Feeder Pathways for Glycolysis
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Feeder Pathways for Glycolysis

A feeder pathway for glycolysis, at least as we'll be describing it now, is something that basically feeds glucose or another starting material into the pathway. As we will see the starting material may be a polysaccharide (such as glycogen or starch), a disaccharaide, or a monosaccharide. Also, the "starting material" may or may not enter the pathway at the beginning, as we shall see.

To begin our discussion, let's consider the introduction of glycogen and starch to the pathway. To handle this chore, three enzymes are needed, all of which we'll discuss below. During this time, keep in mind that glycogen and starch are both storage forms of glucose.

Glycogen Phosphorylase
This enzyme removes the terminal glucose units of glycogen (a similar enzyme works on starch). The enzyme uses pyridoxal phosphate as a cofactor and relies on an attack by an inorganic phosphate. Once this phosphate attacks the bond between successive glucose residues, the terminal one is removed with a phosphate ester. This phosphate ester preserves some of the energy initially stored in the chain. The enzyme will continue in this manner, taking successive glucose molecules off of the end of the chain, until it reaches four residues from a branch point.

This enzyme works at the branch point. There are, quite frankly, no words that adequately explain how it works. Please see the image included in this section's jpg file. In coordination with glycogenphosphorylase, these two enzymes create glucose-1-P. But remember! We wanted Glu-6-P.

Phosphoglucomutase handles the 1-P to 6-P conversion for us. This allows the newly-freed glucose molecules to go through glycolysis. However, phosphoglucomutase doesn't just keep churning out the 6-P on its own. It requires glucose-1,6-bP as a cofactor, which again has some interesting ramifications that will be discussed in the next section.

Monosaccharides are handled somewhat differently; instead of simply being broken down and fed into the beginning of the cycle they must be converted and then plugged in. While not all examples will be discussed below, hopefully enough will be to give you something of an idea of what is going on.

Remember this molecule? That's right, it is in charge of phosphorylating glucose that enters the cell. Well, that is not all it does. Still using Mg2+ as a cofactor and ATP as a phosphate source, it can phosphorylate fructose (another hexose) to produce Fru-1-P. As you (hopefully) recall, Fru-1-P is a molecule which is an intermediate in glycolysis, and the cycle can simply go from there.

Fructokinase, Fru-1-P aldolase, and triose kinase
Hexokinase is not the only enzyme that can act on fructose with ATP, however. In the liver, something entirely different occurs. Fructose and ATP are recombined to again form Fru-1-P and ADP. However, from here the path diverges from that seen with hexokinase. Here, Fru-1-P aldolase forms glyceraldehyde and dihydroxy... yes! You recognize those names! We saw them before, they are simply farther down the cycle. From there triose kinase acts and the cycle proceeds.

It begins with galactose and ends with Glu-1-P. It uses UDP as a cofactor, which is all of an introduction to UDP you get right now (although it will come up more later).

Phosphomannose isomerase
Beginning with Man-6-P, this enzyme is capable of creating Fru-6-P in a single bound.

Finally, disaccharides can also be fed into the pathway. As I'm sure you've guessed by now, this is done simply by breaking down the dissarchride (lactase breaks down lactose, sucrase hits sucrose, etc) and then dealing with the resulting monomers as discussed above.