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- EMDB-53198: Structure of the Complement classical and lectin pathway proconve... -

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Basic information

Entry
Database: EMDB / ID: EMD-53198
TitleStructure of the Complement classical and lectin pathway proconvertase, C4b2
Map dataSharpened map of Complement C4b in complex with C2
Sample
  • Complex: Classical and lectin pathway C3 proconvertase
    • Protein or peptide: Complement C4-A
    • Protein or peptide: Complement C2
  • Ligand: 2-acetamido-2-deoxy-beta-D-glucopyranose
  • Ligand: MAGNESIUM ION
KeywordsProconvertase / Complement / classical pathway / C4b2 / IMMUNE SYSTEM
Function / homology
Function and homology information


classical-complement-pathway C3/C5 convertase / complement component C1q complex binding / response to thyroid hormone / positive regulation of apoptotic cell clearance / Activation of C3 and C5 / complement activation, GZMK pathway / symbiont cell surface / complement activation / Initial triggering of complement / endopeptidase inhibitor activity ...classical-complement-pathway C3/C5 convertase / complement component C1q complex binding / response to thyroid hormone / positive regulation of apoptotic cell clearance / Activation of C3 and C5 / complement activation, GZMK pathway / symbiont cell surface / complement activation / Initial triggering of complement / endopeptidase inhibitor activity / complement activation, classical pathway / response to nutrient / Regulation of Complement cascade / Post-translational protein phosphorylation / response to bacterium / Regulation of Insulin-like Growth Factor (IGF) transport and uptake by Insulin-like Growth Factor Binding Proteins (IGFBPs) / blood microparticle / response to lipopolysaccharide / endoplasmic reticulum lumen / inflammatory response / serine-type endopeptidase activity / axon / neuronal cell body / synapse / dendrite / cell surface / proteolysis / : / extracellular exosome / extracellular region / metal ion binding / plasma membrane
Similarity search - Function
: / : / Complement C4, MG1 domain / Complement C4A/B CUB C-terminal domain / Complement B/C2 / Alpha-2-macroglobulin, conserved site / Alpha-2-macroglobulin family thiolester region signature. / Anaphylatoxin, complement system domain / : / Alpha-macro-globulin thiol-ester bond-forming region ...: / : / Complement C4, MG1 domain / Complement C4A/B CUB C-terminal domain / Complement B/C2 / Alpha-2-macroglobulin, conserved site / Alpha-2-macroglobulin family thiolester region signature. / Anaphylatoxin, complement system domain / : / Alpha-macro-globulin thiol-ester bond-forming region / Anaphylatoxin domain signature. / Anaphylatoxin, complement system / Anaphylatoxin/fibulin / Anaphylotoxin-like domain / Anaphylatoxin domain profile. / Anaphylatoxin homologous domain / Netrin C-terminal Domain / Netrin module, non-TIMP type / UNC-6/NTR/C345C module / Macroglobulin domain MG4 / Macroglobulin domain MG4 / Alpha-macroglobulin, receptor-binding / Alpha-macroglobulin, receptor-binding domain superfamily / Macroglobulin domain MG3 / : / A-macroglobulin receptor binding domain / Macroglobulin domain MG3 / A-macroglobulin receptor / Netrin domain / NTR domain profile. / Alpha-2-macroglobulin / Macroglobulin domain / Tissue inhibitor of metalloproteinases-like, OB-fold / Alpha-2-macroglobulin, bait region domain / Alpha-macroglobulin-like, TED domain / Alpha-2-macroglobulin family / MG2 domain / A-macroglobulin TED domain / Alpha-2-macroglobulin bait region domain / Alpha-2-Macroglobulin / Alpha-2-macroglobulin family / Sushi repeat (SCR repeat) / Domain abundant in complement control proteins; SUSHI repeat; short complement-like repeat (SCR) / Sushi/SCR/CCP domain / Sushi/CCP/SCR domain profile. / Sushi/SCR/CCP superfamily / von Willebrand factor type A domain / Terpenoid cyclases/protein prenyltransferase alpha-alpha toroid / VWFA domain profile. / von Willebrand factor (vWF) type A domain / von Willebrand factor, type A / von Willebrand factor A-like domain superfamily / Serine proteases, trypsin family, histidine active site / Serine proteases, trypsin family, serine active site / Serine proteases, trypsin family, histidine active site. / Peptidase S1A, chymotrypsin family / Serine proteases, trypsin family, serine active site. / Serine proteases, trypsin domain profile. / Trypsin-like serine protease / Serine proteases, trypsin domain / Trypsin / Immunoglobulin-like fold / Peptidase S1, PA clan, chymotrypsin-like fold / Peptidase S1, PA clan
Similarity search - Domain/homology
Complement C2 / Complement C4-A
Similarity search - Component
Biological speciesHomo sapiens (human)
Methodsingle particle reconstruction / cryo EM / Resolution: 3.9 Å
AuthorsDe la O Becerra KI / Brondijk THC / Gros P
Funding support Mexico, Netherlands, European Union, 3 items
OrganizationGrant numberCountry
Consejo Nacional de Ciencia y Tecnologia (CONACYT)CVU 604718 Mexico
Netherlands Organisation for Scientific Research (NWO)01.80.104.00 Netherlands
European Research Council (ERC)787241European Union
CitationJournal: Nat Commun / Year: 2025
Title: Structural insights into C3 convertase activity of the classical pathway of complement.
Authors: Karla I De la O Becerra / T Harma C Brondijk / Itziar Serna Martin / Piet Gros /
Abstract: Immune protection by the complement system depends on C3 cleavage by C3 convertases that is critical to all three activation pathways. Structural data on convertase formation in the classical pathway ...Immune protection by the complement system depends on C3 cleavage by C3 convertases that is critical to all three activation pathways. Structural data on convertase formation in the classical pathway and on C3-substrate binding to convertases is lacking. We present the cryo-EM structures of the proconvertase (C4b2), convertase (C4b2b), and convertase-substrate complex (C4b2b-C3) of the classical pathway. The data show that C2 and C4b form proconvertases and convertases like factor B and C3b of the alternative pathway. Substrate C3 binds C4b of the convertase through two interfaces: one also found in the SCIN-inhibited C3bBb dimer, and another facilitated by conformational changes in C3. Bending of C3 and swinging of the C2 protease bring the C3-scissile loop into the active site. The second, charged, C4b-interaction site favors C3- substrate binding, but upon cleavage repels product C3b. Thus, a charge switch-over mechanism effects the catalytic turnover of the convertases producing opsonin C3b.
History
DepositionMar 18, 2025-
Header (metadata) releaseDec 24, 2025-
Map releaseDec 24, 2025-
UpdateMar 4, 2026-
Current statusMar 4, 2026Processing site: PDBe / Status: Released

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Structure visualization

Supplemental images

emd_53198.png

Downloads & links

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Map

FileDownload / File: emd_53198.map.gz / Format: CCP4 / Size: 421.9 MB / Type: IMAGE STORED AS FLOATING POINT NUMBER (4 BYTES)
AnnotationSharpened map of Complement C4b in complex with C2
Projections & slices

Image control

Size
Brightness
Contrast
Others
AxesZ (Sec.)Y (Row.)X (Col.)
0.84 Å/pix.
x 480 pix.
= 401.28 Å		0.84 Å/pix.
x 480 pix.
= 401.28 Å		0.84 Å/pix.
x 480 pix.
= 401.28 Å

Surface

Projections

Slices (1/3)

Slices (1/2)

Slices (2/3)

Images are generated by Spider.

Voxel sizeX=Y=Z: 0.836 Å
Density

Histogram

Histogram (log scale)
Contour LevelBy AUTHOR: 0.065
Minimum - Maximum-0.445866 - 0.7746618
Average (Standard dev.)-0.00014405981 (±0.012284191)
SymmetrySpace group: 1
Details

EMDB XML:

Map geometry
Axis orderXYZ
Origin000
Dimensions480480480
Spacing480480480
CellA=B=C: 401.28 Å
α=β=γ: 90.0 °

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Supplemental data

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Mask #1

Fileemd_53198_msk_1.map
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Mask #2

Fileemd_53198_msk_2.map
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Additional map: Unsharpened map of Complement C4b in complex with C2

Fileemd_53198_additional_1.map
AnnotationUnsharpened map of Complement C4b in complex with C2
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Additional map: Local resolution map of Complement C4b in complex with C2

Fileemd_53198_additional_2.map
AnnotationLocal resolution map of Complement C4b in complex with C2
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Half map: Half map A of Complement C4b in complex with C2

Fileemd_53198_half_map_1.map
AnnotationHalf map A of Complement C4b in complex with C2
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Half map: Half map B of Complement C4b in complex with C2

Fileemd_53198_half_map_2.map
AnnotationHalf map B of Complement C4b in complex with C2
Projections & Slices
AxesZYX

Projections

Slices (1/2)
Density Histograms

Histogram

Histogram (log scale)

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Sample components

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Entire : Classical and lectin pathway C3 proconvertase

EntireName: Classical and lectin pathway C3 proconvertase
Components
  • Complex: Classical and lectin pathway C3 proconvertase
    • Protein or peptide: Complement C4-A
    • Protein or peptide: Complement C2
  • Ligand: 2-acetamido-2-deoxy-beta-D-glucopyranose
  • Ligand: MAGNESIUM ION

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Supramolecule #1: Classical and lectin pathway C3 proconvertase

SupramoleculeName: Classical and lectin pathway C3 proconvertase / type: complex / ID: 1 / Parent: 0 / Macromolecule list: #1-#2
Source (natural)Organism: Homo sapiens (human) / Tissue: blood plasma
Molecular weightTheoretical: 288 KDa

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Macromolecule #1: Complement C4-A

MacromoleculeName: Complement C4-A / type: protein_or_peptide / ID: 1 / Number of copies: 3 / Enantiomer: LEVO
Source (natural)Organism: Homo sapiens (human) / Organ: blood plasma
Molecular weightTheoretical: 193.007234 KDa
SequenceString: MRLLWGLIWA SSFFTLSLQK PRLLLFSPSV VHLGVPLSVG VQLQDVPRGQ VVKGSVFLRN PSRNNVPCSP KVDFTLSSER DFALLSLQV PLKDAKSCGL HQLLRGPEVQ LVAHSPWLKD SLSRTTNIQG INLLFSSRRG HLFLQTDQPI YNPGQRVRYR V FALDQKMR ...String:
MRLLWGLIWA SSFFTLSLQK PRLLLFSPSV VHLGVPLSVG VQLQDVPRGQ VVKGSVFLRN PSRNNVPCSP KVDFTLSSER DFALLSLQV PLKDAKSCGL HQLLRGPEVQ LVAHSPWLKD SLSRTTNIQG INLLFSSRRG HLFLQTDQPI YNPGQRVRYR V FALDQKMR PSTDTITVMV ENSHGLRVRK KEVYMPSSIF QDDFVIPDIS EPGTWKISAR FSDGLESNSS TQFEVKKYVL PN FEVKITP GKPYILTVPG HLDEMQLDIQ ARYIYGKPVQ GVAYVRFGLL DEDGKKTFFR GLESQTKLVN GQSHISLSKA EFQ DALEKL NMGITDLQGL RLYVAAAIIE SPGGEMEEAE LTSWYFVSSP FSLDLSKTKR HLVPGAPFLL QALVREMSGS PASG IPVKV SATVSSPGSV PEVQDIQQNT DGSGQVSIPI IIPQTISELQ LSVSAGSPHP AIARLTVAAP PSGGPGFLSI ERPDS RPPR VGDTLNLNLR AVGSGATFSH YYYMILSRGQ IVFMNREPKR TLTSVSVFVD HHLAPSFYFV AFYYHGDHPV ANSLRV DVQ AGACEGKLEL SVDGAKQYRN GESVKLHLET DSLALVALGA LDTALYAAGS KSHKPLNMGK VFEAMNSYDL GCGPGGG DS ALQVFQAAGL AFSDGDQWTL SRKRLSCPKE KTTRKKRNVN FQKAINEKLG QYASPTAKRC CQDGVTRLPM MRSCEQRA A RVQQPDCREP FLSCCQFAES LRKKSRDKGQ AGLQRALEIL QEEDLIDEDD IPVRSFFPEN WLWRVETVDR FQILTLWLP DSLTTWEIHG LSLSKTKGLC VATPVQLRVF REFHLHLRLP MSVRRFEQLE LRPVLYNYLD KNLTVSVHVS PVEGLCLAGG GGLAQQVLV PAGSARPVAF SVVPTAAAAV SLKVVARGSF EFPVGDAVSK VLQIEKEGAI HREELVYELN PLDHRGRTLE I PGNSDPNM IPDGDFNSYV RVTASDPLDT LGSEGALSPG GVASLLRLPR GCGEQTMIYL APTLAASRYL DKTEQWSTLP PE TKDHAVD LIQKGYMRIQ QFRKADGSYA AWLSRDSSTW LTAFVLKVLS LAQEQVGGSP EKLQETSNWL LSQQQADGSF QDP CPVLDR SMQGGLVGND ETVALTAFVT IALHHGLAVF QDEGAEPLKQ RVEASISKAN SFLGEKASAG LLGAHAAAIT AYAL TLTKA PVDLLGVAHN NLMAMAQETG DNLYWGSVTG SQSNAVSPTP APRNPSDPMP QAPALWIETT AYALLHLLLH EGKAE MADQ ASAWLTRQGS FQGGFRSTQD TVIALDALSA YWIASHTTEE RGLNVTLSST GRNGFKSHAL QLNNRQIRGL EEELQF SLG SKINVKVGGN SKGTLKVLRT YNVLDMKNTT CQDLQIEVTV KGHVEYTMEA NEDYEDYEYD ELPAKDDPDA PLQPVTP LQ LFEGRRNRRR REAPKVVEEQ ESRVHYTVCI WRNGKVGLSG MAIADVTLLS GFHALRADLE KLTSLSDRYV SHFETEGP H VLLYFDSVPT SRECVGFEAV QEVPVGLVQP ASATLYDYYN PERRCSVFYG APSKSRLLAT LCSAEVCQCA EGKCPRQRR ALERGLQDED GYRMKFACYY PRVEYGFQVK VLREDSRAAF RLFETKITQV LHFTKDVKAA ANQMRNFLVR ASCRLRLEPG KEYLIMGLD GATYDLEGHP QYLLDSNSWI EEMPSERLCR STRQRAACAQ LNDFLQEYGT QGCQV

UniProtKB: Complement C4-A

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Macromolecule #2: Complement C2

MacromoleculeName: Complement C2 / type: protein_or_peptide / ID: 2 / Number of copies: 1 / Enantiomer: LEVO / EC number: classical-complement-pathway C3/C5 convertase
Source (natural)Organism: Homo sapiens (human) / Organ: blood plasma
Molecular weightTheoretical: 83.363766 KDa
SequenceString: MGPLMVLFCL LFLYPGLADS APSCPQNVNI SGGTFTLSHG WAPGSLLTYS CPQGLYPSPA SRLCKSSGQW QTPGATRSLS KAVCKPVRC PAPVSFENGI YTPRLGSYPV GGNVSFECED GFILRGSPVR QCRPNGMWDG ETAVCDNGAG HCPNPGISLG A VRTGFRFG ...String:
MGPLMVLFCL LFLYPGLADS APSCPQNVNI SGGTFTLSHG WAPGSLLTYS CPQGLYPSPA SRLCKSSGQW QTPGATRSLS KAVCKPVRC PAPVSFENGI YTPRLGSYPV GGNVSFECED GFILRGSPVR QCRPNGMWDG ETAVCDNGAG HCPNPGISLG A VRTGFRFG HGDKVRYRCS SNLVLTGSSE RECQGNGVWS GTEPICRQPY SYDFPEDVAP ALGTSFSHML GATNPTQKTK ES LGRKIQI QRSGHLNLYL LLDCSQSVSE NDFLIFKESA SLMVDRIFSF EINVSVAIIT FASEPKVLMS VLNDNSRDMT EVI SSLENA NYKDHENGTG TNTYAALNSV YLMMNNQMRL LGMETMAWQE IRHAIILLTD GKSNMGGSPK TAVDHIREIL NINQ KRNDY LDIYAIGVGK LDVDWRELNE LGSKKDGERH AFILQDTKAL HQVFEHMLDV SKLTDTICGV GNMSANASDQ ERTPW HVTI KPKSQETCRG ALISDQWVLT AAHCFRDGND HSLWRVNVGD PKSQWGKEFL IEKAVISPGF DVFAKKNQGI LEFYGD DIA LLKLAQKVKM STHARPICLP CTMEANLALR RPQGSTCRDH ENELLNKQSV PAHFVALNGS KLNINLKMGV EWTSCAE VV SQEKTMFPNL TDVREVVTDQ FLCSGTQEDE SPCKGESGGA VFLERRFRFF QVGLVSWGLY NPCLGSADKN SRKRAPRS K VPPPRDFHIN LFRMQPWLRQ HLGDVLNFLP L

UniProtKB: Complement C2

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Macromolecule #3: 2-acetamido-2-deoxy-beta-D-glucopyranose

MacromoleculeName: 2-acetamido-2-deoxy-beta-D-glucopyranose / type: ligand / ID: 3 / Number of copies: 3 / Formula: NAG
Molecular weightTheoretical: 221.208 Da
Chemical component information

ChemComp-NAG:
2-acetamido-2-deoxy-beta-D-glucopyranose

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Macromolecule #4: MAGNESIUM ION

MacromoleculeName: MAGNESIUM ION / type: ligand / ID: 4 / Number of copies: 1 / Formula: MG
Molecular weightTheoretical: 24.305 Da

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Experimental details

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Structure determination

Methodcryo EM
Processingsingle particle reconstruction
Aggregation stateparticle

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Sample preparation

Concentration0.2 mg/mL
BufferpH: 7.4
Component:
ConcentrationFormulaName
150.0 mMNaClsodium chloride
20.0 mMHEPES
2.0 mMMgCl2Magnesium chloride
GridModel: Quantifoil R1.2/1.3 / Material: GOLD / Mesh: 200 / Support film - Material: CARBON / Support film - topology: HOLEY / Pretreatment - Type: GLOW DISCHARGE / Pretreatment - Time: 30 sec. / Pretreatment - Atmosphere: AIR
VitrificationCryogen name: ETHANE / Chamber humidity: 95 % / Chamber temperature: 277 K / Instrument: FEI VITROBOT MARK IV
DetailsComplex was crosslinked with glutaraldehyde 0.2%

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Electron microscopy

MicroscopeTFS KRIOS
Image recordingFilm or detector model: GATAN K3 (6k x 4k) / Number grids imaged: 1 / Number real images: 4662 / Average exposure time: 2.24 sec. / Average electron dose: 50.0 e/Å2
Electron beamAcceleration voltage: 300 kV / Electron source: FIELD EMISSION GUN
Electron opticsC2 aperture diameter: 50.0 µm / Illumination mode: FLOOD BEAM / Imaging mode: BRIGHT FIELD / Cs: 2.7 mm / Nominal defocus max: 2.2 µm / Nominal defocus min: 1.0 µm / Nominal magnification: 105000
Sample stageCooling holder cryogen: NITROGEN
Experimental equipment
Model: Titan Krios / Image courtesy: FEI Company

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Image processing

Particle selectionNumber selected: 2647189
CTF correctionSoftware - Name: cryoSPARC (ver. v3.2/3) / Type: NONE
Startup modelType of model: NONE
Final reconstructionNumber classes used: 3 / Applied symmetry - Point group: C1 (asymmetric) / Resolution.type: BY AUTHOR / Resolution: 3.9 Å / Resolution method: FSC 0.143 CUT-OFF / Software - Name: cryoSPARC (ver. v3.2/3) / Number images used: 31895
Initial angle assignmentType: MAXIMUM LIKELIHOOD / Software - Name: cryoSPARC (ver. v3.2/3)
Final angle assignmentType: MAXIMUM LIKELIHOOD / Software - Name: cryoSPARC (ver. v3.2/3)
FSC plot (resolution estimation)

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Atomic model buiding 1

Initial model
PDB IDChain

5jtw

chain_id: A, source_name: PDB, initial_model_type: experimental model

5jtw

chain_id: B, source_name: PDB, initial_model_type: experimental model

5jtw

chain_id: C, source_name: PDB, initial_model_type: experimental model

3erb

chain_id: D, source_name: PDB, initial_model_type: experimental model

2i6q

chain_id: D, source_name: PDB, initial_model_type: experimental model
RefinementSpace: REAL / Protocol: FLEXIBLE FIT
Output model

PDB-9qj4:
Structure of the Complement classical and lectin pathway proconvertase, C4b2

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