What are enzyme cascades?
1 Answer
A series of linked enzymatic reactions in which the product of one reaction is the substrate for the next.
Explanation:
An enzyme cascade consists of a series of enzymes in which the product of one enzyme is the substrate for the next.
In an enzyme cascade, you may have a rate-limiting step which consists of the enzyme catalysing the slowest reaction of the cascade. You may also have regulation through feedback of products inhibiting the activity of enzymes further back in the cascade. You may also have control through a "feed-forward" mechanism.
In some enzyme cascades you may also get an amplification of the signal. For example, in G-protein coupled receptor cell signalling you have a ligand (primary message) outside the cell that binds to one receptor. Hence, one ligand activates one receptor.
The activated receptor then stimulates the exchange of GTP for GDP on the G protein (the G protein is an enzyme as it has GTPase activity), which will then activate an enzyme such as adenylyl cyclase to produce many molecules of cAMP.
So, one ligand has activated many G proteins, which in turn have activated many molecules of adenylyl cyclase, which produced many molecules of cAMP.
The molecules a cAMP can then go on to activate many molecules of protein kinase A (another enzyme in the cascade), which in turn can phosphorylate many proteins. and these phosphorylation's may regulate enzymes within the cell.
Hence, we have gone from one molecule of a ligand outside the cell activating one receptor to potentially many hundreds, if not thousands, of proteins being activated inside the cell. This is an example of signal amplification within an enzyme cascade.
