Messenger Substances Second Messengers

The intracellular messengers are diffusible signal molecules and reach their target proteins mostly by diffusion. Close spatial proximity of the signal components, as achieved for transmembrane receptors and their effector proteins with the help of membrane anchoring or with specific protein-protein modules (see Chapter 5, Chapter 8), is not necessarily required for this type of signal transduction. 

The intracellular second messengers are characterized by a series of properties that 

3 -5 -cyclic AMP is a central intracellular second messenger that influences many cellular fimctions, such as gluconeogenesis, glycolysis, lipogenesis, muscle contraction, membrane secretion, learning processes, ion transport, differentiation, growth control and apoptosis. 

The general role of cAMP formed by adenylyl cyclase (see Fig. 5.21) is to activate protein kinases. Protein kinases regulated by cAMP are also classified as protein kinase A (see 7.5). 

The mechanism of activation of protein kinases of type A by cAMP is schematically represented in Fig. 6.2. In the absence of cAMP, protein kinase A exists as a tetramer, composed of two regulatory (R) and two catalytic (C) subimits. In the tetrameric R2C2 form, protein kinase A is inactive since the catalytic center of the C subimit is blocked by the R subimit. 

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