Table II. NDPK binding and activation of α-(R_p)-borano thymidine nucleotide analogues.

Nucleotide		Catalytic efficiency (M–1s–1)a	KD (µM)b
TTP	–	1 × 106	1.2
AZT triphosphate	–	80	30.0
	α-borano	375	2.0
d4T triphosphate	–	800	2.0
	α-borano	6000	0.25
TDP	–	2 × 106	–
	α-borano	27 × 106	–
AZT diphosphate	–	200	–
	α-borano	2000	–
d4T diphosphate	–	2600	–
	α-borano	ND	–

^aThe phosphotransfer reaction between human NDPK A and thymidine derivatives was studied in both directions as described in Figure 3. Note that the α-borano group increases the catalytic efficiency 4- to 10-fold.

^bThe F64W-H122G variant of Dictyostelium NDPK is designed to measure NTP binding in the absence of phosphotransfer. Protein fluorescence (λ_exc = 310 nm, λ_em = 330 nm) increases by 50% upon binding saturating amounts of NTP at 20°C in 50 mM Tris–HCl, 5 mM MgCl₂ and 75 mM KCl pH 7.5. K_D values are estimated by fitting the binding curve to a quadratic equation.