Table 1.

Comparison of apparent steady-state kinetics (k_cat and K_m), BIEs, forward commitment factors, KIEs, and BIEs with SAM and SeAM as cofactors (cosubstrates) in SET8-catalyzed H4K20 monomethylation

Parameters	SAM	SeAM
kcat, min−1	7.0 ± 0.8	13 ± 2*
Km, μM	16 ± 6	16 ± 6*
CD3 or 13CD3 BIE	0.959 ± 0.006	0.962 ± 0.004
CT3 or 14CT3 BIE	0.979 ± 0.009	NA
Cf	0.10 ± 0.01	0.10 ± 0.01
${}^{C{D}_3}\left(V/K\right)$	0.878 ± 0.008	0.99 ± 0.02
${}^{C{D}_3}k$	0.90 ± 0.01	1.02 ± 0.02
${}^{C{D}_3}k,$ calculated	0.896†	1.018‡
${}^{{}^{13}C{D}_3}\left(V/K\right)$	0.907 ± 0.007	1.04 ± 0.01
${}^{{}^{13}C{D}_3}k$	0.94 ± 0.02	1.09 ± 0.02
${}^{{}^{13}C{H}_3}k$	1.04 ± 0.02	1.06 ± 0.02
${}^{{}^{13}C{H}_3}k,$ calculated	1.058†	1.061‡
${}^{C{T}_3/{}^{14}C{H}_3}\left(V/K\right)$	0.785 ± 0.009	NA
${}^{C{T}_3/{}^{14}C{H}_3}k$	0.78 ± 0.01	NA
${}^{C{T}_3/{}^{14}C{H}_3}k,$ calculated	0.773†	NA

NA, not applicable.

^*These values were calculated on the basis of k_cat and K_m values of SAM, ${}^{\text{SeAM}/\text{SAM}}(k_{\text{cat}}/K_{\text{m,cofactor}})$ and ${}^{\text{SeAM}/\text{SAM}}(1/K_{\text{m,cofactor}})$ (1.94 and 1.03 for the latter two, respectively).

^†The experimental SAM KIEs match TS structures with d_C-S = 2.00–2.05 Å and d_C-N = 2.35–2.40 Å. The calculated KIEs are listed for a representative TS structure with d_C-S = 2.05 Å (bond order 0.845), and d_C-N = 2.38 Å (bond order 0.117).

^‡The experimental SeAM KIEs match a wide range of TS structures with d_C-Se + d_C-N = 4.8–4.9 Å. The calculated KIEs are listed for a representative TS structure with d_C-Se = 2.50 Å and d_C-N = 2.35 Å.