Crystallographic Structure of Human β-Hexosaminidase A: Interpretation of Tay-Sachs Mutations and Loss of G_M2 Ganglioside Hydrolysis

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• 作者：M. Joanne Lemieux ; Brian L. Mark ; Maia M. Cherney ; Stephen G. Withers ; Don J. Mahuran and Michael N.G. James
• 刊名：Journal of Molecular Biology
• 出版年：2006
• 出版时间：16 June 2006
• 年：2006
• 卷：359
• 期：4
• 页码：913-929
• 全文大小：2348 K

文摘

Lysosomal β-hexosaminidase A (Hex A) is essential for the degradation of G_M2 gangliosides in the central and peripheral nervous system. Accumulation of G_M2 leads to severely debilitating neurodegeneration associated with Tay-Sachs disease (TSD), Sandoff disease (SD) and AB variant. Here, we present the X-ray crystallographic structure of Hex A to 2.8 Å resolution and the structure of Hex A in complex with NAG-thiazoline, (NGT) to 3.25 Å resolution. NGT, a mechanism-based inhibitor, has been shown to act as a chemical chaperone that, to some extent, prevents misfolding of a Hex A mutant associated with adult onset Tay Sachs disease and, as a result, increases the residual activity of Hex A to a level above the critical threshold for disease. The crystal structure of Hex A reveals an αβ heterodimer, with each subunit having a functional active site. Only the α-subunit active site can hydrolyze G_M2 gangliosides due to a flexible loop structure that is removed post-translationally from β, and to the presence of αAsn423 and αArg424. The loop structure is involved in binding the G_M2 activator protein, while αArg424 is critical for binding the carboxylate group of the N-acetyl-neuraminic acid residue of G_M2. The β-subunit lacks these key residues and has βAsp452 and βLeu453 in their place; the β-subunit therefore cleaves only neutral substrates efficiently. Mutations in the α-subunit, associated with TSD, and those in the β-subunit, associated with SD are discussed. The effect of NGT binding in the active site of a mutant Hex A and its effect on protein function is discussed.