Biomedical Engineering Reference
In-Depth Information
where
R
is the radius of the cell,
F
is the secretion rate, and
D
is the diffusion coefficient of the
growth factor. The distance that a signal reaches when
c
¼
K
a
is estimated by
r
critical
=
R
¼
a
ð
6
:
16
Þ
Thus, the maximal secretion rate is given by the ratio of two time constants
a
, the time constant for
2
/
diffusion away from the cell (
). Since it takes an infi-
nite amount of time to reach a steady state, a more reasonable estimate of the signal propagation
distance is the time at which the signal reaches one-half of its ultimate value. This leads to a time
constant estimate for the signaling process of about 20 minutes and a maximal distance of about
200
R
D
), and the secretion time constant (
K
a
R
/
F
m
m. This distance is shortened proportionally to the secretion rate (
F
) and inversely propor-
tionally to the affinity (
K
a
), both of which are limited.
The binding of a growth factor to its receptor triggers a complex signal transduction
process (Figure 6.20). Typically the receptor complex changes in such a way that its intra-
cellular components take on catalytic activities. These activities are in many cases due to
IFN
γ
(dimer)
Dimeric IFN
γ
receptor
α
subunit
β
subunit
P
P
AT P
AT P
ADP
P
P
JAK 1
JAK 2
ADP
SH2
SH2
P
P
SH2
SH2
P
Nucleus
P
SH2
GAF
FIGURE 6.20
A schematic representation of the interferon gamma signal transduction pathway.
From [10].
