Whole Cells - Molecular Theory of the Living Cell - page 459

a Signal: Instruction

S1

1

R1

2

S2

R2

3 S3

R3

b

Signal: Decodification 1

R1

S1

1

S2

R2

1

1

R3 S3

c

Signal: Decodification 2

1

R3 S3

R3 S3

2

S3

3

R3

Fig. 12.34 Three mechanisms of transmembrane signal transduction according to Barbieri
(2003). (a) The one-to-one signal transduction, where one primary messenger produces a
corresponding secondary messenger. (b) The one-to-many signal transduction, where one primary
messenger produces more than one secondary messenger. (c) The many-to-one signal transduc-
tion, where many different primary messengers produce an identical secondary messenger

We may refer to these functions as the triadic functions of membrane receptors .
Since all molecular machines must work in more than one cycle in a given
direction, their operation cannot be driven by random thermal fluctuations or
Brownian motions alone but must ultimately be driven by free energy dissipation.
There are two free energy sources for membrane receptors - (1) ATP or GTP for
Mechanisms 2, 3, and 5 (see Table 12.14 ), and (2) transmembrane ion gradients for
Mechanism 4.
The signal transduction machinery (STM) is more complex than membrane
receptors and the interactions with their ligands. The STM of the cell may be viewed
as composed of at least eight distinct components as depicted in Fig. 12.35 . Common
examples of the first three components of STM are listed in Table 12.14 .