Defect, enzyme
An abnormality in the protein (enzyme) important in catalyzing a normal biochemical reaction in the body.
Disorders result from a deficiency (or functional abnormality) of an enzyme.
Archibald Garrod in 1902 was the first to attribute a disease to an enzyme defect: an inborn error of metabolism.
Today, newborns are routinely screened for certain enzyme defects such as phenylketonuria (PKU) and galactosemia, an error in the handling (metabolism) of the sugar galactose.