|Lecture Notes: 5 November||
Last time we looked at the three Phases of the Glycolysis Pathway. Now let's note the energy and kinetic relationships of this pathway as shown in Table I. 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!
|Glycolytic Enzymes||.||.||Brain||.||Skeletal Muscle||.||RBC||.|
|.||G°', kJ||K'||Q||Max Act||Q||Max Act||Q||Max Act|
Now look at the K' and Q values: remember that K' gives equilibrium values under standard conditions (with pH = 7), 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? (overhead 13.7, MvH; P-H 15.23)
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.
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.
The mechanism here is based on the Lobry-de-Bruyn von Ekenstein mechanism. This base catalyzed reaction sequence interconverts three of the major hexoses, and can be used in understanding some isomerase enzyme mechanisms. The mechanism is symmetrical. You should finish the second half on your own.
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-glu diad 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-glu diad (glu acts to enhance his catalysis much as in catalytic triad of serine proteases, Lecture 21) 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 21.)
Last modified 5 November 2008