It would most likely render the protein nonfunctional or mis-functional.
The mutation could result in three outcomes:
Silent mutation, which changes the codon to the same amino acid. (AAA->AAG, both are lysine). But since the problem specified that it has a "slightly different amino acid sequence," we can assume this doesn't happen.
Nonsense mutation, which changes a codon to a stop codon. This would end the chain of amino acids, making the protein potentially nonfunctional.
Missense mutation, which changes a codon to another completely different codon. This can be harmful, as in sickle-cell disease, where just one amino acid, glutamic acid, is changed to valine.
