Table 2.

Kinetic Parameters of the Native and Chimeric Form of ATCase^a

	nucleotide effectorsb
	none	ATP	CTP	UTP	C + U
Native Sm Holoenzyme
Vmaxc	22.4 ± 0.5	25.6 ± 1.2	25.0 ± 0.4	23.1 ± 0.6	30.2 ± 0.4
[Asp]0.5d	39.2 ± 0.3	17.7 ± 3.7	22.0 ± 1.8	41.0 ± 3.1	25.7 ± 1.0
nHe	1.9 ± 0.1	2.5 ± 0.1	2.7 ± 0.1	2.2 ± 0.1	2.2 ± 0.1
[NTP]0.5f	udg	1.8	0.5	ud	ndh
Native Ec Holoenzyme
Vmax	24.9 ± 1.2	24.8 ± 0.6	18.9 ± 1.9	25.1 ± 0.2	16.7 ± 0.5
[Asp]0.5	16.6 ± 1.4	9.9 ± 1.9	27.3 ± 2.0	15.4 ± 2.7	31.8 ± 2.0
nH	3.2 ± 0.1	2.5 ± 0.3	3.7 ± 0.5	3.1 ± 0.3	3.1 ± 0.2
[NTP]0.5	ud	1.0	0.1	ud	nd
Chimera SM:rS5′ec Holoenzyme
Vmax	18.5 ± 1.1	25.0 ± 0.6	25.5 ± 0.3	25.9 ± 0.4	23.8 ± 0.5
[Asp]0.5	4.7 ± 0.1	2.8 ± 0.1	9.0 ± 0.4	4.6 ± 0.1	12.0 ± 1.2
nH	2.1 ± 0.1	1.5 ± 0.1	2.5 ± 0.2	2.2 ± 0.1	2.2 ± 0.1
[NTP]0.5	ud	0.3	0.2	ud	nd

^aAll assays were carried out in the tripartate buffer (pH 8.3).

^bThe values of V_max, [Asp]_0.5, and n_H were determined at a nucleotide effector concentration of 2 mM. The single exception was ATP, which was 4 mM in the assays with the native Sm holoenzyme.

^cMaximal velocity, estimated at saturating concentrations of substrate, is given as micro-moles per hour per milligram.

^dConcentration of aspartate (mM) required to produce half-maximal velocity.

^eHill coefficient (n_H) estimated from aspartate saturation kinetics (determined by the Hill plot).

^f[NTP]_0.5 is the nucleotide concentration (mM) at which the nucleotide exhibits half of its maximum effect. The values were calculated from Figure 5.

^gud indicates the value is unable to be determined.

^hnd indicates the value is not determined.