Table 2. A comparison of primary and secondary KIE values for a variety of glycosidic bond formation and cleavage reactions.

Reaction	Mediator	Primary isotope effect [1-12C, 1-13C]	Secondary isotope effect [1-1H, 1-2H]	Initial orientation of leaving group*
4-Thiouridine synthesis	4-Thiouridine synthase (RNA)	1.002 ± 0.003	1.15 ± 0.012	α
RNA depurination (8)†	Ricin A-chain (protein)	0.996 ± 0.004	1.111 ± 0.009	β
Deoxyuridine hydrolysis (20)	Uracil-DNA glycosylase (protein)	1.010	1.201 ± 0.021	β
OMP synthesis (6)†‡	OPRTase (protein)	1.021 ± 0.004	1.135 ± 0.007	(β)
IMP synthesis (5)†	HGPRTase (protein)	1.026	1.20	α
AMP hydrolysis (24)†	Acid	1.026 ± 0.009	1.159 ± 0.01	β
Uridine hydrolysis (25)	Acid	ND	1.11	β
PRPP hydrolysis (5)†	Acid	1.01	1.27	α

The KIE effects observed for our ribozyme-promoted nucleotide synthesis reaction are highlighted in bold. OMP, orotidine 5′-monophosphate; OPRTase, orotate phosphoribosyltransferase; HGPRTase, hypoxanthine/guanine phosphoribosyltransferase; ND, not done.

^*The orientation of the leaving group (α or β) might have a subtle effect on chemistry. For example, 1-glucopyranosyl fluorides have differing KIE values depending on the orientation of the fluoride leaving group (22).

^†Tritium and ¹⁴C isotope experiments that have been converted to the corresponding ²H and ¹³C values by using the approximation of Swain and Schaad (28).

^‡KIE effects for OPRTase were studied by using the reverse reaction and phosphonoacetate (6).