Fold-recognition and comparative modeling of human α2,3-sialyltransferases reveal their sequence and structural similarities to CstII from Campylobacter jejuni

Table 2 Roles deduced for some of the residues which are conserved in the eukaryotic SiaT superfamily and whose mutations have been experimentally characterized^#

Mutant	Equivalent residue in ST3Gal I	K_m(μM) Donor/Acceptor	Activity	Role deduced for the mutated residue from the modeled 3-D structures^‡
Mutations in L motif^¶
Wild type		50/331	100%
C181A	C142	-/-	< 5%	Structural role: involved in disulphide bridge
V184A	V145	300/372	~45%	Structural role: part of hydrophobic core^†
L190A	L151	597/188	~28%	Structural role: part of hydrophobic core^†
R207A	R168	-/-	< 5%	Structural role: is buried and hydrogen bonds with side chains of N147 and E178 (ST3Gal I numbering). N147 is replaced by Ser in ST3Gal V, but Ser does not form hydrogen bond with Arg
V220A	V181	343/260	30%	Structural role: part of the hydrophobic core^†
S222A	T183	-/-	< 5%	Structural role: These are at beginning of strand β4 and are solvent exposed
K223A	K184	330/400	32%
T225A	T186	160/200	27%
Mutations in S motif^§
Wild type		50/330	100
P318A	P267	-/-	very low	Structural role: helix J nucleator
S319A	S268	-/-	< 5%	Functional role: close to ribose
C332A	C281	-/-	< 5%	Structural role: forms disulphide bridge
V335L	V284	80/322	79%	Structural role: part of the hydrophobic core
V335A	V284	89/947	83%
Mutations in motif-III*
H299A		-/-	loss of activity	Functional role: close to phosphate
Y300A		-/-

^#Residues that are either strictly conserved or have conservative replacements in the eukaryotic SiaT superfamily are from [11].
^¶Data corresponds to mutants of ST6Gal I [8]. Hyphen (" - ") indicates values could not be determined.
^§Data corresponds to mutants of ST6Gal I [9]. Hyphen (" - ") indicates values could not be determined.
*Data corresponds to mutants of ST3Gal I [7]. Hyphen (" - ") indicates values could not be determined.
^‡The mutation is considered structural if it destabilizes the structure and is far from functional site and it is termed functional if it is part of the substrate binding site.
^†Are part of the same hydrophobic core.

ISSN: 1472-6807