 In AK-HSD expressing organisms, one of the threonine binding sites is found in the linker region between AK and HSD, suggesting potential allosteric inhibition of both enzymes.However, some threonine-resistant HSD forms exist that require concentrations of threonine much greater than physiologically present for inhibition.These threonine-insensitive forms of HSD are used in genetically engineered plants to increase both threonine and methionine production for higher nutritional value.