glycolysis

Initially there is energy input corresponding to cleavage of two ~P bonds of ATP.

The reaction catalyzed by Hexokinase is highly spontaneous.

A phosphoanhydride bond of ATP (~P) is cleaved.

The phosphate ester formed in glucose-6-phosphate has a lower DG of hydrolysis.

Induced fit:

Glucose binding to Hexokinase stabilizes a conformation in which:

w  the C6 hydroxyl of the bound glucose is close to  the terminal phosphate of  ATP, promoting catalysis.

w  water is excluded from the active site.

            This prevents the enzyme from catalyzing ATP hydrolysis, rather than transfer of phosphate to glucose. 

It is a common motif for an enzyme active site to be located at an interface between protein domains that are connected by a flexible hinge region.

The structural flexibility allows access to the active site, while permitting precise positioning of active site residues, and in some cases exclusion of water, as substrate binding promotes a particular conformation.

2. Phosphoglucose Isomerase catalyzes:

     glucose-6-P (aldose) ?à fructose-6-P (ketose)

The mechanism involves acid/base catalysis, with ring opening, isomerization via an enediolate intermediate, and then ring closure.  A similar reaction catalyzed by Triosephosphate Isomerase will be presented in detail.

3. Phosphofructokinase catalyzes:

    fructose-6-P + ATP à  fructose-1,6-bisP + ADP

This highly spontaneous reaction has a mechanism similar to that of Hexokinase.

The Phosphofructokinase reaction is the rate-limiting step of Glycolysis.

The enzyme is highly regulated, as will be discussed later.

4. Aldolase catalyzes: fructose-1,6-bisphosphate ?à dihydroxyacetone-P + glyceraldehyde-3-P

The reaction is an aldol cleavage, the reverse of an aldol condensation. 

Note that C atoms are renumbered in products of Aldolase.