it, is the key enzyme in glycolysis.
Voet and Voet follow essentially the same pattern as above:
The metabolic flux through an entire pathway is determined by its rate-determining step (or steps) which,
by definition, is much slower than the following reaction step(s). (p. 471)
PFK, an elaborately regulated enzyme functioning far from equilibrium, evidently is the major control point for glycolysis
in muscle under most conditions. (p. 472, italics in the original)
However, pyruvate kinase also has its partisans, as on p. 588 of Garrett and Grisham:
The large negative ΔG of this reaction makes pyruvate kinase a suitable target site for regulation of glycolysis.
Stryer hedges his bets:
Phosphofructokinase is the key enzyme in the control of glycolysis (p. 493)
but (p. 495)...
Hexokinase and pyruvate kinase also set the pace of glycolysis.
The following statement from Abeles, Frey and Jencks
(p. 589) is true enough as
far as it goes, but it actually says very little, as it is equally true that glucose would be prevented from entering glycolysis
if the reactions of phosphoglucomutase or aldolase were shut off, and no one calls them the rate-limiting enzymes of glycolysis:
An important control point in glycolysis is the phosphorylation of fructose-6-P catalyzed by
phosphofructokinase. If that reaction were shut off, glucose would be prevented from entering glycolysis.
What happens in reality?
The standard idea is that phosphofructokinase controls flux through glycolysis, but as seen in the examples above there are
two other popular candidates, hexokinase and pyruvate kinase.
In any case, how true is it that phosphofructokinase, or any other enzyme, controls the glycolytic flux? The crucial experiment was done by
Heinisch
before any of these books were written; he found that overexpressing phosphofructokinase 3.5-fold in yeast had
no measurable effect on the flux to ethanol. Other experiments have been done since then in other organisms.
Why, then, do textbooks continue to claim that phosphofructokinase controls the glycolytic flux?
As the pages in this section of this site are intended to stick to facts
and to discuss interpretations only insofar they are uncontroversial,
that is as far as I shall go in this page about what happens in
reality. I do, nonetheless, hold opinions about how the control and
regulation of glycolysis should be understood, and anyone interested
in these can find them in other pages on this site, e.g. those based on
Chapter 10 of my book
Fundamentals of Enzyme Kinetics.
Why does it matter?
The misconception at the heart of this question is at the heart of the failure of genetic engineering during the quarter century
since it became technically feasible to produce useful effects on industrially important metabolic fluxes.
Textbook checklist
Reference
J. Heinisch (1986)
Isolation and characterisation of the two structural
genes coding for phosphofructokinase in yeast
Mol. Gen Genet. 202, 75–82
Other common errors in textbooks
List of books considered
