Title : Crystallographic structure of human
beta-hexosaminidase A : interpretation of Tay-Sachs mutations and loss of
GM2 ganglioside hydrolysis
Abstract :
- Lysosomal beta-hexosaminidase A (Hex A) is essential for the degradation of GM2 gangliosides in the central and peripheral nervous system
- Accumulation of GM2 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 A resolution and the structure of Hex A in complex with NAG-thiazoline, (NGT) to 3.25 A 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 alphabeta heterodimer , with each subunit having a functional active site
- Only the alpha-subunit active site can hydrolyze GM2 gangliosides due to a flexible loop structure that is removed post-translationally from beta, and to the presence of alphaAsn423 and alphaArg424
- The loop structure is involved in binding the GM2 activator protein , while alphaArg424 is critical for binding the carboxylate group of the N-acetyl-neuraminic acid residue of GM2.
- The beta-subunit lacks these key residues and has betaAsp452 and betaLeu453 in their place; the beta-subunit therefore cleaves only neutral substrates efficiently
- Mutations in the alpha-subunit , associated with TSD, and those in the beta-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