ABSTRACT
This chapter describes the most abundant and important switching reactions, and provides the reader with an "Ariadne's thread" to help find a path through the maze of details. Frequently the energy-supplying biochemical reactions coupled with signal transduction generate covalent changes of the signal-transducing protein. Moreover, in a network of switching units, the input signal becomes dispersed rapidly and fades away; therefore, it has to be permanently amplified. An additional cause of selectivity is targeted interactions of nitric oxide synthases with the substrate proteins of nitrosylation. Thus G-proteins are not only amplifiers and rectifiers but also adjustable timers. In an exemplary fashion G-proteins demonstrate two basic features of signal transduction: one, connection of signaling processes with an energy-supplying biochemical reaction and two, spatial and temporal control of signaling. These are, in particular, redox reactions, the hydrolysis of nucleoside triphosphates and other energy-rich compounds, the degradation of macromolecules, and ion flows along electrochemical gradients across membranes.
