Evolutionary classification of protein domain structures
alpha arraysalpha-helix arrays with mainly perpendicular helices
alpha bundlesalpha-helix bundles with mainly parallel helices
alpha superhelicesrepeating alpha-helix hairpins form a superhelix
alpha duplicates or obligate multimersalpha duplicates or monomers associate together to form a compact domain
alpha complex topologyalpha domain with complex and unique topology that cannot be described as array or bundle or superhelix
beta barrelssingle beta-sheet folded upon itself to form a barrel
beta meanderssingle beta-sheet with mainly meander topology
beta sandwichestwo beta-sheets stacked together to form a sandwich
beta duplicates or obligate multimersbeta duplicates or monomers associate together to form a compact domain
beta complex topologybeta domains with complex and unique topology that cannot be described as barrel or sandwich or meander
a+b two layersone alpha-helix layer and one beta-sheet layer




Representatives:
e1i58A1[A:352-540]
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CHEMOTAXIS PROTEIN CHEA ; ligands: ACP,ACT,MG ; X-RAY DIFFRACTION 1.6Å
e1id0A1[A:335-480]
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PHOQ HISTIDINE KINASE ; ligands: ANP,MG ; X-RAY DIFFRACTION 1.6Å
e1r62A1[A:194-292,A:313-349]
nonrep
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Nitrogen regulation protein NR(II) ; X-RAY DIFFRACTION 1.6Å
e1ysrA1[A:299-446]
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SENSOR-TYPE HISTIDINE KINASE PRRB ; ligands: GOL,TRS ; X-RAY DIFFRACTION 1.78Å
e1pvgA2[A:7-245]
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DNA topoisomerase II ; ligands: ANP,MG ; X-RAY DIFFRACTION 1.80Å
e2c2aA2[A:321-481]
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SENSOR HISTIDINE KINASE ; ligands: ADP ; X-RAY DIFFRACTION 1.9Å
e1s14A1[A:1027-1217]
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Topoisomerase IV subunit B ; ligands: NOV ; X-RAY DIFFRACTION 2.0Å
e2hkjA3[A:10-228]
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TYPE II DNA TOPOISOMERASE VI SUBUNIT B ; ligands: MG,RDC ; X-RAY DIFFRACTION 2.0Å
e1gkzA2[A:186-306,A:332-378]
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[3-METHYL-2-OXOBUTANOATE DEHYDROGENASE [LIPOAMIDE]] KINASE ; ligands: ADP,CL,K,MG ; X-RAY DIFFRACTION 2.200Å
e1ei1A2[A:2-220]
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DNA GYRASE B ; ligands: ANP,GOL,SO4 ; X-RAY DIFFRACTION 2.3Å
e1y8oA2[A:177-401]
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[PYRUVATE DEHYDROGENASE [LIPOAMIDE]] KINASE ISOZYME 3 ; ligands: ADP,K,MG ; X-RAY DIFFRACTION 2.48Å
e1jm6A2[A:1177-1366]
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PYRUVATE DEHYDROGENASE KINASE, ISOZYME 2 ; ligands: ADP,MG ; X-RAY DIFFRACTION 2.5Å
e4bizD1[D:304-393,D:420-456]
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SENSOR PROTEIN CPXA ; ligands: SO4 ; X-RAY DIFFRACTION 2.65Å
e4xivB1[B:355-540]
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Chemotaxis protein CheA ; ligands: ADP ; X-RAY DIFFRACTION 2.800Å
e3ehfD2[D:244-367]
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SENSOR KINASE (YOCF PROTEIN) ; ligands: ACP,MG ; X-RAY DIFFRACTION 3.1Å

Pseudo group of domains that do not map to PFam family profiles. Domains inside should not be considered as in one family.
a+b three layersone mainly antiparallel beta-sheet layer sandwiched between two alpha-helix layers
a+b four layerstwo beta-sheet layers sandwiched between two alpha-helix layers
a+b complex topologyalpha and beta domain with complex and unique topology that cannot be described in layers
a+b duplicates or obligate multimersalpha and beta duplicates or monomers associated together to form a compact domain
a/b barrelsrepeating beta-alpha units form a barrel
a/b three-layered sandwichesrepeating beta-alpha units form a sandwich with a mainly parallel beta-sheet layer stacked between two alpha-helix layers
mixed a+b and a/balpha and beta domain consists of both a+b and a/b regions
few secondary structure elementsstabilized by cofactors or disulfides
extended segmentsextended, non-globular segments, frequently bind to other proteins to form complexes