The lifetime of an intracellular signaling molecule must be short for the cell to be able to cause a change in the concentration
of the signaling molecule in an efficient manner. PKA is more closely related to PKC than to PKG at the amino acid level. Components of intracellular signaling pathways can be small molecules, proteins or lipids. Second messengers can convey a signal via a change in their steady state level. Negative feedback systems play important roles in regulating cellular signaling.
PKA and PKG is both have binding domains to attach with nucleoside monophosphate and help in modulating these enzymes that explains that PKA is more related to the PKG than PKC at the level of amino acid.
At the level of polypeptide chain level PKG has single polypeptide chain whereas in PKA modulatory domains are located on different poly peptide chain. Both are activated by the nucluoside triphosphate, more precisely PKA by cAMP and PKG is by cGMP respectively, whereas PKC is activated by DAG or Ca or both depending on the isofom.
