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Publication Abstract from PubMed

Cell surface glycans are often terminated by sialic acid, which is incorporated onto sugar acceptors by sialyltransferases. The crystal structure of the GT family 42 Campylobacter jejuni alpha-2,3/2,8-sialyltransferase (CstII) provides key insights into the sialyl-transfer mechanism, including tentative identification of His188 as the catalytic base. In support of this hypothesis, the CstII-H188A mutant is able to catalyze sialyl transfer from CMP-Neu5Ac to added anions such as azide and formate, but not to its natural sugar acceptor lactose. Complementing this work, NMR spectroscopy was used to investigate the structure and dynamics of CstII and to measure the intrinsic pKa value of His188 for comparison with the pKa determined from the pH-dependent kcat/KM of enzyme. By systematically introducing point mutations at the subunit interfaces, two active monomeric variants, CstII-F121D and CstII-Y125Q, were obtained and characterized. In contrast to the wild-type tetramer, the monomeric CstII variants yielded good quality 1H/15N-HSQC and 1H/13C methyl-TROSY NMR spectra. However, the absence of signals from approximately one half of the amides in the 1H/15N-HSQC spectra of both monomeric forms suggests that the enzyme undergoes substantial conformational motions on a msec-musec timescale. The histidine pKa values of CstII-F121D in its apo form were measured by monitoring the pH-dependent chemical shifts of [13Cepsilon1]-histidine, biosynthetically incorporated into the otherwise uniformly deuterated protein. Consistent with its proposed catalytic role, the site-specific pKa value ~ 6.6 of His188 matches the apparent pKa value ~ 6.5 governing the pH-dependence of kcat/KM for CstII towards CMP-Neu5Ac in the presence of saturating acceptor substrate.

NMR Spectroscopic Characterization of the Sialyltransferase CstII from Camplyobacter jejuni: Histidine 188 is the General Base.,Chan PH, Lairson LL, Lee HJ, Wakarchuk WW, Strynadka NC, Withers SG, McIntosh LP Biochemistry. 2009 Oct 13. PMID:19824695^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.