Most proteins have to adopt folded structures in order to perform a complex function. But not all proteins able to adopt folded structures will be able to perform a complex function. IOW, for most proteins, being able to adopt folded structures is a necessary condition for performing a complexfunction, but it is not sufficient. Now, once a protein has emerged that is able to perform a complex function, it will be very, very difficult for it to perform another complex function. What you may observe of course is some tweaking of the existing function, aka micro evolution. But that’s all.