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1 Introduction

Figure 1.2: Signaling between cells; (a) Neuronal

signaling; (b) Endocrine signaling.

reaction. The product of that reaction then binds to another enzyme, which activates it

to perform another reaction, whose product binds and activates another enzyme, and

so on. This process is called a signal cascade.

Why does a signal cascade take place, instead of just one single reaction? Doesn’t

such a complex process create a greater risk of something going wrong? In fact, the

opposite is the case: Each of the steps in the signal cascade can be regulated, thus allow-

ing the signal effect to be tightly controlled. Also, a signal cascade creates many signals

for a lot of proteins in a short time, and thus creates signal amplification, which allows

for fast, coordinated action. For example, within a muscle, a large number of heads of

myosin have to be activated at the same time, otherwise only a few muscle fibers would

contract instead of the complete muscle.

We will look at two common signal transduction pathways as examples. The first is

the adenylate cyclase pathway (Figure 1.3). The transmembrane receptor is associated

with a G-protein (a protein that hydrolyses guanine triphosphate (GTP) to guanosine

diphosphate (GDP)). These complexes are called G-protein-coupled receptors, or GPCR.