Signal transduction mechanisms of Ca²⁺ mobilizing hormones: The case of gonadotropin-releasing hormone

Hormones, neurotransmitters, growth factors, antigens, and light exert their diverse biological effects by producing “second messenger” molecules which amplify and propagate the receptor signal into the cell biochemical network. The “second messengers” [e.g. cAMP, cGMP, Ca²⁺, and diacylglycerol (DG)] activate respective protein kinases which are the key enzymes in the regulatory network of the living organism (1). Protein kinases (~100 have been described) are divided into serine/threonine and tyrosine-specific kinases (2). The main members of the first family are activated by second messengers, whereas several members of the tyrosine-specific kinases are associated with growth factor receptors or are encoded by retroviral oncogenes or the cellular homolog counterparts. The kinases catalyze the transfer of phosphate from ATP to serine, threonine, or tyrosine in the substrate proteins, and phosphatases remove the phosphate group (3). The phosphorylation-dephosphorylation cycle enables the cell to alternate from resting to activated state (or vice versa) according to the stimuli input.