The investment stage

Glycolysis can be thought of as much like investing in the stock market. Initially an investment has to be made before realising a profit. In glycolysis the first few reactions are the investment stage. In this stage, 2 molecules of ATP are invested to drive glycolysis.

The first ATP is used by hexokinase to synthesise glucose-6-phosphate (G6P). This reaction is important for a number of reasons. First, this reaction has a large –ΔG’° and is irreversible, initiating glycolysis. Second, the addition of phosphate to glucose traps the glucose in the cell. G6P is then converted to fructose-6-phosphate (FGP).

The second ATP is used by phosphofructokinase (PFK) to add a second phosphate to FGP to give fructose-1,6-bisphosphate (FBP). This reaction is also an irreversible reaction and PFK is a key control point for glycolysis.

The addition of these two phosphates activates the sugar molecule and allows for the creation of high-energy phosphate compounds later in glycolysis, which will be able to drive the formation of ATP.

Finally, the 6-carbon FBP is split by the enzyme aldolase into two 3-carbon molecules, glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). However, DHAP is not a substrate for the next enzyme in the pathway, GAP dehydrogenase (GAPDH). The enzyme triose phosphate isomerase is able to interconvert GAP and DHAP. As GAP is being consumed by the rest of the glycolytic pathway this drives the conversion of DHAP to GAP.

Making an ATP profit

As can be seen in Figure 16-2 all further steps of glycolysis occur twice for each glucose molecule that enters the pathway. In this stage of glycolysis there are three key reactions that ensure the creation of more ATP than the initial investment made in the early stages.

The first of these important reactions is that catalysed by GAPDH. This enzyme oxidises GAP and also adds a second phosphate molecule. This achieves two things: first, the electrons gained from the oxidation are passed to NAD⁺ to give NADH, and this NADH can then ferry these electrons to the electron transport chain (ETC) to make ATP (see below); second, the addition of the another phosphate group to GAP forms the ‘high-energy’ phosphate compound 1,3-bisphosphoglycerate (1,3-BPG).

In the second important reaction of this stage of glycolysis the enzyme phosphoglycerate kinase uses 1,3-BPG to make ATP. The removal of phosphate from 1,3-BPG is very energetically favourable and has a larger –ΔG’° than the +ΔG° required for the formation of ATP and so can be used to drive ATP synthesis. This is the first of two substrate-level phosphorylation reactions that occur in glycolysis. The product of the reaction, 3-phosphoglycerate, still has one remaining phosphate group but this is not a high-energy phosphate compound; it has to undergo rearrangement and dehydration before becoming the high-energy compound phosphoenolpyruvate (PEP).

The second substrate-level phosphorylation occurs as part of the third important reaction of this stage of glycolysis. Pyruvate kinase (PK) uses PEP to catalyse the formation of ATP and creates the product pyruvate. Similar to the removal of phosphate from 1,3-BPG the –ΔG° of this step is large enough to drive ATP synthesis. This also makes the step irreversible.

The final reckoning

If glycolysis as a whole is considered, it can be seen that for every glucose molecule used there is a net gain of ATP. Although 2 ATP has to be used in the initial stages 4 ATP is gained in the final stages to give an overall yield of 2 ATP. There is also the potential to create more ATP by using the electrons carried by NADH; however, as will be seen later in this chapter, this is dependent on the availability of oxygen and the transport of those electrons into the mitochondrion.

Controlling glycolysis

As with any metabolic pathway, glycolysis needs to be controlled so that it responds to changes in the physiological environment of the cell. It is of particular importance in this case as some of the enzymes of glycolysis are shared by the competing pathway gluconeogenesis (see Ch 17).

As was seen in Chapter 14 the activity of just one enzyme in a pathway can affect flux through that pathway, and that the controlling enzymes are those that catalyse the non-equilibrium, irreversible reactions. In glycolysis the enzymes HK, PFK and PK all catalyse irreversible reactions, are unique to glycolysis and are control points. The control of glycolysis in humans occurs at three levels: at the level of the enzyme, at the level of the cell and at the level of the body.

At the level of the enzyme the activity of glycolysis, and all of the enzymes within it, is controlled by the availability of substrate. If glucose is not present in the cell then glycolysis cannot function.

At the level of the cell it is the enzyme PFK that is the key to control. ATP, PEP and citrate inhibit PFK, whereas ADP and AMP stimulate it (Fig 16-3). This allows PFK to respond to a number of things:

FIGURE 16-3 Phosphofructokinase is a key control point for glycolysis.

[Based on Campbell & Reece, 2005, Biology, 7th edition, Pearson Benjamin Cummings]

At the level of the body the activity and amount of the enzymes HK, PFK and PK are all affected by the hormones insulin and glucagon, which work in concert to regulate metabolism throughout the whole body.