2.5 Regulation of enzyme activity

The enzyme phosphofructokinase catalyses the following reaction:

Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

In Figure 2.5-1, the substrates are located next to each other within the active site of the dimeric enzyme. This reaction is an early step in the degradation of glucose, which ultimately serves to replenish ATP from ADP and phosphate. It therefore makes good biological sense that phosphofructokinase should be stimulated by ADP.

To accomplish this stimulation, ADP binds to another site that is far away from the active site; it therefore clearly does not directly participate in the reaction. Instead, ADP binding changes the conformation of the entire enzyme molecule. The change will also affect the active site and enhance the efficiency of catalysis there. This mode of action is known as allosteric regulation and is exceedingly common. Allosteric effectors can be either stimulatory (as ADP is in this example) or inhibitory. E.g., ATP does not only bind to the active site but also acts as an allosteric inhibitor of phosphofructokinase.

The workings of allosteric regulation are schematically depicted Figure 2.5-2a-c. The enzyme has two possible conformations that are in equilibrium with each other (Figure 2.5-2a). An allosteric activator will bind selectively to the regulatory site as it occurs in the active conformation (Figure 2.5-2b) and thereby shift the equilibrium towards this conformation. Conversely, an inhibitor would bind selectively to the inactive conformation and thereby stabilize it. As you can see, activators and inhibitors may share the same regulatory site; this is the case in the above example of phosphofructokinase with ATP and ADP. Note, however, that phosphofrucokinase has additional allosteric sites that permit regulation by other effectors (cf. Figure 7.3.2-1).

Although it is not theoretically necessary, it seems that all allosteric enzymes occur as oligomers. In Figure 2.5-2c, a dimeric enzyme is shown, but often the number of subunits is considerably higher. Their oligomeric nature enables enzymes to react more sensitively to changes in effector concentration.

Another important means of enzyme regulation consists in phosphorylation. This occurs by protein kinases, which transfer a phosphate group from ATP to a specific site on the regulated enzyme. The mechanism of regulation by phosphorylation is not really that different – it also works by selective stabilization of either the active or the inactive conformation. The only difference is that the regulator is more stably attached, so that the regulatory effect may last longer. Like allosteric regulation, it can be inhibitory (as shown in Figure 2.5-2d) or stimulatory. Many enzymes are subject to regulation both by allosteric effectors and by phosporylation.

While all mechanisms discussed so far modulate the activity of existing enzyme molecules, the overall enzyme activity may also be varied by changing their abundance:

The transcription of the gene encoding the enzyme in question can be turned on or off. This mechanism is employed by many hormones, in particular steroid hormones (e.g., cortisone) and thyroid hormones.
The stability of the enzyme mRNA’s of the enzymes can be changed.

Hormones may affect the activity of an enzyme at more than one level. For example, insulin increases the activity of glycogen synthase by way of transcriptional induction, increased mRNA stability, and protein phosphorylation.