What is the general effect of phosphorylation on enzyme activity in insulin signaling?

• A. Phosphorylation has no effect on enzyme activity in insulin signaling.
• B. Phosphorylation always activates enzymes in insulin signaling.
• C. Phosphorylation always inactivates enzymes in insulin signaling.
• D. Phosphorylation alters enzyme activity, which can lead to either activation or inactivation depending on the enzyme.

Answer: (D)