Where are creatinine, ammonium ions, small amounts of urea, and drugs such as penicillin typically secreted?

You are studying a protein that you call protein x. there is an aspartic acid at a key position in protein x which is important in the folding and stabilization of that protein. if this aspartic acid is changed to a different amino acid, which type of amino acid substitutions is most likely to allow the protein to fold normally?