Having problems with ode solution

Hello,

So I have this:

function [dS] = Mechanism(~, S, E, K, k)

S1 = S(1); S2 = S(2); S3 = S(3);
E1 = E(1); E2 = E(2);
K1 = K(1); K2 = K(2);
k1 = k(1); k2 = k(2);

v1 = (k1*E1*S1)/(K1+S1);
v2 = (k2*E2*S2)/(K2+S2);

dS1 = -v1;
dS2 = +v1 -v2;
dS3 = +v2;

dS(1,:) = dS1;
dS(2,:) = dS2;
dS(3,:) = dS3;

end

Just two Michaelis Menten Equations for rates v1 and v2 of this reaction S1 -> S2 -> S3.

And I am trying to solve it with:

S = [100 1 1]; % Substrate Conc.
E = [10 10]; % Enzyme Conc.
K = [200 50]; % Km
k = [5 10]; % kcat
tspan = [0 20];
options = odeset('NonNegative',1:3);

[t,y] = ode15s(@(t,y)Mechanism(t,S,E,K,k), tspan, S, options);

figure
plot(t, y);
xlabel('Time / s');
ylabel('Concentration / uM');
title('S1 -> S3');
legend('S1', 'S2', 'S3');
set(legend,'Location','northwest');

Now I would expect the plot to show S1 going down whilst S2 and S3 are increasing and then S2 going down - something like that.

But the plot which is created shows S1 going to 0 but S2 and S3 continuing linearly upwards without stopping.

So I am confused.

Why is this so? Did I write something incorrectly? Or is somehow my Matlab broken?

I also tried something similar which I copied 1:1 from a youtube video https://www.youtube.com/watch?v=Ksi1R2pcE1M - where it functioned without problems - but it didn't work. That video used MATLAB R2016b - so maybe something to do with that?

Thank you for reading and hoping you can help, Andrej

Answer 1

[Torsten]

Use

[t,y] = ode15s(@(t,y)Mechanism(t,y,E,K,k), tspan, S, options);

instead of

[t,y] = ode15s(@(t,y)Mechanism(t,S,E,K,k), tspan, S, options);

Best wishes

Torsten.

[Andrej Musikic]

Thank you!

I feel like an idiot right now ^^'

Best wishes, Andrej