Final answer:
The two important second messengers in the G protein-linked receptor mechanism are cyclic AMP and calcium ions, with calcium ions being the correct answer to complete the statement.
Therefore, option 1) is correct.
Step-by-step explanation:
The two very important second messengers used in the G protein-linked receptor mechanism are cyclic AMP and calcium ions. Cyclic AMP, or cAMP, is synthesized by the enzyme adenylyl cyclase from ATP, and its main role is to activate a type of enzyme called cAMP-dependent kinase, which then phosphorylates various proteins within the cell. However, not all hormones use the cAMP system. An alternative mechanism involves G proteins activating the enzyme phospholipase C (PLC), which cleaves a membrane-bound phospholipid into diacylglycerol (DAG) and inositol trisphosphate (IP3). IP3 then causes the release of calcium ions from storage sites within the cell, and these calcium ions become second messengers that can directly influence cellular activities or bind to proteins like calmodulin, which modulates protein kinase activity within the cell. Consequently, the correct answer to the question is calcium ions.