A DNA expression construct comprising a polynucleotide encoding an unnatural UstD enzyme, the unnatural enzyme itself, and a method of making gamma-hydroxy amino acids by contacting an aldehyde-containing substrate, an amino acid, and the unnatural, purified UstD enzyme under conditions and for a time sufficient to react at least a portion of the aldehyde-containing substrate with at least a portion of the amino acid, to yield a gamma-hydroxy amino acid product.