Amino acids can be present as -D or -L isomers or both of them. Isomers structures seems to be quite similar or can be said isomers are actually mirror image of each other.

All amino acids in animals are -L isomers; because -D isomers don’t have a biological function. But methionine is exception, birds or poultry able to use its -D and -L forms. There are indications that -D isomers of methionine are less effective when the amount of protein or amino acid in the diet is low. Among analogues, methionine analogues are mostly used to replace the methionine amino acid, and -LD is best hydroxy analog.