The enzyme takes the end methyl residue from the methionine side chain .
The enzyme adds it to the side chain carboxyl group of L - isoaspartate or D - aspartate to create a methyl ester .
The enzyme takes the end methyl residue from the methionine side chain and adds it to the side chain carboxyl group of L - isoaspartate or D - aspartate to create a methyl ester .