dx1/dt=a1*(x2-x1)-ka*vm*x1/(kc*ka+kc*x3+ka*x1);
dx2/dt=a2*(x1-x2);
dx3/dt=b1*(x4-x3)-kc*vm*x3/(kc*ka+kc*x3+ka*x1);
dx4/dt=b2*(x3-x4);
y1=x1;
y2=x4;

