Final answer:
Threonine dehydratase is termed a dehydratase because it catalyzes the removal of water (dehydration) during the non-oxidative deamination of threonine, involving the cofactor PLP.
Step-by-step explanation:
Threonine dehydratase is called a dehydratase rather than a deaminase because it catalyzes a dehydration reaction by removing a water molecule from threonine rather than just removing an amino group, which would be a deamination. Dehydratases facilitate the removal of -OH and -H to form double bonds, as opposed to deaminases which directly remove the amino group. The involvement of pyridoxal phosphate (PLP) as a cofactor indicates the non-oxidative deamination of hydroxy amino acids like serine and threonine, where dehydration is a key step.