Protein function and how it does not get affected with amino acid
The side chains of some amino acids are not reactive and therefore not involved in any covalent chemistry in enzyme active centers.

Isoleucine, an essential amino acid, is one of the three amino acids having branched hydrocarbon side chains. It is usually interchangeable with leucine and occasionally with valine in proteins.

Typical amino acids and their alternatives usually have similar physicochemical properties.

Amino acid replacement is a change from one amino acid to a different amino acid in a protein due to point mutation in the corresponding DNA sequence. It is caused by nonsynonymous missense mutation which changes the codon sequence to code for another amino acid instead of the original.

The effects of amino acid replacement may vary depending on how similar or dissimilar the replaced amino acids are, as well as on their position in the sequence or the structure.

References:

http://www.biology.arizona.edu/biochemistry/problem_sets/aa/Isoleusine.html
https://en.wikipedia.org/wiki/Amino_acid_replacement