Glycosidase mechanisms

Current Opinion in Chemical Biology

Volumn 6, Issue 5, 2002, Pages 619-629

  Vasella, Andrea ^a   Davies, Gideon J ^b   Böhm, Matthias ^a  

^a ETH ZURICH   (Switzerland)

^b UNIVERSITY OF YORK   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

1 DEOXYNOJIRIMYCIN; 2 DIMETHYLAMINO 5,6 DIHYDROXYTETRALIN; CYCLOPENTANE DERIVATIVE; GLYCOSIDASE; GLYCOSIDASE INHIBITOR; ISOFAGOMINE; LIGAND; PYRIDAZINONE DERIVATIVE; PYRROLIDINE DERIVATIVE; TETRAHYDROPYRIDAZINONE; UNCLASSIFIED DRUG;

COMPLEX FORMATION; DRUG DEVELOPMENT; ENZYME CONFORMATION; ENZYME INHIBITION; ENZYME MECHANISM; ENZYME STRUCTURE; NONHUMAN; REVIEW;

EID: 0036802905     PISSN: 13675931     EISSN: None     Source Type: Journal    
DOI: 10.1016/S1367-5931(02)00380-0     Document Type: Review

References (99)

1. Withers S.G. Mechanisms of glycosyl transferases and hydrolases. Carbohydr Polym. 44:2001;325-337.
2. Rye C.S., Withers S.G. Glycosidase mechanisms. Curr Opin Chem Biol. 4:2000;573-580.
3. Zechel D.L., Withers S.G. Glycosidase mechanisms: anatomy of a finely tuned catalyst. Acc Chem Res. 33:2000;11-18.
4. Henrissat B., Davies G. Glycosidase families. Biochem Soc Trans. 26:1998;153-156.
5. Markovic-Housley Z., Miglierini G., Soldatova L., Rizkallah P.J., Müller U., Schirmer T. Crystal structure of hyaluronidase, a major allergen of bee venom. Structure. 8:2000;1025-1035. Another fine example of an enzyme utilising substrate-assisted catalysis via acetamido group participation.
6. Mark B.L., Vocadlo D.J., Knapp S., Triggs-Raine B.L., Withers S.G., James N.G.J. Crystallographic evidence for substrate-assisted catalysis in a bacterial β-hexosaminidase. J Biol Chem. 276:2001;10330-10337.
7. Mark B.L., Vocadlo D.J., Zhao D.L., Knapp S., Withers S.G., James M.N.G. Biochemical and structural assessment of the 1-N-azasugar GalNAc-isofagomine as a potent family 20 β-N-acetylhexosaminidase inhibitor. J Biol Chem. 276:2001;42131-42137.
8. Papanikolaou Y., Prag G., Tavlas G., Vorgias C.E., Oppenheim A.B., Petratos K. High resolution structural analyses of mutant chitinase A complexes with substrates provide new insight into the mechanism of catalysis. Biochemistry. 40:2001;11338-11343.
9. van Aalten D.M.F., Komander D., Synstad B., Gåseidnes S., Peter M.G., Eijsink V.G.H. Structural insights into the catalytic mechanism of a family 18 exo-chitinase. Proc Natl Acad Sci USA. 98:2001;8979-8984.
10. 2+, long acknowledged from kinetic studies, in substrate binding and catalysis by this exceptionally important family of enzymes.
11. Numao S., He S.M., Evjen G., Howard S., Tollersrud O.K., Withers S.G. Identification of Asp197 as the catalytic nucleophile in the family 38 α-mannosidase from bovine kidney lysosomes. FEBS Lett. 484:2000;175-178.
12. Vallée F., Lipari F., Yip P., Sleno B., Herscovics A., Howell P.L. Crystal structure of a class I α1,2-mannosidase involved in N-glycan processing and endoplasmic reticulum quality control. EMBO J. 19:2000;581-588.
13. 4 conformation.
14. Ly H.D., Howard S., Shum K., He S., Zhu A., Withers S.G. The synthesis, testing and use of 5-fluoro-α-D-galactosyl fluoride to trap an intermediate on green coffee bean α-galactosidase and identify the catalytic nucleophile. Carbohydr Res. 329:2000;539-547.
15. Garman S.C., Hannick L., Zhu A., Garboczi D.N. The 1.9 Å structure of α-N-acetylgalactosaminidase: molecular basis of glycosidase deficiency diseases. Structure. 10:2002;425-434.
16. Egloff M.-P., Uppenberg J., Haalck L., van Tilbeurgh H. Crystal structure of maltose phosphorylase from Lactobacillus brevis: unexpected evolutionary relationship with glucoamylases. Structure. 9:2001;689-697. An exciting structure that reveals the evolutionary relationship between the inverting hydrolase glucoamylase and this inverting phosphorylase.
17. Nurizzo D., Nagy T., Gilbert H.J., Davies G.J. The structural basis for catalysis and specificity of the Pseudomonas cellulosa α-glucuronidase, GlcA67A. Structure. 10:2002;547-556. A fine example of an inverting glycosidase, which despite numerous crystal structures of complexes and kinetics of mutants still leaves questions as to the exact nature of the general base assistance to nucleophilic attack.
18. Nurizzo D., Turkenburg J.P., Charnock S.J., Roberts S.M., Dodson E.J., McKie V.A., Taylor E.J., Gilbert H.J., Davies G.J. Cellvibrio japonicus α-l-arabinanase 43A has a novel five-blade propeller fold. Nat Struct Biol. 9:2002;665-668.
19. Michel G., Chantalat L., Fanchon E., Henrissat B., Kloareg B., Dideberg O. The ι-carrageenase of Alteromonas fortis - a β-helix fold-containing enzyme for the degradation of a highly polyanionic polysaccharide. J Biol Chem. 276:2001;40202-40209.
20. Vocadlo D.J., Davies G.J., Laine R., Withers S.G. Catalysis by hen egg-white lysozyme proceeds via a covalent intermediate. Nature. 412:2001;835-838. This paper finally shows that hen egg white lysozyme, in common with other retaining hydrolases, performs catalysis via formation of a covalent intermediate in contrast to the mechanism shown in most textbooks.
21. Namchuk N.M., McCarter J.D., Becalski A., Andrews T., Withers S.G. The role of sugar substituents in glycoside hydrolysis. J Am Chem Soc. 122:2000;1270-1277. The relative rates of hydrolysis of deoxy- and deoxyfluoro-pyranosides are correlated with the influence of the substituents on the stability of the intermediate oxycarbenium cation.
22. Zhu J., Bennet A.J. Solvolyses of 2-deoxy-α- and β-D-glucopyranosyl 4′-bromoisoquinolinium tetrafluoroborates. J Org Chem. 65:2000;4423-4430.
23. Namchuk M.N., Withers S.G. Mechanism of Agrobacterium β-glucosidase: kinetic analysis of the role of noncovalent enzyme/substrate interactions. Biochemistry. 34:1995;16194-16202.
24. Nishio T., Hakamata W., Kimura A., Chiba S., Kawachi R., Oku T. Glycon specificity profiling of α-glucosidases using monodeoxy and mono-O-methyl derivatives of p-nitrophenyl α-D-glucopyranoside. Carbohydr Res. 337:2002;629-634.
25. Frandsen T.P., Palcic M.M., Svensson B. Substrate recognition by three family 13 yeast α-glucosidases - evaluation of deoxygenated and conformationally biased isomaltosides. Eur J Biochem. 269:2002;728-734.
26. Sierks M.R., Svensson B. Energetic and mechanistic studies of glucoamylase using molecular recognition of maltose OH groups coupled with site-directed mutagenesis. Biochemistry. 39:2000;8585-8592.
27. Tanaka S.E., Zhu J., Huang X., Lipari F., Bennet A.J. Glycosidase-catalysed hydrolysis of 2-deoxyglucopyranosyl pyridinium salts: effect of the 2-OH group on binding and catalysis. Can J Chem. 78:2000;577-582.
28. Werner R.M., Stivers J.T. Kinetic isotope effect studies of the reaction catalyzed by uracil DNA glycosylase: evidence for an oxocarbenium ion - uracil anion intermediate. Biochemistry. 39:2000;14054-14064. The geometric interpretation of KIEs for the DNA repair enzyme indicates a conformation with maximal stabilisation of a cationic TST.
29. Chen X.-Y., Berti P.J., Schramm V.L. Ricin A-chain: kinetic isotope effects and transition state structure with stem-loop RNA. J Am Chem Soc. 122:2000;1609-1617. A detailed analysis of KIEs evidences a cationic intermediate.
30. Knier B.L., Jencks W.P. Mechanism of reactions of N-(methoxymethyl)-N,N-dimethylanilinium ions with nucleophilic reagents. J Am Chem Soc. 102:1980;6789-6798.
31. Parikh S.S., Walcher G., Jones G.D., Slupphaug G., Krokan H.E., Blackburn G.M., Tainer J.A. Uracil-DNA glycosylase-DNA substrate and product structures: Conformational strain promotes catalytic efficiency by coupled stereoelectronic effects. Proc Natl Acad Sci USA. 97:2000;5083-5088.
32. Schramm V.L. Transition state variation in enzymatic reactions. Curr Opin Chem Biol. 5:2001;556-563.
33. Tews I., Perrakis A., Oppenheim A., Dauter Z., Wilson K.S., Vorgias C.E. Bacterial chitobiase structure provides insight into catalytic mechanism and the basis of Tay-Sachs disease. Nat Struct Biol. 3:1996;638-648.
34. Davies G.J., Mackenzie L., Varrot A., Dauter M., Brzozowski A.M., Schülein M., Withers S.G. Snapshots along an enzymatic reaction coordinate: analysis of a retaining β-glycoside hydrolase. Biochemistry. 37:1998;11707-11713.
35. Sulzenbacher G., Driguez H., Henrissat B., Schülein M., Davies G.J. Structure of the Fusarium oxysporum endoglucanase I with a nonhydrolyzable substrate analogue: Substrate distortion gives rise to the preferred axial orientation for the leaving group. Biochemistry. 35:1996;15280-15287.
36. Fort S., Varrot A., Schülein M., Cottaz S., Driguez H., Davies G.J. Mixed-linkage cellooligosaccharides: a new class of glycoside hydrolase inhibitors. Chem Bio Chem. 5:2001;319-325. The observation that a mixed α- and β-linked oligosaccharide may prove a strong glycosidase inhibitor by mimicking the axial bond of a distorted substrate. It remains to be seen if such facets may be harnessed in better TST mimics.
37. 2,5B conformation for the glycosyl-enzyme intermediate revealed by the structure of the Bacillus agaradhaerens family 11 xylanase. Chem Biol. 6:1999;483-492.
38. 2,5B conformations at the active center. Acta Crystallogr Sect D Biol Cryst. D57:2001;1344-1347.
39. Sidhu G., Withers S.G., Nguyen N.T., McIntosh L.P., Zisler L., Brayer D.T. Sugar ring distortion in the glycosyl-enzyme intermediate of a family G/11 xylanase. Biochemistry. 38:1999;5346-5354.
40. 2,5 transition-state.
41. Winkler D.A., Holan G. Design of potential anti-HIV agents. 1. Mannosidase inhibitors. J Med Chem. 32:1989;2084-2089.
42. Wolfenden R., Lu X., Young G. Spontaneous hydrolysis of glycosides. J Am Chem Soc. 120:1998;6814-6815.
43. Lillelund V.H., Jensen H.H., Liang X.F., Bols M. Recent developments of transition-state analogue glycosidase inhibitors of non-natural product origin. Chem Rev. 102:2002;515-553.
44. Asano N., Nash R.J., Molyneux R.J., Fleet G.W.J. Sugar-mimic glycosidase inhibitors: natural occurrence, biological activity and prospects for therapeutic application. Tetrahedron Asymmetry. 11:2000;1645-1680.
45. Asano N. Alkaloidal sugar mimetics: biological activities and therapeutic applications. J Enzym Inhib. 15:2000;215-234.
46. El Ashry ESH, Rashed N, Shobier AHS: Glycosidase inhibitors and their chemotherapeutic value, part 1-3. Pharmazie 2000, 55:251-262, 331-348, 403-415.
47. Ermert P., Vasella A., Weber M., Rupitz K., Withers S.G. Configurationally selective transition state analogue inhibitors of glycosidases. A study with nojiritetrazoles, a new class of glycosidase inhibitors. Carbohydr Res. 250:1993;113-128.
48. Yuasa H., Takada J., Hashimoto H. Glycosidase inhibition by cyclic sulfonium compounds. Bioorg Med Chem Lett. 11:2001;1137-1139.
49. Muraoka O., Ying S., Yoshikai K., Matsuura Y., Yamada E., Minematsu T., Tanabe G., Matsuda H., Yoshikawa M. Synthesis of a nitrogen analogue of salacinol and its α-glucosidase inhibitory activity. Chem Pharm Bull. 49:2001;1503-1505.
50. Ghavami A., Johnston B.D., Jensen M.T., Svensson B., Pinto B.M. Synthesis of nitrogen analogues of salacinol and their evaluation as glycosidase inhibitors. J Am Chem Soc. 123:2001;6268-6271.
51. Britton D., Dunitz J.D. Directional preferences of approach of nucleophiles to sulfonium ions. Helv Chim Acta. 63:1980;1068-1073.
52. Heightman T.D., Vasella A.T. Recent insights into inhibition, structure, and mechanism of configuration-retaining glycosidases. Angew Chem Int Ed Engl. 38:1999;750-770.
53. Vonhoff S., Piens K., Pipelier M., Braet C., Claeyssens M., Vasella A. Inhibition of cellobiohydrolases from Trichoderma reesei. Synthesis and evaluation of some glucose, cellobiose, and cellotriose derived hydroximolactams and imidazoles. Helv Chim Acta. 82:1999;963-980.
54. Notenboom V., Williams S.J., Hoos R., Withers S.G., Rose D.R. Detailed structural analysis of glycosidase/inhibitor interactions: complexes of Cex from Cellulomonas fimi with xylobiose-derived aza-sugars. Biochemistry. 39:2000;11553-11563.
55. Williams S.J., Hoos R., Withers S.G. Nanomolar versus millimolar inhibition by xylobiose-derived azasugars: significant differences between two structurally distinct xylanases. J Am Chem Soc. 122:2000;2223-2235. The differences of inhibition strength is correlated with the syn- or anti-protonation trajectory.
56. Bülow A., Plesner I.W., Bols M. Slow inhibition of almond β-glucosidase by azasugars: determination of activation energies for slow binding. Biochim Biophys Acta. 1545:2001;207-215.
57. Bülow A., Plesner I.W., Bols M. A large difference in the thermodynamics of binding of isofagomine and 1-deoxynojirimycin to β-glucosidase. J Am Chem Soc. 122:2000;8567-8568. A stimulating and controversial paper highlighting the importance of determining enthalpic and entropic contributions to the binding of inhibitors.
58. Horn J.R., Russel D., Lewis E.A., Murphy K.P. van't Hoff and calorimetric enthalpies from isothermal titration calorimetry: are there significant discrepancies? Biochemistry. 40:2001;1774-1778.
59. Naghibi H., Tamura A., Sturtevant J.M. Significant discrepancies between van't Hoff and calorimetirc enthalpies. Proc Natl Acad Sci USA. 92:1995;5597-5599.
60. Wolfenden R., Snider M.J., Ridgway C., Miller B. The temperature dependence of enzyme rate enhancements. J Am Chem Soc. 34:1999;7419-7420.
61. Wolfenden R., Snider M.J. The depth of chemical time and the power of enzymes as catalysts. Acc Chem Res. 34:2001;938-945.
62. Kondo K., Adachi H., Shitara E., Kojima F., Nishimura Y. Glycosidase inhibitors of gem-diamine 1-N-iminosugars: structures in media of enzyme assays. Bioorg Med Chem. 9:2001;1091-1095.
63. Lorthiois E, Meyyappan M, Vasella A: β-glycosidase inhibitors mimicking the pyranoside boat conformation. Chem Commun 2000:1829-1830. The first selective inhibitor mimicking a distorted conformation of a substrate.
64. Boss O., Leroy E., Blaser A., Reymond J.-L. Synthesis and evaluation of iminocyclopentitol inhibitors of β-glucosidases. Org Lett. 2:2000;151-154.
65. Kleban M., Hilgers P., Greul J.N., Kugler R.D., Li J., Picasso S., Vogel P., Jager V. Amino(hydroxymethyl)cyclopen-tanetriols, an emerging class of potent glycosidase inhibitors - part I: synthesis and evaluation of β-D-pyranoside analogues in the manno, gluco, galacto, and GlcNAc series. ChemBioChem. 2:2001;365-368.
66. Panday N., Meyyappan M., Vasella A. A comparison of glucose- and glucosamine-related inhibitors: probing the interaction of the 2-hydroxy group with retaining β-glucosidases. Helv Chim Acta. 83:2000;513-538. The inhibition of a series of 2-amino sugars evidences lateral protonation and determines the extent to which the interaction of C(2)OH with the cat. nucleophile contributes to inhibition.
67. Williams S.J., Notenboom V., Wicki J., Rose D.R., Withers S.G. A new, simple, high-affinity glycosidase inhibitor: analysis of binding through X-ray crystallography, mutagenesis, and kinetic analysis. J Am Chem Soc. 122:2000;4229-4230.
68. Ramana C.V., Vasella A. Sugar-derived di- and tetrahydropyridazinones: synthesis of new glycosidase inhibitors. Helv Chim Acta. 83:2000;1599-1610.
69. Liu H., Liang X., Søhoel H., Bülow A., Bols M. Noeuromycin, a glycosyl cation mimic that strongly inhibits glycosidases. J Am Chem Soc. 123:2001;5116-5117.
70. Panday N., Canac Y., Vasella A. Very strong inhibition of glucosidases by C(2)-substituted tetrahydroimidazopyridines. Helv Chim Acta. 83:2000;58-79. The introduction of a flexible hydrophobic aglycon-mimic led to the strongest known inhibitor of β-glucosidases.
71. Legler G. Glycoside hydrolases: mechanistic information from studies with reversible and irreversible inhibitors. Adv Carbohydr Chem Biochem. 48:1990;319-384.
72. Blaser A., Reymond J.L. Aminocyclopentitol inhibitors of α-l-fucosidases. Helv Chim Acta. 84:2001;2119-2131.
73. Wrodnigg T.M., Withers S.G., Stutz A.E. Novel, lipophilic derivatives of 2,5-dideoxy-2,5-imino-D-mannitol (DMDP) are powerful β-glucosidase inhibitors. Bioorg Med Chem Lett. 11:2001;1063-1064.
74. Hermetter A., Scholze H., Stütz A.E., Withers S.G., Wrodnigg T.M. Powerful probes for glycosidases: novel, fluorescent tagged glycosidase inhibitors. Bioorg Med Chem. 11:2001;1339-1342.
75. Popowycz F., Gerber-Lemaire S., Demange R., Rodriguez-Garcia E., Asenjo A.T.C., Robina I., Vogel P. Derivatives of (2R,3R,4S)-2-aminomethylpyrrolidine-3,4-diol are selective α-mannosidase inhibitors. Bioorg Med Chem Lett. 11:2001;2489-2493.
76. Saotome C., Wong C.-H., Kanie O. Combinatorial library of five-membered iminocyclitol and the inhibitory activities against glyco-enzymes. Chem Biol. 8:2001;1061-1070.
77. Chand P., Kotian P.L., Dehghani A., El-Kattan Y., Lin T.-H., Hutchison T.L., Babu Y.S., Bantia S., Elliott A.J., Montgomery J.A. Systematic structure-based design and stereoselective synthesis of novel multisusbstituted cyclopentane derivatives with potent anti-influenza activity. J Med Chem. 44:2001;4379-4392.
78. Liu J., Shikman A.R., Lotz M.K., Wong C.-H. Hexosaminidase inhibitors as new drug candidates for the therapy of osteoarthritis. Chem Biol. 8:2001;701-711.
79. Mackenzie L.F., Wang Q., Warren R.A.J., Withers S.G. Glycosynthases: mutant glycosidases for oligosaccharide synthesis. J Am Chem Soc. 120:1998;5583-5584.
80. Faijes M., Fairweather J.K., Driguez H., Planas A. Oligosaccharide synthesis by coupled endo-glycosynthases of different specificity: a straightforward preparation of two mixed-linkage hexasaccharide substrates of 1,3/1,4-β- glucanases. Chem Eur ET J. 7:2001;4651-4655. A particularly striking example for the use of glycosynthases in the preparation of oligosaccharides.
81. Fort S., Boyer V., Greffe L., Davies G.J., Moroz O., Christiansen L., Schülein M., Cottaz S., Driguez H. Highly efficient synthesis of β(1→4)-oligo- and -polysaccharides using a mutant cellulase. J Am Chem Soc. 122:2000;5429-5437.
82. Trincone A., Perugino G., Rossi M., Moracci M. A novel thermophilic glycosynthase that effects branching glycosylation. Bioorg Med Chem Lett. 10:2000;365-368.
83. Blanchard J.E., Withers S.G. Rapid screening of the aglycone specificity of glycosidases: applications to enzymatic synthesis of oligosaccharides. Chem Biol. 8:2001;627-633.
84. Corbett K., Fordham-Skelton A.P., Gatehouse J.A., Davis B.G. Tailoring the substrate specificity of the β-glycosidase from the thermophilic archaeon Sulfolobus solfataricus. FEBS Lett. 509:2001;355-360.
85. Zechel D.L., Withers S.G. Dissection of nucleophilic and acid-base catalysis in glycosidases. Curr Opin Chem Biol. 5:2001;643-649.
86. HA differences of cat. acid and nucleophile.
87. HA of the cat. acid and nucleophile and its consequences for catalysis.
88. Rau A., Hogg T., Marquardt R., Hilgenfeld R. A new lysozyme fold - crystal structure of the muramidase from Streptomyces coelicolor at 1.65 Å resolution. J Biol Chem. 276:2001;31994-31999.
89. Hogg D., Woo E.J., Bolam D.N., McKie V.A., Gilbert H.J., Pickersgill R.W. Crystal structure of mannanase 26A from Pseudomonas cellulosa and analysis of residues involved in substrate binding. J Biol Chem. 276:2001;31186-31192.
90. Lobsanov Y.D., Vallée F., Imberty A., Yoshida T., Yip P., Herscovics A., Howell P.L. Structure of Penicillium citrinum α1,2-mannosidase reveals the basis for differences in specificity of the endoplasmic reticulum and Golgi class I enzymes. J Biol Chem. 277:2002;5620-5630.
91. Imamura H., Fushinobu S., Yamamoto M., Kumasaka T., Wakagi T., Matsuzawa H. Reaction mechanism and crystal structure of 4-α-glucanotransferase from a hyperthermophilic archaeon, Thermococcus litoralis. J Appl Glycosci. 48:2001;171-175.
92. Przylas I., Terada Y., Fujii K., Takaha T., Saenger W., Sträter N. X-ray structure of acarbose bound to amylomaltase from Thermus aquaticus - implications for the synthesis of large cyclic glucans. Eur J Biochem. 267:2000;6903-6913.
93. Crennell S., Takimoto T., Portner A., Taylor G. Crystal structure of the multifunctional paramyxovirus hemagglutinin-neuraminidase. Nat Struct Biol. 7:2000;1068-1074.
94. Davies G.J., Sinnott M.L., Withers S.G. Glycosyl transfer. Sinnott M.L., Comprehensive Biological Catalysis. 1:1997;119-209 Academic Press, London.
95. Burmeister W.P., Cottaz S., Rollin P., Vasella A., Henrissat B. High resolution X-ray crystallography shows that ascorbate is a cofactor for myrosinase and substitutes for the function of the catalytic base. J Biol Chem. 275:2000;39385-39393. As indicated by the title.
96. Hrmova M., Varghese J.N., De Gori R., Smith B.J., Driguez H., Fincher G.B. Catalytic mechanism and reaction intermediates along the hydrolytic pathway of a plant β-D-glucan glucohydrolase. Structure. 9:2001;1005-1016.
97. Varrot A., Schülein M., Fruchard S., Driguez H., Davies G.J. Atomic resolution structure of endoglucanase Cel5A in complex with methyl 4,4″,4′″,4′v-tetrathio-α-cellopentoside highlights the alternative binding modes targeted by substrate mimics. J Am Chem Soc. 121:2001;2621-2622.
98. Zou J.-Y., Kleywegt G.J., Ståhlberg J., Driguez H., Nerinckx W., Claeyssens M., Koivula A., Teeri T.T., Jones T.A. Crystallographic evidence for substrate ring distortion and protein conformational changes during catalysis in cellobiohydrolase Cel6A from Trichoderma reesei. Structure. 7:1999;1035-1045.
99. Prag G., Papanikolaou Y., Tavlas G., Vorgias C.E., Petratos K., Oppenheim A.B. Structures of chitobiase mutants complexed with the substrate di-N-acetyl-D-glucosamine: the catalytic role of the conserved acidic pair, aspartate 539 and glutamate 540. J Mol Biol. 300:2000;611-617.