Final answer:
The second step of the Signal Transduction Pathway (STP) involves the activation of a kinase activity through autophosphorylation of the receptor.
Step-by-step explanation:
The second step of the Signal Transduction Pathway (STP) is the activation of a kinase activity by a receptor that has undergone a conformational change due to ligand binding. This activation occurs through autophosphorylation of the receptor, meaning the receptor adds a phosphate group to itself.
For example, when a hormone binds to its receptor on the cell surface, the receptor changes shape and activates a kinase activity. The receptor then undergoes autophosphorylation, where it adds a phosphate group to itself. This autophosphorylation step is crucial for propagating the signal downstream in the signaling pathway.