Enzyme kinetics: NTKase and NTKase deficiency

NTKA is a newly discovered (pretend) excitatory neurotransmitter. After its release into the synaptic cleft, NTKA is cleaved into two components, NT and KA, by an enzyme known as NTKAse. A "new" disease, with an autosomal recessive mode of inheritance, is characterized by NTKA deficiency. Autosomal recessive NTKA deficiency is caused by a hyperactive NTKAse enzyme, “$∆$‍NTKAse,” which binds NTKA with a much higher affinity than wild-type NTKAse. Interestingly, $∆$‍NTKAse and NTKAse do not appear to have different rates of catalysis at saturating substrate concentrations (or different “k${}_{cat}$‍” values).

Researchers performed the following analyses to further characterize NTKA deficiency and $∆$‍NTKAse.

Researchers isolated a large quantity of NTKAse protein from three different patients with a family history of NTKA deficiency. Patients 1 and 2 are of unknown genotype, while Patient 3 is homozygous recessive. Figure 1 shows a Lineweaver-Burke plot of NTKAse activity for each patient. The NTKAse activity of Patient 3 was measured in the presence of a drug known to limit the activity of $∆$‍NTKAse.

Figure 1 A Lineweaver-Burke analysis of NTKAse activity in Patients 1, 2, and 3

Researchers cloned the gene encoding NTKAse (WT-NTKAse) and constructed several NTKAse variants. The dissociation constants (K${}_d$‍) of NTKA for each of the NTKAse variants were determined (Table 1).

Table 1 Dissociation constants (K${}_d$‍) of NTKA for Various NTKAse variants