9 - Signal Transduction

Abstract

Signal transduction is the process where an external chemical signal elicits an intracellular metabolic change. The process begins with the binding of specific ligands to receptors located at the surface of the plasma membrane. The receptor responds to the binding of agonists in several different ways. This chapter discusses two mechanisms of signal transduction: generation of second messengers, and receptor phosphorylation. Most signaling involves receptor activation of a GTP-binding protein (G protein). The activated G proteins interact with enzymes that produce second messenger molecules. The chapter explains that changes in cellular enzyme activities lead directly to a biological response by the cell. Another signaling pathway regulated by the action of G proteins is the protein kinase (PKA) pathway. A second mechanism of signal transduction involves cell surface receptors that have tyrosine kinase activities associated with their cytoplasmic domains. Binding of agonist to these receptors directly activates their kinase domains. The kinase activity catalyzes the phosphorylation of intracellular proteins and often causes autophosphorylation. Modulation of ion channels occurs as the result of the binding of extracellular signals to their receptors. In these cases, ions act as second messengers. This process is the least understood mechanism of signal transduction.