CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Can protein model accuracy be identified? Morten Nielsen, CBS, BioCentrum, DTU NO!

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Identification of Protein- model accuracy Why is it important? What is accuracy –RMSD, fraction correct,… Protein model correctness/quality –Procheck, Whatif, ProsaII, Verify3d Prediction of protein model accuracy –ProQ server

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Why is it so important Reliable fold recognition –P-value, E-value, Z-score… –Tells you if you should believe in the fold!! Alignment (model construction) –No obvious method to estimate reliability of alignment Number of gaps, length of gaps Amino acids in protein core and loops –% id is too conservative Many low homology models are accurate, and some high homology model are wrong Correct fold, wrong alignment => Terrible model How to gain confidence in a protein model?

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Model accuracy. Swiss-model models sharing 25-95% sequence identity with the submitted sequences (

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU What is protein model accuracy Model quality (correctness) –Does the model look like a protein? Hydrophobic residues in core, hydrophilic on surface Backbone geometry (phi/psi angles, bond-length) Amino acid environment A correct model can be completely wrong Structure accuracy (if we know the answer) –RMSD –Fraction of correct modeled residues

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Amino acid environment of different protein sequences (Swissprot) different solved protein structures (PDB) 600 different protein folds => Typical amino acid environment

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Model accuracy Fraction correct = N c /N N c = number correct (dij<4Å) d ij Blue model Yellow structure

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Evaluation of model quality Check for proper protein stereochemistry –ProCheck ( Ramachandran plot, bond-length, … –Whatif ( Packing quality –Both web-servers Fitness of sequence to structure –ProsaII ( Program runs on Linux and Unix –Verify3D ( Web-server

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU ProCheck Peptide backbone geometry Peptide planes –C  NCC  Dihedral angles   degrees –  strand  degrees –  helix From s peedy.st-and.ac.uk/.../lectures/ 3014/lecture/dars1.htm 

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU B. Beta strand A. Right handed helix L. Left handed helix Color coding –White. Disallowed –Red. Most favorable –Yellow. Allowed region Glycine triangles Ramachandran plot B A L

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Find the wrong structure 1RIP Ribosomal protein. 1PLC Electron transport protein

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Procheck. Bond length 1plc

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU

1plc

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU What-if. Fine packing Quality Statistical description of local chemical environment in high quality protein structures –Superimpose tryptophans and find average local environment. Same for other amino acids –Full atom model G. Vriend and C. Sander, 1992

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Example. Casp Model T0133 T0133 Casp5 target Modeled by X3M ( CPHModels-2.0, Lund O., 2002) RMSD=7.3

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Casp Model - Fine packing quality ---Residue----- State AllAll BB-BB BB-SC SC-BB SC-SC ILE ( 33 ) SER ( 34 ) … ALA ( 296 ) GLU ( 297 ) HIS ( 298 ) ============================================================ All contacts : Average = Z-score = BB-BB contacts : Average = Z-score = BB-SC contacts : Average = Z-score = SC-BB contacts : Average = Z-score = SC-SC contacts : Average = Z-score = ============================================================ Average protein values ("Z-score for all contacts") can be read as follows: -5.0 Guaranteed wrong structure. Bad structure or poor model -3.0 Probably bad structure or unrefined model. Doubtful structure or model -2.0 Structure OK or good model. Good structures 0.0 Good structures. 2.0 Good structures. Unusually Good structures 4.0 Probably a strange model of a perfect helix Bad model BB: Backbone SC: Sidechain

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU T0133 structure - Fine packing quality ---Residue----- State AllAll BB-BB BB-SC SC-BB SC-SC ILE ( 33 ) A SER ( 34 ) A … ALA ( 296 ) A GLU ( 297 ) A HIS ( 298 ) A ============================================================ All contacts : Average = Z-score = BB-BB contacts : Average = Z-score = BB-SC contacts : Average = Z-score = 0.90 SC-BB contacts : Average = Z-score = SC-SC contacts : Average = Z-score = 0.02 ============================================================ Average protein values ("Z-score for all contacts") can be read as follows: -5.0 Guaranteed wrong structure. Bad structure or poor model -3.0 Probably bad structure or unrefined model. Doubtful structure or model -2.0 Structure OK or good model. Good structures 0.0 Good structures. 2.0 Good structures. Unusually Good structures 4.0 Probably a strange model of a perfect helix Good model

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Sippl, J.M. (1990) J. Mol. Biol. 213, (1990). ProsaII (Potential of Mean Force) Likelihood of amino acid packing Method developed by Manfred Sippl., 1993 Works for C  -models For high quality protein structure estimate nearest neighbor counts for all aa E = -log(P(N|a)/P(N)) Hydrophobic residues tend to have many neighbors (buried) Hydrophilic residues tend to have fewer N (exposed) Finding an hydrophilic aa with many NN can indicate wrong model Exposure potential for D D is a charged aa

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU ProsaII (Potential of Mean Force) Likelihood of amino acid packing E = -log(P(r|abs)/P(r|s)) If D and E are close in sequence (s=3), then they prefer to be close in distance d~5.5Å Hydrogen bonds? Sippl, J.M. (1990) J. Mol. Biol. 213, (1990). a b r s Pair potential for D, E. s=3

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Verify 3D (Eisenberg et al. 1997) Closely related to ProsaII exposure potential. How well does aa fit its local environment (hydrophobic/hydrophilic) –T0133 Casp5 target –Modeled by X3M (Lund, O., 2002) –RMSD=7.3 –Red: Crystal structure, –Blue: Model

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Model T0133. Verify 3D Sequence has poor match to structure

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU ProQ. Prediction of Model accuracy Neural network to identify correct protein models. –B. Wallner and Arne Elofsson, 2003 – Input, a pdb structure/model Output, accuracy measure –LGscore –Maxsub score

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU ProQ Input to neural net –Atom-atom contacts C, N, O How often is C in contact with N? –Residue-residue contacts How often is E in contact with D? –Solvent accessibility surface Average exposure of L’s –Secondary structure prediction How consistent is prediction with model?

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Casp model T0113

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Structure 1RIP

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU LifeBench data Models 220 targets Modeled by Pcons Incorrect model Lgscore <1.5 Maxsub < 0.1

CENTER FOR BIOLOGICAL SEQUENCE ANALYSISTECHNICAL UNIVERSITY OF DENMARK DTU Conclusions Correct protein models cannot (yet!) reliably be identified!! Many methods from the protein crystallography world are useful to identify wrong models Bad models can however pass all filters ProQ is a first attempt of an “accuracy prediction server” –Can integrate information from many sources –Future will show if this approach can provide reliable prediction of model accuracy