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:
e3ezuA1[A:186-341] nonrep Image Pymol Jmol Links: PDB, SCOP, CATH
GGDEF domain protein ; X-RAY DIFFRACTION 1.95Å
e1w25A1[A:294-455] nonrep Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
STALKED-CELL DIFFERENTIATION CONTROLLING PROTEIN ; ligands: C2E ; X-RAY DIFFRACTION 2.700Å
e4zmmA1[A:178-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; ligands: C2E ; X-RAY DIFFRACTION 2.5Å
e4zmmB1[B:181-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; ligands: C2E ; X-RAY DIFFRACTION 2.5Å
e2v0nA1[A:294-454] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
RESPONSE REGULATOR PLED ; ligands: C2E,CL,GAV,MG ; X-RAY DIFFRACTION 2.70Å
e2v0nB1[B:294-454] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
RESPONSE REGULATOR PLED ; ligands: C2E,GAV,MG ; X-RAY DIFFRACTION 2.70Å
e1w25B1[B:294-455] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
STALKED-CELL DIFFERENTIATION CONTROLLING PROTEIN ; ligands: C2E ; X-RAY DIFFRACTION 2.700Å
e3hvwA1[A:171-333] Image Pymol Jmol Links: PDB, SCOP, CATH
Diguanylate-cyclase (DGC) ; X-RAY DIFFRACTION 1.7Å
e3mtkA1[A:186-353] Image Pymol Jmol Links: PDB, SCOP, CATH
Diguanylate cyclase/phosphodiesterase ; X-RAY DIFFRACTION 2.240Å
e2wb4A1[A:294-459] Image Pymol Jmol Links: PDB, SCOP, CATH
DIGUANYLATE CYCLASE ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.8Å
e3qyyA1[A:134-290] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Response regulator ; ligands: C2E,MG,PEG ; X-RAY DIFFRACTION 1.90Å
e3iclA1[A:0-161] Image Pymol Jmol Links: PDB, SCOP, CATH
EAL/GGDEF domain protein ; ligands: SO4 ; X-RAY DIFFRACTION 2.000Å
e3ignA1[A:152-317] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase ; ligands: C2E ; X-RAY DIFFRACTION 1.83Å
e3tvkA1[A:126-297] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DgcZ ; ligands: C2E,MG,TAR ; X-RAY DIFFRACTION 1.800Å
e3i5aA1[A:174-334] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Response regulator/GGDEF domain protein ; ligands: C2E ; X-RAY DIFFRACTION 2.796Å
e2wb4B1[B:294-454] Image Pymol Jmol Links: PDB, SCOP, CATH
DIGUANYLATE CYCLASE ; ligands: C2E,MG,SO4 ; X-RAY DIFFRACTION 2.8Å
e3mtkB1[B:186-353] Image Pymol Jmol Links: PDB, SCOP, CATH
Diguanylate cyclase/phosphodiesterase ; X-RAY DIFFRACTION 2.240Å
e3iclB1[B:7-161] Image Pymol Jmol Links: PDB, SCOP, CATH
EAL/GGDEF domain protein ; ligands: SO4 ; X-RAY DIFFRACTION 2.000Å
e3hvaA1[A:266-427] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
PROTEIN FIMX ; X-RAY DIFFRACTION 2.04Å
e3hvaB1[B:266-427] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
PROTEIN FIMX ; X-RAY DIFFRACTION 2.04Å
e3qyyB1[B:130-290] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Response regulator ; ligands: C2E,MG,PEG ; X-RAY DIFFRACTION 1.90Å
e3pjxA1[A:219-399] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Cyclic dimeric GMP binding protein ; ligands: EDO ; X-RAY DIFFRACTION 2.0Å
e4h54A1[A:127-296] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE YDEH ; ligands: C2E,GAV,MG ; X-RAY DIFFRACTION 3.9Å
e4iobA1[A:254-414] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase TpbB ; ligands: GOL,TBU ; X-RAY DIFFRACTION 2.78Å
e3pjwA3[A:241-407] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Cyclic dimeric GMP binding protein ; X-RAY DIFFRACTION 3.1Å
e4urgA1[A:90-239] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; ligands: C2E ; X-RAY DIFFRACTION 1.90Å
e4urgB1[B:91-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 1.90Å
e4urqU1[U:93-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4urqV1[V:94-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4urqW1[W:93-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4urqX1[X:93-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4urqY1[Y:93-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4urqZ1[Z:94-246] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; X-RAY DIFFRACTION 2.50Å
e4ursA1[A:87-248] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; ligands: C2E,GOL,MES ; X-RAY DIFFRACTION 2.27Å
e4ursB1[B:87-248] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE ; ligands: C2E,GOL,MES ; X-RAY DIFFRACTION 2.27Å
e4rnfA2[A:1-171] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Motility regulator ; X-RAY DIFFRACTION 2.850Å
e4rnhA2[A:977-1148] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Motility regulator ; X-RAY DIFFRACTION 2.45Å
e4ymeA1[A:372-527] Image Pymol Jmol Links: PDB, SCOP, CATH
sensory box/GGDEF family protein ; X-RAY DIFFRACTION 1.40Å
e3i5bA1[A:169-338] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
WspR response regulator ; ligands: TLA ; X-RAY DIFFRACTION 2.03Å
e3i5bB1[B:169-337] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
WspR response regulator ; ligands: TLA ; X-RAY DIFFRACTION 2.03Å
e3breA2[A:176-339] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Probable two-component response regulator ; ligands: C2E ; X-RAY DIFFRACTION 2.390Å
e3breB1[B:176-340] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Probable two-component response regulator ; ligands: C2E ; X-RAY DIFFRACTION 2.390Å
e4zveA1[A:298-458] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; ligands: CIT,EDO ; X-RAY DIFFRACTION 1.20Å
e4zvfA1[A:297-460] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; ligands: CA,EDO,GAV ; X-RAY DIFFRACTION 1.15Å
e4zvgA1[A:297-461] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; X-RAY DIFFRACTION 2.20Å
e4zvgB1[B:298-461] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; X-RAY DIFFRACTION 2.20Å
e4zvhA1[A:298-460] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; ligands: SO4 ; X-RAY DIFFRACTION 3.30Å
e4zvhB1[B:298-458] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase DosC ; ligands: SO4 ; X-RAY DIFFRACTION 3.30Å
e4wxoA1[A:302-483] Image Pymol Jmol Links: PDB, SCOP, CATH
Uncharacterized protein ; X-RAY DIFFRACTION 2.80Å
e4wxoB1[B:302-483] Image Pymol Jmol Links: PDB, SCOP, CATH
Uncharacterized protein ; X-RAY DIFFRACTION 2.80Å
e4wxoC1[C:302-483] Image Pymol Jmol Links: PDB, SCOP, CATH
Uncharacterized protein ; X-RAY DIFFRACTION 2.80Å
e4wxwA1[A:323-487] Image Pymol Jmol Links: PDB, SCOP, CATH
Uncharacterized protein ; ligands: MG ; X-RAY DIFFRACTION 1.8Å
e5euhA1[A:339-502] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF domain membrane protein ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.989Å
e5euhB1[B:339-502] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF domain membrane protein ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.989Å
e5euhC1[C:340-502] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF domain membrane protein ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.989Å
e5euhD1[D:340-502] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF domain membrane protein ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.989Å
e4zmuA1[A:180-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; X-RAY DIFFRACTION 2.5Å
e4zmuB1[B:180-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; X-RAY DIFFRACTION 2.5Å
e4zmuC2[C:180-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; X-RAY DIFFRACTION 2.5Å
e4zmuD2[D:180-341] Image Pymol Jmol Links: PDB, SCOP, CATH
diguanylate cyclase ; X-RAY DIFFRACTION 2.5Å
e5llwB2[B:524-683] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; X-RAY DIFFRACTION 2.800Å
e5llwA2[A:524-683] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; X-RAY DIFFRACTION 2.800Å
e5llxA2[A:524-682] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; ligands: GTP,MG ; X-RAY DIFFRACTION 2.800Å
e5llxB2[B:524-682] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; ligands: GTP,MG ; X-RAY DIFFRACTION 2.800Å
e4h54B1[B:127-297] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
DIGUANYLATE CYCLASE YDEH ; ligands: GAV,MG ; X-RAY DIFFRACTION 3.9Å
e5xgbA2[A:372-557] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Uncharacterized protein ; X-RAY DIFFRACTION 2.28Å
e5m3cA1[A:809-974] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase ; ligands: CA,GTP ; X-RAY DIFFRACTION 2.800Å
e5xgdA3[A:372-557] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Uncharacterized protein PA0861 ; ligands: GTP,MG ; X-RAY DIFFRACTION 2.800Å
e5xgeA1[A:372-557] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Uncharacterized protein PA0861 ; X-RAY DIFFRACTION 3.310Å
e5m3cB1[B:809-974] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase ; ligands: CA,GTP ; X-RAY DIFFRACTION 2.800Å
e6eibB1[B:2-161] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Sensory box/GGDEF family protein ; ligands: ACT,EDO,SO4 ; X-RAY DIFFRACTION 1.94Å
e6et7B3[B:524-682] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; X-RAY DIFFRACTION 2.850Å
e6eibA1[A:1-159] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Sensory box/GGDEF family protein ; ligands: ACT,EDO,SO4 ; X-RAY DIFFRACTION 1.94Å
e6eibC1[C:3-159] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Sensory box/GGDEF family protein ; ligands: ACT,EDO,SO4 ; X-RAY DIFFRACTION 1.94Å
e6eibD1[D:4-159] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Sensory box/GGDEF family protein ; ligands: ACT,EDO,SO4 ; X-RAY DIFFRACTION 1.94Å
e6et7A4[A:524-682] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase (GGDEF) domain-containing protein ; X-RAY DIFFRACTION 2.850Å
e6d9mA2[A:3-163] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Fusion protein of Endolysin,Response receiver sensor diguanylate cyclase, GAF domain-containing ; ligands: GMP,GOL,GTP,MG,POP ; X-RAY DIFFRACTION 1.350Å
e6hbzA1[A:134-309] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF domain protein ; ligands: C2E,CL ; X-RAY DIFFRACTION 1.790Å
e6hbzB1[B:135-310] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF domain protein ; ligands: C2E,CL,EDO ; X-RAY DIFFRACTION 1.790Å
e6pwjA2[A:242-401] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF and EAL domain-containing protein ; X-RAY DIFFRACTION 2.700Å
e6pwjB2[B:221-396] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF and EAL domain-containing protein ; X-RAY DIFFRACTION 2.700Å
e6pwkA1[A:222-397] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF and EAL domain-containing protein ; ligands: C2E ; X-RAY DIFFRACTION 2.6Å
e6pwkB2[B:222-397] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
GGDEF and EAL domain-containing protein ; ligands: C2E ; X-RAY DIFFRACTION 2.6Å
e6khuA1[A:60-190] Image Pymol Jmol Links: PDB, SCOP, CATH
Diguanylate cyclase/phosphodiesterase-domain containing protein ; ligands: ZN ; X-RAY DIFFRACTION 2.0980Å
e6ttsA1[A:176-345] Image Pymol Jmol Links: PDB, SCOP, CATH
GGDEF diguanylate cyclase DgcB ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.5Å
e6ttsB1[B:170-345] Image Pymol Jmol Links: PDB, SCOP, CATH
GGDEF diguanylate cyclase DgcB ; ligands: C2E,SO4 ; X-RAY DIFFRACTION 2.5Å
e6zxmA2[A:141-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e6zxmC1[C:141-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e6zxmD2[D:141-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e6zxmE2[E:141-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e6zxmF1[F:141-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e6ttrA1[A:180-343] Image Pymol Jmol Links: PDB, SCOP, CATH
GGDEF diguanylate cyclase DgcB ; ligands: C2E,PO4 ; X-RAY DIFFRACTION 2.85Å
e7a7eA1[A:250-421] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase TpbB ; ligands: MG ; X-RAY DIFFRACTION 2.800Å
e7a7eB1[B:245-420] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase TpbB ; ligands: MG ; X-RAY DIFFRACTION 2.800Å
e7a7eC1[C:250-421] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Diguanylate cyclase TpbB ; ligands: MG ; X-RAY DIFFRACTION 2.800Å
e6zxbA1[A:129-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: EDO,GH3,MG,SO4 ; X-RAY DIFFRACTION 2.19Å
e6zxbB1[B:129-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: EDO,GH3,MG,SO4 ; X-RAY DIFFRACTION 2.19Å
e6zxcA1[A:129-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: GH3,MG ; X-RAY DIFFRACTION 2.70Å
e6zxcB1[B:134-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: GH3,MG ; X-RAY DIFFRACTION 2.70Å
e6zxcC1[C:134-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: GH3,MG ; X-RAY DIFFRACTION 2.70Å
e6zxcD1[D:129-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: GH3,MG ; X-RAY DIFFRACTION 2.70Å
e6zxmB1[B:129-298] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Putative GGDEF/response regulator receiver domain protein ; ligands: C2E ; X-RAY DIFFRACTION 3.300Å
e8c05A1[A:123-312] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
; ligands: DPO,MG ; X-RAY DIFFRACTION 2.5Å
e3i5cA1[A:142-337] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Fusion of General control protein GCN4 and WSPR response regulator protein ; ligands: C2E,MG ; X-RAY DIFFRACTION 1.94Å
e3i5cB1[B:142-339] Image Pymol Jmol Links: PDB, SCOP, CATH, PubMed, DOI
Fusion of General control protein GCN4 and WSPR response regulator protein ; ligands: C2E ; X-RAY DIFFRACTION 1.94Å
Pseudo group of domains that do not map to PFam family profiles. Domains inside should not be considered as in one family.
looks like an elaborated ferredoxin
contains a HTH motif
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