Table 1. Kinetic Parameters for Activation of ScTIM by Oxydianions (Scheme 2) and Derived Parameters for the Binding of Dianions to [E·S]^‡ (Scheme 5)^a.

dianion	(kcat/Km)E (M–1 s–1)b	KX (mM)c,d	(kcat/Km)E·X (M–1 s–1)c,e	(kcat/Km)E·X/KX (M–2 s–1)	(K‡)X (M)f	RT ln(K‡)X (kcal/mol)g
none	0.062
HPO32–		18 ± 3	48 ± 4	2700	2.3 × 10–5	–6.3
FPO32–		5.1 ± 1.3	10 ± 0.7	2000	3.1 × 10–5	–6.1
HOPO32–		n.dh	n.dh	70 ± 10i	8.8 × 10–4	–4.2
SO42–		21 ± 4	2.8 ± 0.3	130	4.7 × 10–4	–4.5
S2O32–		7 ± 1.0	7.3 ± 0.3	1200	5.2 × 10–5	–5.8

^aReactions of 20 mM [1-¹³C]-GA in D₂O at pD 7.0 (20 mM imidazole), 25 °C and I = 0.1 (NaCl). The quoted uncertainty in the kinetic parameters is the standard error determined for the nonlinear least-squares fits of these data.

^bSecond-order rate constant for the ScTIM-catalyzed reaction of [1-¹³C]-GA determined for a reaction in the absence of dianion activator.

^cKinetic parameter determined from the fit of data shown in Figure 3 to eq 3.

^dDissociation constant for release of the oxydianion from ScTIM.

^eSecond-order rate constant for the reactions of [1-¹³C]-GA catalyzed by the enzyme–oxydianion complex (Scheme 2).

^fDissociation constant for release of the oxydianion from the transition state complex, calculated using eq 6, derived for Scheme 5.

^gIntrinsic Gibbs dianion binding free energy.

^hNot determined: the plot for activation by HOPO₃^2– (Figure 3) is linear through [HOPO₃^2–] = 14 mM.

ⁱThe slope of the linear correlation from Figure 3.