Which of the following are among the most common second messengers?

kinase and phosphatase

G proteins and GTP

kinase and phosphate groups

calcium ion and cAMP

GTP and GDP

The correct answer would be calcium ion and cAMP.

Secondary messengers refers to the intracellular signaling molecules which pass the signals from the receptors located on the surface of the cell to the target molecule present within the cell.

The secondary messengers can be classified into three major class:

Cyclic nucleotide for example cAMP (cyclic adenosine monophosphate), cGMP (cyclic guanosine monophosphate) DAG (diacylglycerol) and IP₃ (inositol triphosphate) Calcium ions (Ca⁺⁺)

Hence, from the given options, calcium ion and cAMP would make the correct choice.