Answer:
A protein kinase is an enzyme which catalysis the phosphorylation reaction.
The phosphorylation can either activate or deactivate other proteins or receptors.
A protein kinase cascade refers to the series of events in which a chain reaction of phosphorylation takes place by which thousands of proteins are phosphorylated.
It helps in signal transduction through which signals (usually hormone) from outside the cell are amplified and transferred to the target molecule present inside the cell.
For example, tyrosine kinase cascade.
