'''Crystal structure analysis of sialic acid synthase (NeuB)from Neisseria meningitidis, bound to Mn2+, Phosphoenolpyruvate, and N-acetyl mannosaminitol'''
===Crystal structure analysis of sialic acid synthase (NeuB)from Neisseria meningitidis, bound to Mn2+, Phosphoenolpyruvate, and N-acetyl mannosaminitol===
==Overview==
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In Neisseria meningitidis and related bacterial pathogens, sialic acids play critical roles in mammalian cell immunity evasion and are synthesized by a conserved enzymatic pathway that includes sialic acid synthase (NeuB, SiaC, or SynC). NeuB catalyzes the condensation of phosphoenolpyruvate (PEP) and N-acetylmannosamine, directly forming N-acetylneuraminic acid (or sialic acid). In this paper we report the development of a coupled assay to monitor NeuB reaction kinetics and an 18O-labeling study that demonstrates the synthase operates via a C-O bond cleavage mechanism. We also report the first structure of a sialic acid synthase, that of NeuB, revealing a unique domain-swapped homodimer architecture consisting of a (beta/alpha)8 barrel (TIM barrel)-type fold at the N-terminal end and a domain with high sequence identity and structural similarity to the ice binding type III antifreeze proteins at the C-terminal end of the enzyme. We have determined the structures of NeuB in the malate-bound form and with bound PEP and the substrate analog N-acetylmannosaminitol to 1.9 and 2.2 A resolution, respectively. Typical of other TIM barrel proteins, the active site of NeuB is located in a cavity at the C-terminal end of the barrel; however, the positioning of the swapped antifreeze-like domain from the adjacent monomer provides key residues for hydrogen bonding with substrates in the active site of NeuB, a structural feature that leads to distinct modes of substrate binding from other PEP-utilizing enzymes that lack an analogous antifreeze-like domain. Our observation of a direct interaction between a highly ordered manganese and the N-acetylmannosaminitol in the NeuB active site also suggests an essential role for the ion as an electrophilic catalyst that activates the N-acetylmannosamine carbonyl to the addition of PEP.
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==About this Structure==
==About this Structure==
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[[Category: Domain-swapped dimer]]
[[Category: Domain-swapped dimer]]
[[Category: Tim barrel]]
[[Category: Tim barrel]]
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Revision as of 13:44, 29 July 2008
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Structural and mechanistic analysis of sialic acid synthase NeuB from Neisseria meningitidis in complex with Mn2+, phosphoenolpyruvate, and N-acetylmannosaminitol., Gunawan J, Simard D, Gilbert M, Lovering AL, Wakarchuk WW, Tanner ME, Strynadka NC, J Biol Chem. 2005 Feb 4;280(5):3555-63. Epub 2004 Oct 29. PMID:15516336
