Constraining Polyfit with known coefficients

Hi. I am performing a 5th order polynomial fit to some data X,Y and use the following syntax

p5=polyfit(x,y,5)
f5=polyval(p5,x);

This performs the best fit and gives me coefficient of all the x's.

How can I restrain this as I know for sure that x^2 & x^4 = 0 (so their coefficients can be set to zero) and the coefficient infront of X is 26. I then want to perform the fit, but only allow the routine to adjust X^5, & X^3 coefficients.

Thanks Jason

[Jason]

I've done it (I think)

 ft=fittype('a*x^5+b*x^3+26*x');
 fitobj,~,~,~]=fit(x,y,ft) %[fitobj, goodness, output, convmsg]=fit(x,y,ft)
      coeff1=fitobj.a;
      coeff2=fitobj.b;  

     yfit=coeff1*x.^5+coeff2*x.^3+26*x;     %Remember dot notation
      r=y-yfit;                            %Calc residuals