. 2019 Mar 13;6:8. doi: 10.3389/fmolb.2019.00008

Table 2.

Kinetic indication that mutations of SR residues in the “triple serine loop” to the corresponding aspartate racemase (AR) residues tends to bias enzyme activity toward racemization over β-elimination.

Variant	Position				Racemization Efficiency^‡	$k_{c a t} (r a c) : k_{c a t} (β - e l i m)$	$K_{c a t} / K_{m} (r a c) : k_{c a t} / K_{m} (β - e l i m)$	References
	$\overset{}{\overset{︷}{\begin{matrix} 152 & 153 & 154 & 155 \end{matrix}}}$
“Triple Ser Loop”- Relation to Racemase Activity
hSR⁺	H	P	N	Q	100	1:4	1:3.7	Hoffman et al., 2009; Nelson et al., 2017; Canosa et al., 2018
mSR⁺	H	P	N	Q	133	1:2.3	1:1.3	Uda et al., 2016
mSR	S	P	N	Q	–	–	1:1.4	Foltyn et al., 2005
mSR	S	P	N	Q	121	rac only^*	rac only^*	Uda et al., 2017
mSR	H	S	N	Q	–	–	4:1	Foltyn et al., 2005
mSR	H	S	N	Q	1140	23:1	2:1	Uda et al., 2017
mSR	H	P	S	Q	174	6.8:1	2.3:1	Uda et al., 2017
mSR	H	P	N	D	46 (700)^‡	7:1	7.3:1	Foltyn et al., 2005
mSR	S	S	N	Q	100	rac only^*	rac only^*	Uda et al., 2017
mSR	H	S	S	Q	376	3.5:1	1:12.8	Uda et al., 2017
mSR	S	P	S	Q	160	rac only^*	rac only^*	Uda et al., 2017
mSR	S	S	S	Q	55	rac only^*	rac only^*	Uda et al., 2017
spSR⁺	P	P	Y	D	^**	1:29^¶	1:26^§	Yamauchi et al., 2009
amAR⁺	H	S	S	D	24	8.6:1	2.2:1	Uda et al., 2017
cgAR⁺	S	S	S	D	18	rac only^*	rac only^*	Uda et al., 2017

Residue numbering corresponds to the hSR sequence. SR wild type residues are in blue; AR from Acropora millepora (amAR) and Crassostrea gigas (CgAR) are being compared here with the CgAR wild type residues appearing in green. Emboldened entries indicate enzymes for which racemization is preferred over elimination.

⁺

designates the wild type enzyme.

^‡

These values are normalized to the average value of k_cat for racemization for wt hSR (30 ± 15 min^-1) which is arbitrarily set at 100.

no β-elimination activity observed.

the value in parentheses here represents the value of kcat(rac) for this mutant normalized to the k_cat(rac) of the wt-mSR reported by these authors which is more than an order of magnitude lower than that determined by other groups.

^**

these authors report “V_max = 30 U/mg,” but this is actually a specific activity reported for a standard assay ^@ [L-Ser] = 10 mM. Since this concentration is very close to K_m for L-Ser, the velocities reported are well-below V_max.

^¶

this value represents relative velocity for racemization vs. elimination at 10 mM concentration rather than a ratio of kcat values.

^§

whereas the K_m values for these activities are reported, k_cat(V_max) values are not. For the latter, reported relative velocity for racemization vs. elimination at 10 mM concentration is given.