Computational Methods for Protein Structure Prediction

Transcription

1 Computational Methods for Protein Structure Prediction Ying Xu 2017/12/6 1

2 Outline introduction to protein structures the problem of protein structure prediction why it is possible to predict protein structures protein tertiary structure prediction protein threading 2017/12/6 2

3 Protein Sequence, Structure and Function Protein >1MBN:_ MYOGLOBIN (154 AA) sequence MVLSEGEWQLVLHVWAKVEADVAGHGQDILIRLFKSHPETLEKFDRFKHL KTEAEMKASEDLKKAGVTVLTALGAILKKKGHHEAELKPLAQSHATKHKI PIKYLEFISEAIIHVLHSRHPGNFGADAQGAMNKALELFRKDIAAKYKEL GYQG Protein structure Protein function Oxygen storage 2017/12/6 3

4 Protein Structure protein sequence folds into a unique shape ( structure ) that minimizes its free potential energy 2017/12/6 4

5 Protein Structures Primary sequence MTYKLILNGKTKGETTTEAVDAATAEKVFQYANDNGVDGEWTYTE Secondary structure -helix anti-parallel -sheet parallel 2017/12/6 5

6 Protein Structures Tertiary structure Quaternary structure 2017/12/6 6

7 2017/12/6 7

8 Why Protein Structure? Importance of protein structure knowledge of the structure of a protein enable us to understand its function and functional mechanism design better mutagenesis experiments structure-based rational drug design 2017/12/6 8

9 Protein Structure Prediction Problem: Given the amino acid sequence of a protein, computationally predict its 3-dimensional shape? MVLSEGEWQLVLHVWAKVEADVAGHGQDILIRLFKSHPETLEKFDRFKHL KTEAEMKASEDLKKAGVTVLTALGAILKKKGHHEAELKPLAQSHATKHKI PIKYLEFISEAIIHVLHSRHPGNFGADAQGAMNKALELFRKDIAAKYKEL GYQG? /12/6 9

10 Why We Can Predict Structure In theory, a protein structure can be solved computationally A protein folds into a 3D structure to minimizes its free potential energy Anfinsen s classic experiment on Ribonuclease A folding in the 1960 s energy functions This problem can be formulated as an optimization problem protein folding problem, or ab initio folding 2017/12/6 10

11 Why We Can Predict Structure The problem is exceedingly difficult to solve the search space is defined by psi/phi angles of backbone and side-chain positions the search space is enormous even for small proteins! the number of local minima increases exponentially of the number of residues Theoretically solvable but practically infeasible! 2017/12/6 11

12 ab initio Methods for Protein Structure Prediction use first principles to fold proteins does not require templates high computational complexity homology modeling similar sequence similar structures practically very useful, need homologues protein threading many proteins share the same structural fold a folding problem becomes a fold recognition problem 2017/12/6 12

13 Why We Can Predict Structure Theoretical studies suggest that the vast majority of the proteins in nature fall into not much more than 1,000 structural folds This realization has fundamentally changed how protein structures can be predicted The structure prediction problem becomes that for a protein sequence, find which of the structural folds the protein can fold into, plus possibly some structural refinement MTYKLILN. NGVDGEWTYTE 2017/12/6 13

14 Protein Threading Basic premise The number of unique structural (domain) folds in nature is fairly small (possibly a few thousand) Statistics from Protein Data Bank (~37,229 structures) 90% of new structures submitted to PDB in the past three years have similar structural folds in PDB Chances for a protein to have a native-like structural fold in PDB are quite good (estimated to be 60-70%) Proteins with similar structural folds could be homologues or analogues 2017/12/6 14

15 Protein Threading The goal: find the correct sequence-structure alignment between a target sequence and its native-like fold in PDB MTYKLILN. NGVDGEWTYTE Energy function knowledge (or statistics) based rather than physics based Should be able to distinguish correct structural folds from incorrect structural folds Should be able to distinguish correct sequence-fold alignment from incorrect sequence-fold alignments 2017/12/6 15

16 Protein Threading energy function MTYKLILNGKTKGETTTEAVDAATAEKVFQYANDNGVDGEWTYTE how preferable to put two particular residues nearby: E_p alignment gap penalty: E_g how well a residue fits a structural environment: E_s total energy: E_p + E_s + E_g find a sequence-structure alignment to minimize the energy function 2017/12/6 16

17 Protein Threading energy function A singleton energy measures each residue s preference in a specific structural environments secondary structure solvent accessibility Compare actual occurrence against its expected value by chance where 2017/12/6 17

18 Protein Threading energy function A simple definition of structural environment secondary structure: alpha-helix, beta-strand, loop solvent accessibility: 0, 10, 20,, 100% of accessibility each combination of secondary structure and solvent accessibility level defines a structural environment E.g., (alpha-helix, 30%), (loop, 80%), E_s: a scoring matrix of 30 structural environments by 20 amino acids E.g., E_s ((loop, 30%), A) singleton energy term 2017/12/6 18

19 Protein Threading energy function Helix Sheet Loop Buried Inter Exposed Buried Inter Exposed Buried Inter Exposed ALA ARG ASN ASP CYS GLN GLU GLY HIS ILE LEU LYS MET PHE PRO SER THR TRP TYR VAL /12/6 19

20 Protein Threading energy function It measures the preference of a pair of amino acids to be close in 3D space. Observed occurrence of a pair compared with its expected occurrence uniform state model pair-wise interaction energy term 2017/12/6 20

21 Protein Threading energy function ALA -140 ARG ASN ASP CYS GLN GLU GLY HIS ILE LEU LYS MET PHE PRO SER THR TRP TYR VAL ALA ARG ASN ASP CYS GLN GLU GLY HIS ILE LEU LYS MET PHE PRO SER THR TRP TYR VAL 2017/12/6 21

22 Threading Parameter Optimization How to determine the weight of different energy term? Etotal = s E singleton + p E pairwise + g E gap + ss E ss Select the weights to give the best threading performance on a training set (fold recognition and alignment accuracy) Different weights for different classes? (superfamily, fold) pair-wise may contribute more for fold level threading mutation/profile terms dominate in superfamily level threading 2017/12/6 22

23 Protein Threading mathematical formulation Etotal( Tj Tj+2 Tj+4 Tj+7 template structure (T) S T K Y Q C D D A Si Si+8 Target sequence (S) 2017/12/6 23

24 Protein Threading -- algorithm Dynamic programming Heuristic algorithms for pair-wise interactions Frozen approximation algorithm (A. Godzik et al.) Double dynamic programming (D. Jones et al.) Monte carlo sampling (S.H. Bryant et al.) Rigorous algorithms for pair-wise interactions Branch-and-bound (R.H. Lathrop and T.F. Smith) Divide-and-conquer (Y. Xu et al.) --PROSPECT Linear programming (J. Xu et al.) RAPTOR Tree decomposition (L. Cai et al.) Rigorous algorithm for treating backbone and side-chain simultaneously (Li et al.) 2017/12/6 24