C3G-Activation

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Binding Reaction

non-modified model:

$\text{inactiveC3G} + \text{unboundEGFR} \rightleftharpoons \text{activeC3G} + \text{unboundEGFR}$, catalysed by src kinase

modified model:

$\text{inactiveC3G} + \text{unboundEGFR} \rightleftharpoons \text{boundC3G}$

$\text{boundC3G} \rightleftharpoons \text{activeC3G}$, catalysed by src kinase

Kinetic equation

non-modified model:

$v_{\text{C3G-Activation}}=\frac{k_{cat} \cdot [\text{unboundEGFR}] \cdot [\text{inactiveC3G}] \cdot (1-\frac{[\text{activeC3G}]}{[\text{inactiveC3G}] \cdot K_{eq}})}{Km_{\text{inactiveC3G}} \cdot (1 + \frac{[\text{inactiveC3G}]}{Km_{\text{inactiveC3G}}} + \frac{[\text{activeC3G}]}{Km_{\text{activeC3G}}})}$

modified model:

$v_{\text{C3G-Binding}}=k_{ass} \cdot [\text{inactiveC3G}] \cdot [\text{unboundEGFR}]-k_{diss} \cdot [\text{boundC3G}]$

$v_{\text{C3G-Activation}}=\frac{v_{max} \cdot [\text{boundC3G}] \cdot (1-\frac{[\text{activeC3G}]}{K_{eq}})}{Km_{\text{boundC3G}} \cdot (1 + \frac{[\text{boundC3G}]}{Km_{\text{boundC3G}}} + \frac{[\text{activeC3G}]}{Km_{\text{activeC3G}}})}$

General Information

C3G is like Sos a GNRP. It binds to the N-terminal SH3 domain of Crk, an adaptor protein which has no catalytic domain, but 1 SH2 and 2 SH3 domains (Feller et al. 1995, York et al. 1998 ). Unlike Sos however C3G is not mainly activated due to the recruitment to the membrane but due to the phosphorylation by Src-kinases. The way of how C3G processes the input seems to be similar to Sos, because the C-terminus is homolog to Cdc25, the catalytic domain of Sos.(Ichiba et al. 1999)) However the main substrate of C3G is not Ras, but Rap1.(York et al. 1998)

This activation process is once modelled as single Michaelis-Menten reaction (non-modified model) and once as a two step reaction: first binding, then phosphorylation (modified model). For the phosphorylation step data found at (BRENDA EC 2.7.10.2) is used because the src is the main kinase that activates C3G.
For the binding the same parameters as for the Sos-Binding process are used. (see here)

final Parameter

Binding Step:

$k_{\text{C3G-binding}}= 1.47 \cdot 10^2 \pm 6.1310 \cdot 10^1\ \frac{1}{\mu M \cdot s}$

$k_{\text{C3G-release}} = 0.1488 \pm 3.965 \cdot 10^{-2}\ \frac{1}{s}$

Phosphorylation Step:(used in the modified and non-modified model)

$k_{cat} = 20.5485 \pm 88.375 \cdot 1.5\ \frac{1}{s} = 20.5485 \pm 132.56\ \frac{1}{s}$

$K_{m} = 1.348 \cdot 10^{-1} \pm 1.601 \cdot 10^{1} \cdot 1.5\ \mu M = 1.348 \cdot 10^{-1} \pm 2.402 \cdot 10^{-1}\ \mu M$

Parameter Calculation

The values that are used can be found in the screenshot below. All values except the ones in which ATP is used as substrate are averaged and the standard deviation is calculated.

BRENDA EC 2.7.10.2 - Screenshots

2.7.10.2 - non-specific protein-tyrosine kinase

• 

  K_m

• 

  k_cat

Km_P value and equilibrium constant

The K_m value for the product is assumed to be similar to but slightly higher than the the K_m value of the substrate because of the similarity of the both species. Therefore the K_m value of the substrate is multiplied by 1.05 to gain the one of the product and the uncertainty is increased by increasing the error on Km_substrate by 50%.

For information about the equilibrium constant please see here.