PDBsum entry 2bat

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Hydrolase(o-glycosyl) PDB id
Protein chain
388 a.a. *
NAG ×2
Waters ×116
* Residue conservation analysis
PDB id:
Name: Hydrolase(o-glycosyl)
Title: The structure of the complex between influenza virus neurami sialic acid, the viral receptor
Structure: Neuraminidase n2. Chain: a. Engineered: yes
Source: Influenza a virus (a/tokyo/3/1967(h2n2 organism_taxid: 380960. Strain: a/tokyo/3/1967(h2n2)
Biol. unit: Tetramer (from PQS)
2.00Å     R-factor:   0.210    
Authors: J.N.Varghese,P.M.Colman
Key ref: J.N.Varghese et al. (1992). The structure of the complex between influenza virus neuraminidase and sialic acid, the viral receptor. Proteins, 14, 327-332. PubMed id: 1438172
10-Aug-92     Release date:   31-Jan-94    
Go to PROCHECK summary

Protein chain
Pfam   ArchSchema ?
P06820  (NRAM_I67A0) -  Neuraminidase
469 a.a.
388 a.a.
Key:    PfamA domain  Secondary structure  CATH domain

 Enzyme reactions 
   Enzyme class: E.C.  - Exo-alpha-sialidase.
[IntEnz]   [ExPASy]   [KEGG]   [BRENDA]
      Reaction: Hydrolysis of alpha-(2->3)-, alpha-(2->6)-, alpha-(2->8)-glycosidic linkages of terminal sialic residues in oligosaccharides, glycoproteins, glycolipids, colominic acid and synthetic substrates.
 Gene Ontology (GO) functional annotation 
  GO annot!
  Cellular component     membrane   3 terms 
  Biological process     carbohydrate metabolic process   1 term 
  Biochemical function     exo-alpha-sialidase activity     1 term  


Proteins 14:327-332 (1992)
PubMed id: 1438172  
The structure of the complex between influenza virus neuraminidase and sialic acid, the viral receptor.
J.N.Varghese, J.L.McKimm-Breschkin, J.B.Caldwell, A.A.Kortt, P.M.Colman.
Crystallographic studies of neuraminidase-sialic acid complexes indicate that sialic acid is distorted on binding the enzyme. Three arginine residues on the enzyme interact with the carboxylate group of the sugar which is observed to be equatorial to the saccharide ring as a consequence of its distorted geometry. The glycosidic oxygen is positioned within hydrogen-bonding distance of Asp-151, implicating this residue in catalysis.

Literature references that cite this PDB file's key reference

  PubMed id Reference
23250062 P.M.Colman (2013).
Early days in drug discovery by crystallography - personal recollections.
  Acta Crystallogr A, 69, 60-62.  
21489803 A.Albohy, S.Mohan, R.B.Zheng, B.M.Pinto, and C.W.Cairo (2011).
Inhibitor selectivity of a new class of oseltamivir analogs against viral neuraminidase over human neuraminidase enzymes.
  Bioorg Med Chem, 19, 2817-2822.  
21303099 C.J.Woods, M.Malaisree, S.Hannongbua, and A.J.Mulholland (2011).
A water-swap reaction coordinate for the calculation of absolute protein-ligand binding free energies.
  J Chem Phys, 134, 054114.  
20981560 M.Sarkar, A.S.Agrawal, R.Sharma Dey, S.Chattopadhyay, R.Mullick, P.De, S.Chakrabarti, and M.Chawla-Sarkar (2011).
Molecular characterization and comparative analysis of pandemic H1N1/2009 strains with co-circulating seasonal H1N1/2009 strains from eastern India.
  Arch Virol, 156, 207-217.  
20572070 K.L.Ngai, W.Y.Lam, N.Lee, T.F.Leung, D.S.Hui, and P.K.Chan (2010).
Allele-specific conventional reverse-transcription polymerase chain reaction as a screening assay for discriminating influenza a H1N1 (H275Y) oseltamivir-resistant and wild-type viruses.
  J Med Virol, 82, 1295-1298.  
20538598 S.J.Gamblin, and J.J.Skehel (2010).
Influenza hemagglutinin and neuraminidase membrane glycoproteins.
  J Biol Chem, 285, 28403-28409.  
20410266 Y.P.Lin, V.Gregory, P.Collins, J.Kloess, S.Wharton, N.Cattle, A.Lackenby, R.Daniels, and A.Hay (2010).
Neuraminidase receptor binding variants of human influenza A(H3N2) viruses resulting from substitution of aspartic acid 151 in the catalytic site: a role in virus attachment?
  J Virol, 84, 6769-6781.  
19563830 L.Jiang, S.Duriseti, P.Sun, and L.H.Miller (2009).
Molecular basis of binding of the Plasmodium falciparum receptor BAEBL to erythrocyte receptor glycophorin C.
  Mol Biochem Parasitol, 168, 49-54.  
19254207 P.M.Colman (2009).
New antivirals and drug resistance.
  Annu Rev Biochem, 78, 95.  
19390154 P.M.Dominiak, A.Volkov, A.P.Dominiak, K.N.Jarzembska, and P.Coppens (2009).
Combining crystallographic information and an aspherical-atom data bank in the evaluation of the electrostatic interaction energy in an enzyme-substrate complex: influenza neuraminidase inhibition.
  Acta Crystallogr D Biol Crystallogr, 65, 485-499.  
17939760 A.Moscona (2008).
Medical management of influenza infection.
  Annu Rev Med, 59, 397-413.  
18175324 M.Malaisree, T.Rungrotmongkol, P.Decha, P.Intharathep, O.Aruksakunwong, and S.Hannongbua (2008).
Understanding of known drug-target interactions in the catalytic pocket of neuraminidase subtype N1.
  Proteins, 71, 1908-1918.  
18404209 N.A.Ilyushina, E.A.Govorkova, T.E.Gray, N.V.Bovin, and R.G.Webster (2008).
Human-like receptor specificity does not affect the neuraminidase-inhibitor susceptibility of H5N1 influenza viruses.
  PLoS Pathog, 4, e1000043.  
18480754 P.J.Collins, L.F.Haire, Y.P.Lin, J.Liu, R.J.Russell, P.A.Walker, J.J.Skehel, S.R.Martin, A.J.Hay, and S.J.Gamblin (2008).
Crystal structures of oseltamivir-resistant influenza virus neuraminidase mutants.
  Nature, 453, 1258-1261.
PDB codes: 3ckz 3cl0 3cl2
18218621 S.L.Newstead, J.A.Potter, J.C.Wilson, G.Xu, C.H.Chien, A.G.Watts, S.G.Withers, and G.L.Taylor (2008).
The structure of Clostridium perfringens NanI sialidase and its catalytic intermediates.
  J Biol Chem, 283, 9080-9088.
PDB codes: 2bf6 2vk5 2vk6 2vk7
18049471 M.von Itzstein (2007).
The war against influenza: discovery and development of sialidase inhibitors.
  Nat Rev Drug Discov, 6, 967-974.  
17085491 O.Aruksakunwong, M.Malaisree, P.Decha, P.Sompornpisut, V.Parasuk, S.Pianwanit, and S.Hannongbua (2007).
On the lower susceptibility of oseltamivir to influenza neuraminidase subtype N1 than those in N2 and N9.
  Biophys J, 92, 798-807.  
17705169 P.A.Reece (2007).
Neuraminidase inhibitor resistance in influenza viruses.
  J Med Virol, 79, 1577-1586.  
17140356 A.C.Hurt, H.T.Ho, and I.Barr (2006).
Resistance to anti-influenza drugs: adamantanes and neuraminidase inhibitors.
  Expert Rev Anti Infect Ther, 4, 795-805.  
16641465 A.C.Hurt, P.Iannello, K.Jachno, N.Komadina, A.W.Hampson, I.G.Barr, and J.L.McKimm-Breschkin (2006).
Neuraminidase inhibitor-resistant and -sensitive influenza B viruses isolated from an untreated human patient.
  Antimicrob Agents Chemother, 50, 1872-1874.  
16929094 B.J.Smith, T.Huyton, R.P.Joosten, J.L.McKimm-Breschkin, J.G.Zhang, C.S.Luo, M.Z.Lou, N.E.Labrou, and T.P.Garrett (2006).
Structure of a calcium-deficient form of influenza virus neuraminidase: implications for substrate binding.
  Acta Crystallogr D Biol Crystallogr, 62, 947-952.
PDB code: 2b8h
17163384 F.G.Hayden, and A.T.Pavia (2006).
Antiviral management of seasonal and pandemic influenza.
  J Infect Dis, 194, S119-S126.  
16912325 H.L.Yen, E.Hoffmann, G.Taylor, C.Scholtissek, A.S.Monto, R.G.Webster, and E.A.Govorkova (2006).
Importance of neuraminidase active-site residues to the neuraminidase inhibitor resistance of influenza viruses.
  J Virol, 80, 8787-8795.  
15890915 B.Lu, H.Zhou, D.Ye, G.Kemble, and H.Jin (2005).
Improvement of influenza A/Fujian/411/02 (H3N2) virus growth in embryonated chicken eggs by balancing the hemagglutinin and neuraminidase activities, using reverse genetics.
  J Virol, 79, 6763-6771.  
16215975 C.P.Lu, C.T.Ren, Y.N.Lai, S.H.Wu, W.M.Wang, J.Y.Chen, and L.C.Lo (2005).
Design of a mechanism-based probe for neuraminidase to capture influenza viruses.
  Angew Chem Int Ed Engl, 44, 6888-6892.  
16189083 H.L.Yen, L.M.Herlocher, E.Hoffmann, M.N.Matrosovich, A.S.Monto, R.G.Webster, and E.A.Govorkova (2005).
Neuraminidase inhibitor-resistant influenza viruses may differ substantially in fitness and transmissibility.
  Antimicrob Agents Chemother, 49, 4075-4084.  
15864320 J.N.Watson, T.L.Knoll, J.H.Chen, D.T.Chou, T.J.Borgford, and A.J.Bennet (2005).
Use of conformationally restricted pyridinium alpha-D-N-acetylneuraminides to probe specificity in bacterial and viral sialidases.
  Biochem Cell Biol, 83, 115-122.  
15918777 P.M.Colman (2005).
Zanamivir: an influenza virus neuraminidase inhibitor.
  Expert Rev Anti Infect Ther, 3, 191-199.  
15159560 B.S.Lommer, S.M.Ali, S.N.Bajpai, W.J.Brouillette, G.M.Air, and M.Luo (2004).
A benzoic acid inhibitor induces a novel conformational change in the active site of Influenza B virus neuraminidase.
  Acta Crystallogr D Biol Crystallogr, 60, 1017-1023.
PDB codes: 1uja 1vcj
15561867 E.A.Govorkova, H.B.Fang, M.Tan, and R.G.Webster (2004).
Neuraminidase inhibitor-rimantadine combinations exert additive and synergistic anti-influenza virus effects in MDCK cells.
  Antimicrob Agents Chemother, 48, 4855-4863.  
15337401 M.Kiso, K.Mitamura, Y.Sakai-Tagawa, K.Shiraishi, C.Kawakami, K.Kimura, F.G.Hayden, N.Sugaya, and Y.Kawaoka (2004).
Resistant influenza A viruses in children treated with oseltamivir: descriptive study.
  Lancet, 364, 759-765.  
15211517 T.Haselhorst, J.C.Wilson, R.J.Thomson, S.McAtamney, J.G.Menting, R.L.Coppel, and M.von Itzstein (2004).
Saturation transfer difference (STD) 1H-NMR experiments and in silico docking experiments to probe the binding of N-acetylneuraminic acid and derivatives to Vibrio cholerae sialidase.
  Proteins, 56, 346-353.  
15016893 V.Zaitsev, M.von Itzstein, D.Groves, M.Kiefel, T.Takimoto, A.Portner, and G.Taylor (2004).
Second sialic acid binding site in Newcastle disease virus hemagglutinin-neuraminidase: implications for fusion.
  J Virol, 78, 3733-3741.
PDB codes: 1usr 1usx
12785732 C.H.Lee, S.I.Kim, K.B.Lee, Y.C.Yoo, S.Y.Ryu, and K.S.Song (2003).
Neuraminidase inhibitors from Reynoutria elliptica.
  Arch Pharm Res, 26, 367-374.  
12974389 E.Tiralongo, I.Martensen, J.Grötzinger, J.Tiralongo, and R.Schauer (2003).
Trans-sialidase-like sequences from Trypanosoma congolense conserve most of the critical active site residues found in other trans-sialidases.
  Biol Chem, 384, 1203-1213.  
11897581 C.Sweet, K.J.Jakeman, K.Bush, P.C.Wagaman, L.A.McKown, A.J.Streeter, D.Desai-Krieger, P.Chand, and Y.S.Babu (2002).
Oral administration of cyclopentane neuraminidase inhibitors protects ferrets against influenza virus infection.
  Antimicrob Agents Chemother, 46, 996.  
12435681 M.Z.Wang, C.Y.Tai, and D.B.Mendel (2002).
Mechanism by which mutations at his274 alter sensitivity of influenza a virus n1 neuraminidase to oseltamivir carboxylate and zanamivir.
  Antimicrob Agents Chemother, 46, 3809-3816.  
11987141 R.Wagner, M.Matrosovich, and H.D.Klenk (2002).
Functional balance between haemagglutinin and neuraminidase in influenza virus infections.
  Rev Med Virol, 12, 159-166.  
11787005 S.V.Vasudevan, and P.V.Balaji (2002).
Molecular dynamics simulations of alpha2 --> 8-linked disialoside: conformational analysis and implications for binding to proteins.
  Biopolymers, 63, 168-180.  
12414967 U.Gulati, C.C.Hwang, L.Venkatramani, S.Gulati, S.J.Stray, J.T.Lee, W.G.Laver, A.Bochkarev, A.Zlotnick, and G.M.Air (2002).
Antibody epitopes on the neuraminidase of a recent H3N2 influenza virus (A/Memphis/31/98).
  J Virol, 76, 12274-12280.  
11274459 B.J.Smith, P.M.Colman, M.Von Itzstein, B.Danylec, and J.N.Varghese (2001).
Analysis of inhibitor binding in influenza virus neuraminidase.
  Protein Sci, 10, 689-696.
PDB codes: 1f8b 1f8c 1f8d 1f8e
11689658 D.Kobasa, K.Wells, and Y.Kawaoka (2001).
Amino acids responsible for the absolute sialidase activity of the influenza A virus neuraminidase: relationship to growth in the duck intestine.
  J Virol, 75, 11773-11780.  
11779391 D.Young, C.Fowler, and K.Bush (2001).
RWJ-270201 (BCX-1812): a novel neuraminidase inhibitor for influenza.
  Philos Trans R Soc Lond B Biol Sci, 356, 1905-1913.  
11557461 E.A.Govorkova, I.A.Leneva, O.G.Goloubeva, K.Bush, and R.G.Webster (2001).
Comparison of efficacies of RWJ-270201, zanamivir, and oseltamivir against H5N1, H9N2, and other avian influenza viruses.
  Antimicrob Agents Chemother, 45, 2723-2732.  
11709315 L.V.Gubareva, R.G.Webster, and F.G.Hayden (2001).
Comparison of the activities of zanamivir, oseltamivir, and RWJ-270201 against clinical isolates of influenza virus and neuraminidase inhibitor-resistant variants.
  Antimicrob Agents Chemother, 45, 3403-3408.  
11308029 R.G.Kleineidam, S.Kruse, P.Roggentin, and R.Schauer (2001).
Elucidation of the role of functional amino acid residues of the small sialidase from Clostridium perfringens by site-directed mutagenesis.
  Biol Chem, 382, 313-319.  
11160691 R.M.Iorio, G.M.Field, J.M.Sauvron, A.M.Mirza, R.Deng, P.J.Mahon, and J.P.Langedijk (2001).
Structural and functional relationship between the receptor recognition and neuraminidase activities of the Newcastle disease virus hemagglutinin-neuraminidase protein: receptor recognition is dependent on neuraminidase activity.
  J Virol, 75, 1918-1927.  
11502530 W.M.Kati, D.Montgomery, C.Maring, V.S.Stoll, V.Giranda, X.Chen, W.G.Laver, W.Kohlbrenner, and D.W.Norbeck (2001).
Novel alpha- and beta-amino acid inhibitors of influenza virus neuraminidase.
  Antimicrob Agents Chemother, 45, 2563-2570.  
11274474 Y.Gholizadeh, M.Prevost, F.Van Bambeke, B.Casadewall, P.M.Tulkens, and P.Courvalin (2001).
Sequencing of the ddl gene and modeling of the mutated D-alanine:D-alanine ligase in glycopeptide-dependent strains of Enterococcus faecium.
  Protein Sci, 10, 836-844.  
10744339 A.J.Oakley, and M.C.Wilce (2000).
Macromolecular crystallography as a tool for investigating drug, enzyme and receptor interactions.
  Clin Exp Pharmacol Physiol, 27, 145-151.  
10654004 M.Tisdale (2000).
Monitoring of viral susceptibility: new challenges with the development of influenza NA inhibitors.
  Rev Med Virol, 10, 45-55.  
  10400772 D.Kobasa, S.Kodihalli, M.Luo, M.R.Castrucci, I.Donatelli, Y.Suzuki, T.Suzuki, and Y.Kawaoka (1999).
Amino acid residues contributing to the substrate specificity of the influenza A virus neuraminidase.
  J Virol, 73, 6743-6751.  
10368285 J.N.Varghese, M.Hrmova, and G.B.Fincher (1999).
Three-dimensional structure of a barley beta-D-glucan exohydrolase, a family 3 glycosyl hydrolase.
  Structure, 7, 179-190.
PDB code: 1ex1
10997129 L.Jindrák, and L.Grubhoffer (1999).
Animal virus receptors.
  Folia Microbiol (Praha), 44, 467-486.  
  9835519 C.Y.Tai, P.A.Escarpe, R.W.Sidwell, M.A.Williams, W.Lew, H.Wu, C.U.Kim, and D.B.Mendel (1998).
Characterization of human influenza virus variants selected in vitro in the presence of the neuraminidase inhibitor GS 4071.
  Antimicrob Agents Chemother, 42, 3234-3241.  
  9517945 D.B.Mendel, C.Y.Tai, P.A.Escarpe, W.Li, R.W.Sidwell, J.H.Huffman, C.Sweet, K.J.Jakeman, J.Merson, S.A.Lacy, W.Lew, M.A.Williams, L.Zhang, M.S.Chen, N.Bischofberger, and C.U.Kim (1998).
Oral administration of a prodrug of the influenza virus neuraminidase inhibitor GS 4071 protects mice and ferrets against influenza infection.
  Antimicrob Agents Chemother, 42, 640-646.  
17092804 D.B.Mendel, and R.W.Sidwell (1998).
Influenza virus resistance to neuraminidase inhibitors.
  Drug Resist Updat, 1, 184-189.  
  9499107 J.L.McKimm-Breschkin, A.Sahasrabudhe, T.J.Blick, M.McDonald, P.M.Colman, G.J.Hart, R.C.Bethell, and J.N.Varghese (1998).
Mutations in a conserved residue in the influenza virus neuraminidase active site decreases sensitivity to Neu5Ac2en-derived inhibitors.
  J Virol, 72, 2456-2462.  
9345622 A.White, and D.R.Rose (1997).
Mechanism of catalysis by retaining beta-glycosyl hydrolases.
  Curr Opin Struct Biol, 7, 645-651.  
  9261394 D.Kobasa, M.E.Rodgers, K.Wells, and Y.Kawaoka (1997).
Neuraminidase hemadsorption activity, conserved in avian influenza A viruses, does not influence viral replication in ducks.
  J Virol, 71, 6706-6713.  
9342319 J.N.Varghese, P.M.Colman, A.van Donkelaar, T.J.Blick, A.Sahasrabudhe, and J.L.McKimm-Breschkin (1997).
Structural evidence for a second sialic acid binding site in avian influenza virus neuraminidases.
  Proc Natl Acad Sci U S A, 94, 11808-11812.
PDB code: 1mwe
  9094607 L.V.Gubareva, M.J.Robinson, R.C.Bethell, and R.G.Webster (1997).
Catalytic and framework mutations in the neuraminidase active site of influenza viruses that are resistant to 4-guanidino-Neu5Ac2en.
  J Virol, 71, 3385-3390.  
9331424 R.C.Wade (1997).
'Flu' and structure-based drug design.
  Structure, 5, 1139-1145.  
9007277 E.C.Lai, S.A.Morris, I.P.Street, and S.G.Withers (1996).
Substituted glycals as probes of glycosidase mechanisms.
  Bioorg Med Chem, 4, 1929-1937.  
  8787876 J.L.McKimm-Breschkin, T.J.Blick, A.Sahasrabudhe, T.Tiong, D.Marshall, G.J.Hart, R.C.Bethell, and C.R.Penn (1996).
Generation and characterization of variants of NWS/G70C influenza virus after in vitro passage in 4-amino-Neu5Ac2en and 4-guanidino-Neu5Ac2en.
  Antimicrob Agents Chemother, 40, 40-46.  
8808739 N.R.Taylor, and M.von Itzstein (1996).
A structural and energetics analysis of the binding of a series of N-acetylneuraminic-acid-based inhibitors to influenza virus sialidase.
  J Comput Aided Mol Des, 10, 233-246.  
8634279 V.Helms, E.Deprez, E.Gill, C.Barret, G.Hui Bon Hoa, and R.C.Wade (1996).
Improved binding of cytochrome P450cam substrate analogues designed to fill extra space in the substrate binding pocket.
  Biochemistry, 35, 1485-1499.  
8710874 Z.Y.Zhu, and S.Karlin (1996).
Clusters of charged residues in protein three-dimensional structures.
  Proc Natl Acad Sci U S A, 93, 8350-8355.  
8560787 A.C.Ward (1995).
Changes in the neuraminidase of neurovirulent influenza virus strains.
  Virus Genes, 10, 253-260.  
  7549872 J.N.Varghese, V.C.Epa, and P.M.Colman (1995).
Three-dimensional structure of the complex of 4-guanidino-Neu5Ac2en and influenza virus neuraminidase.
  Protein Sci, 4, 1081-1087.
PDB codes: 1nnc 7nn9
7895225 S.Sabesan, S.Neira, and Z.Wasserman (1995).
Structural and functional-group tuning in the design of neuraminidase inhibitors.
  Carbohydr Res, 267, 239-261.  
  7920262 A.R.Poirrette, P.J.Artymiuk, H.M.Grindley, D.W.Rice, and P.Willett (1994).
Structural similarity between binding sites in influenza sialidase and isocitrate dehydrogenase: implications for an alternative approach to rational drug design.
  Protein Sci, 3, 1128-1130.  
8134399 B.E.Johansson, and E.D.Kilbourne (1994).
Immunization with purified N1 and N2 influenza virus neuraminidases demonstrates cross-reactivity without antigenic competition.
  Proc Natl Acad Sci U S A, 91, 2358-2361.  
7712292 J.D.McCarter, and S.G.Withers (1994).
Mechanisms of enzymatic glycoside hydrolysis.
  Curr Opin Struct Biol, 4, 885-892.  
  7849585 P.M.Colman (1994).
Influenza virus neuraminidase: structure, antibodies, and inhibitors.
  Protein Sci, 3, 1687-1696.  
8081752 T.Izard, M.C.Lawrence, R.L.Malby, G.G.Lilley, and P.M.Colman (1994).
The three-dimensional structure of N-acetylneuraminate lyase from Escherichia coli.
  Structure, 2, 361-369.
PDB code: 1nal
  7509002 T.Saito, G.Taylor, W.G.Laver, Y.Kawaoka, and R.G.Webster (1994).
Antigenicity of the N8 influenza A virus neuraminidase: existence of an epitope at the subunit interface of the neuraminidase.
  J Virol, 68, 1790-1796.  
  8497041 P.M.Colman, P.A.Hoyne, and M.C.Lawrence (1993).
Sequence and structure alignment of paramyxovirus hemagglutinin-neuraminidase with influenza virus neuraminidase.
  J Virol, 67, 2972-2980.  
8234325 S.J.Crennell, E.F.Garman, W.G.Laver, E.R.Vimr, and G.L.Taylor (1993).
Crystal structure of a bacterial sialidase (from Salmonella typhimurium LT2) shows the same fold as an influenza virus neuraminidase.
  Proc Natl Acad Sci U S A, 90, 9852-9856.
PDB codes: 1sil 1sim
8069621 W.P.Burmeister, B.Henrissat, C.Bosso, S.Cusack, and R.W.Ruigrok (1993).
Influenza B virus neuraminidase can synthesize its own inhibitor.
  Structure, 1, 19-26.
PDB codes: 1nsc 1nsd
The most recent references are shown first. Citation data come partly from CiteXplore and partly from an automated harvesting procedure. Note that this is likely to be only a partial list as not all journals are covered by either method. However, we are continually building up the citation data so more and more references will be included with time. Where a reference describes a PDB structure, the PDB codes are shown on the right.