View source for 1m03

[[Image:1m03.gif|left|200px]]

 {{Structure
|PDB= 1m03 |SIZE=350|CAPTION= <scene name='initialview01'>1m03</scene>, resolution 1.90&Aring;
|SITE= 
|LIGAND= <scene name='pdbligand=NAG:N-ACETYL-D-GLUCOSAMINE'>NAG</scene>, <scene name='pdbligand=CL:CHLORIDE+ION'>CL</scene>, <scene name='pdbligand=SO4:SULFATE+ION'>SO4</scene> and <scene name='pdbligand=GOL:GLYCEROL'>GOL</scene>
|ACTIVITY= [http://en.wikipedia.org/wiki/Beta-N-acetylhexosaminidase Beta-N-acetylhexosaminidase], with EC number [http://www.brenda-enzymes.info/php/result_flat.php4?ecno=3.2.1.52 3.2.1.52] 
|GENE= 
}}

'''Mutant Streptomyces plicatus beta-hexosaminidase (D313A) in complex with product (GlcNAc)'''


==Overview==
SpHex, a retaining family 20 glycosidase from Streptomyces plicatus, catalyzes the hydrolysis of N-acetyl-beta-hexosaminides. Accumulating evidence suggests that the hydrolytic mechanism involves substrate-assisted catalysis wherein the 2-acetamido substituent acts as a nucleophile to form an oxazolinium ion intermediate. The role of a conserved aspartate residue (D313) in the active site of SpHex was investigated through kinetic and structural analyses of two variant enzymes, D313A and D313N. Three-dimensional structures of the wild-type and variant enzymes in product complexes with N-acetyl-d-glucosamine revealed substantial differences. In the D313A variant the 2-acetamido group was found in two conformations of which only one is able to aid in catalysis through anchimeric assistance. The mutation D313N results in a steric clash in the active site between Asn-313 and the 2-acetamido group preventing the 2-acetamido group from providing anchimeric assistance, consistent with the large reduction in catalytic efficiency and the insensitivity of this variant to chemical rescue. By comparison, the D313A mutation results in a shift in a shift in the pH optimum and a modest decrease in activity that can be rescued by using azide as an exogenous nucleophile. These structural and kinetic data provide evidence that Asp-313 stabilizes the transition states flanking the oxazoline intermediate and also assists to correctly orient the 2-acetamido group for catalysis. Based on analogous conserved residues in the family 18 chitinases and family 56 hyaluronidases, the roles played by the Asp-313 residue is likely general for all hexosaminidases using a mechanism involving substrate-assisted catalysis.

==About this Structure==
1M03 is a [[Single protein]] structure of sequence from [http://en.wikipedia.org/wiki/Streptomyces_plicatus Streptomyces plicatus]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1M03 OCA]. 

==Reference==
Aspartate 313 in the Streptomyces plicatus hexosaminidase plays a critical role in substrate-assisted catalysis by orienting the 2-acetamido group and stabilizing the transition state., Williams SJ, Mark BL, Vocadlo DJ, James MN, Withers SG, J Biol Chem. 2002 Oct 18;277(42):40055-65. Epub 2002 Aug 8. PMID:[http://www.ncbi.nlm.nih.gov/pubmed/12171933 12171933]
[[Category: Beta-N-acetylhexosaminidase]]
[[Category: Single protein]]
[[Category: Streptomyces plicatus]]
[[Category: James, M N.G.]]
[[Category: Mark, B L.]]
[[Category: Vocadlo, D J.]]
[[Category: Williams, S J.]]
[[Category: Withers, S G.]]
[[Category: CL]]
[[Category: GOL]]
[[Category: NAG]]
[[Category: SO4]]
[[Category: beta-hexosaminidase]]
[[Category: family 20 glycosidase]]
[[Category: substrate assisted catalysis]]

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