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Glycobiology. 2015 Sep;25(9):992-1006. doi: 10.1093/glycob/cwv034. Epub 2015 May 28.

Human sialic acid acetyl esterase: Towards a better understanding of a puzzling enzyme.

Author information

1
Unit of Biotechnology.
2
Unit of Biology and Genetics, Department of Molecular and Translational Medicine (DMTM), Viale Europa 11, Brescia 25123, Italy.
3
Institute of Biochemistry, Christian-Albrechts University, Otto-Hahn-Platz 9, Kiel 24118, Germany.
4
Department of Translational Immunology, Unit of Glycoimmunology, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg 69120, Germany.
5
Unit of Biotechnology eugenio.monti@unibs.it.

Abstract

Sialic acid acetyl esterase (SIAE) removes acetyl moieties from the hydroxyl groups in position 9 and 4 of sialic acid. Recently, a dispute has been opened on its association to autoimmunity. In order to get new insights on human SIAE biology and to clarify its seemingly contradictory molecular properties, we combined in silico characterization, phylogenetic analysis and homology modeling with cellular studies in COS7 cells. Genomic and phylogenetic analysis revealed that in most tissues only the "long" isoform, originally referred to lysosomal sialic acid esterase, is detected. Using the homology modeling approach, we predicted a model of SIAE 3D structure, which fulfills the topological features of SGNH-hydrolase family. In addition, the model and site-directed mutagenesis experiments allowed the definition of the residues involved in catalysis. SIAE transient expression revealed that the protein is glycosylated and is active in vitro as an esterase with a pH optimum corresponding to 8.4-8.5. Moreover, glycosylation influences the biological activity of the enzyme and is essential for release of SIAE into the culture medium. According to these findings, co-localization experiments demonstrated the presence of SIAE in membranous structures corresponding to endoplasmic reticulum and Golgi complex. Thus, at least in COS7 cells, SIAE behaves as a typical secreted enzyme, subjected to glycosylation and located along the classical secretory route or in the extracellular space. In these environments, the enzyme could act on 9-O-acetylated sialic acid residues, contributing to the fine-tuning of the various functions played by this acidic sugar.

KEYWORDS:

3D model; Neu5Ac; O-acetylation; in silico characterization; phylogenesis; sialic acid acetyl esterase-SIAE; subcellular localization

PMID:
26022516
DOI:
10.1093/glycob/cwv034
[Indexed for MEDLINE]

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