Table 1.

Kinetic constants of spinach PRPP synthase isozyme 4 a

	Kmb	Kappc	Vmax	Vapp
Substrate	μM		μmol/(min × mg of protein)
Rib-5-P	48 ± 12
ATP	77 ± 10		16.2 ± 0.5
dATP		84 ± 5		15 ± 0.2
GTP		490 ± 58		6.4 ± 0.2
CTP		680 ± 130		4.2 ± 0.2
UTP		500 ± 200		6.6 ± 2.1

^a Kinetic constants were determined as described in Materials and Methods. Standard errors are those given by the computer program.

^b The concentration of ATP was varied from 0.05 to 1.00 mM and the concentration of Rib-5-P was varied from 0.125 to 2.00 mM in the presence of 5.0 mM MgCl₂.

^c The concentration of dATP, GTP, CTP or UTP was varied from 0.125 to 8.0 mM at 3.0 mM Rib-5-P and MgCl₂ in at least 2 mM excess of diphosphoryl donor concentration.