SME
Cell & Molecular Biology · Problem type
Signal amplification in a cascade
Use this for “One receptor activates 20 G proteins, each enzyme makes 1000 cAMP a second. How many cAMP a second?”
Chart
A log scale: each grid line up is 10 times the one below.
This is an example. Type your own values to start a new problem.
G
=
R
×
g
A (s⁻¹)
=
G
×
c (s⁻¹)
Formulas
G
=
R
×
g
20 = 1 × 20
A
=
G
×
c
20,000 = 20 × 1,000
Assumptions
•
An active receptor switches on many G proteins before the hormone leaves it.
•
Each active G protein switches on one adenylyl cyclase, which makes cAMP until the G protein shuts off.
•
Each stage multiplies the one before it, so the bars on a log scale climb one step per factor of 10.
Step-by-step
WE KNOW
R = 1, g = 20, c = 1,000 s⁻¹
FIND
active G proteins (G), cAMP per second (A)
Step 1 · Find active G proteins (G)
Use G = R × g
Each
receptor
switches
on
g
G
proteins.
G
=
1
×
20
G
=
20
Step 2 · Find cAMP per second (A)
Use A = G × c
Each
switched-on
enzyme
makes
c
cAMP
a
second.
A
=
20
×
1,000
A
=
20,000
s⁻¹
Check
Put the numbers back in. Both sides should match.
20 = 1 × 20 ✓
20,000 = 20 × 1,000 ✓
Topic check
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