The present invention relates to isolated nucleic acid molecules encoding nitric oxide synthases. The isolated nucleic acid molecules and their encoded protein or polypeptides are useful in methods for attaching a nitrogen group to a target moiety of a compound and for synthesizing a nitrogen-modified compound in a transgenic host cell. The present invention also relates to expression systems and host cells containing the nucleic acids of the present invention, as well as a method of recombinantly producing the nitric oxide synthases of the present invention.