First let's look at Glycolysis to get an overview, then we will look at the reactions and enzymes of this pathway individually. We will then come back and look at the overall regulation and control of this pathway. If we look at the Glycolysis Pathway (Slide), we can break it into three phases:
We have now looked at the overall pathway of glycolysis (Glycolysis Pathway) and its phases. Now let's note the energy and kinetic relationships of this pathway as shown in Table I, below. Note the G°' values: some reactions are quite favorable whereas others are unfavorable, but the overall pathway, including triose isomerase, has a net G°' of -44.65 kJ (Glucose to 2 Pyruvates). So Glycolysis is favorable under standard conditions!
Now look at the K' and Q values: remember that K' gives equilibrium values under standard conditions, while Q gives measured values for real tissues. What we want to pay attention to here is differences between these two values (small variations are expected since tissues are not at standard conditions). Here large differences indicate reactions which are not at equilibrium: these reactions must be controlled in some way by the organism! Thus we see large differences for HK, PFK, and PK in brain, and HK and PFK in RBC's. Muscle is like brain (overhead). The G values are plotted below as well for clarity. Finally the max activity column shows us what kind of flux is possible through these enzymes - what does this indicate about these tissues and glycolysis?.
Figure I. Free Energy changes in rabbit skeletal muscle (Data from Mathews and van Holde, Biochemistry, Benjamin/Cummings (1990))
Now let's look at the individual reactions of Glycolysis.
1) Hexokinase (HK): Glucose to G-6-P. [Figure 11.3]
Here we see a nucleophilic attack by a primary alcohol on the gamma phosphate of ATP (alcoholysis of an acid anhydride). As we would expect this is a very favorable reaction.
2) G-6-P Isomerase: G-6-P to F-6-P.
The mechanism here is based on the Lobry-de-Bruyn von Ekenstein mechanism we looked at earlier (Lecture 18)
Note that this would seem an ideal reaction to catalyze with a general acid/base mechanism. The enzyme has a bell shaped pH profile with pKa's at 7 & 9 and has his and lys residues in the active site.
Let's think about this mechanism for a couple of minutes -talk among yourselves and see what you can come up with.
Hexose Isomerase Mechanism: Based on the data provided you should have come up with a mechanism using histidine as a general base catalyst and lysine as general acid catalyst in the first step of the Lobry-de-Bruyn-van Ekenstein Transformation, with a reversal of roles in the second step. (It turns out its more subtle. In fact the lysine is used as a general acid in catalyzing the ring opening as we saw with the mutarotation of glucose in our study of catalysis; Lecture 15.)
3) PhosphoFructoKinase (PFK)-1: F-6-P to F-1,6-bis P.
The chemical mechanism here will be the same as for HK. Note the requirement for Magnesium, as expected.
Phosphofructokinase Regulation I
PFK is the key regulatory enzyme for Glycolysis: It regulates the flux into pathway (flux generating step) and is the first committed step for Glycolysis.
ATP inhibits, giving sigmoidal kinetics for F-6-P vs. rate. But [ATP] is not important for regulation! (Probably left over from early regulatory system, but under physiological conditions [ATP] doesn't change enough to regulate PFK in most organisms. By the time [ATP] falls significantly, organism is dead.)
AMP releases ATP inhibition, and is an important regulator for mammals (lots of phylogenetic variation).
Why AMP? [ATP]:[AMP] = approx. 50, while [ATP]:[ADP] = approx. 10. Thus [AMP] changes more and is much more sensitive measure of [ATP] change and thus availability (e.g. a change of about 10% in [ATP] will result in a change of about 400% in [AMP]!). [Table 10.3] Of course the problem is where does the AMP come from? Turns out there is an enzyme in most tissues catalyzing the interconversion of ATP, ADP and AMP, Adenylate Kinase:
© R. A. Paselk 2010;
Last modified 13 March 2013