Question

question 18
signal transduction pathways that include a phosphorylation cascade ...
○ are generally initiated by a phosphorylase enzyme.
○ are propagated by steriod hormone receptors.
○ cause a structural change in each phosphorylated protein.
○ are generally inactivated by a kinase enzyme.

Explanation:

1. Analyze Option 1: Phosphorylation cascades in signal transduction are initiated by receptor activation (e.g., growth factor receptors), not typically by a phosphorylase enzyme. Phosphorylases add phosphate groups to substrates like glycogen, not the start of a phosphorylation cascade in signal transduction. So this is incorrect.
2. Analyze Option 2: Steroid hormone receptors are intracellular and act by binding to DNA after hormone binding. Phosphorylation cascades are part of cell - surface receptor - mediated pathways (e.g., RTK - Ras - MAPK pathway). So steroid hormone receptors do not propagate phosphorylation cascades. This is incorrect.
3. Analyze Option 3: When a protein is phosphorylated (addition of a phosphate group), the negative charge of the phosphate group and its interaction with the amino acid residues of the protein cause a conformational (structural) change in the protein. This conformational change can activate or inactivate the protein's function, which is a key aspect of how phosphorylation cascades transmit signals. This is correct.
4. Analyze Option 4: Kinase enzymes are responsible for adding phosphate groups (phosphorylation) and thus activating the phosphorylation cascade. Phosphatases are the enzymes that remove phosphate groups and inactivate the cascade. So this is incorrect.

Answer:

The correct option is: cause a structural change in each phosphorylated protein.