Bioinformatics: Secondary Structure Prediction

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1 Bioinformatics: Secondary Structure Prediction Prof. David Jones

2 LMLSTQNPALLKRNIIYWNNVALLWEAGSD The greatest unsolved problem in molecular biology:the Protein Folding Problem?

3 Entries Why predict structure? Growth of sequence and structure databases Metagenomics Sequences Structures Year

4 Biological Information Flow Gene (DNA) Protein 3D structure Function The unique 3D structure of a protein determines its biochemical function.

5 Protein Secondary Structure HELIX STRAND COIL

6 Secondary Structure Prediction LMLSTQNPALLKRNIIYWNNVALLWEAGSD? LMLSTQNPALL HHHEEEECCCC OUTPUT: 3-letter Secondary Structure Alphabet INPUT: 20-letter Amino Acid Alphabet

7 Goal Take primary structure (sequence) and, using rules derived from known structures, predict the secondary structure that is most likely to be adopted by each residue

8 1st Generation Methods Based on statistical analysis of single amino acid properties Examples: Chou & Fasman (1974) Lim (1974) Garnier, Osguthorpe & Robson (1978)

9 Structural Propensities Due to the size, shape and charge of its side chain, each amino acid may fit better in one type of secondary structure than another Classic example: The rigidity and side chain angle of proline cannot be accommodated in an -helical structure

10 Structural Propensities Two ways to view the significance of this preference (or propensity) It may control or affect the folding of the protein in its immediate vicinity (amino acid determines structure) It may constitute selective pressure to use particular amino acids in regions that must have a particular structure (structure determines amino acid)

11 Chou-Fasman method Uses table of conformational parameters (propensities) determined primarily from measurements of secondary structure by CD spectroscopy Table consists of one likelihood for each structure for each amino acid For amino acid type A (e.g. leucine) and structure type S (e.g. α helix), a propensity score is calculated as follows: P S p( A S) p( A) n A, S n A n n S

12 Chou-Fasman propensities (partial table) Amino Acid P P P t Glu Met Ala Val Ile Tyr Pro Gly

13 Chou-Fasman method A prediction is made for each type of structure for each amino acid Can result in ambiguity if a region has high propensities for both helix and sheet (higher value usually chosen, with exceptions)

14 Chou-Fasman method Calculation rules are somewhat ad hoc Example: Method for helix Search for nucleating region where 4 out of 6 a.a. have P > 1.03 Extend until 4 consecutive a.a. have an average P < 1.00 If region is at least 6 a.a. long, has an average P > 1.03, and average P > average P consider region to be helix

15 2nd Generation Methods Based on peptide segments / residue pairs Examples: GOR III (1987) The BIG NEWS, however, was the appearance of the first examples of MACHINE LEARNING in secondary structure prediction Neural Networks: Qian & Sejnowski (1988), Bohr et al. (1988), Holley & Karplus (1989)

16 Neural Networks Originally, neural networks were developed as simple models of brain function i.e. they were intended to be simulations of real networks of neurons. Hence the term Artificial Neural Network. Today these simple models are obsolete in neuroscience research but instead have become very useful tools for finding patterns in data.

17 Artificial Neural Networks Inputs Output An artificial neural network (ANN) is made up of many switching units (artificial neurons) that are connected together according to a specific network architecture. The objective of an artificial neural network is to learn how to transform inputs into meaningful outputs.

18 Training and Using ANNs We start with complete random connection weights Then we present an input pattern to the network and compare the output we get to what it should be If the output is correct then we don t need to change any weights If the output is WRONG then we make small changes to the connection weights so as to reduce the ERROR We then repeat this for all the examples we have And keep repeating on all the data until we see no further improvement

19 Training and Testing Sets The data we use to adjust the network weights is called the TRAINING SET To make sure we are not over-fitting our network to our training set, we should test the network on a completely separate TESTING SET This splitting of training and testing data is called CROSS- VALIDATION and is an important concept in statistics and machine learning

20 Some real data... Inputs A R N D C Q E G H I L K M F P S T W Y V Coil Strand Coil Strand Strand Helix Helix Coil Strand Helix Helix Helix Strand Helix Helix Strand Strand Strand Coil Helix Helix Coil Coil Coil... And so on... Desired Outputs

21 A Better Scheme for Predicting Secondary Structure by Machine Learning Window of 15 residues Classifier (neural network) Helix Strand Coil MLSPQAMSDFHEELKWLLCNIPGQKLASLANREYT We are predicting the secondary Structure for this central residue

22 Representation Usual representation is to use 21 inputs per amino acid: Ala : Val : :

23 3rd Generation Neural Network Methods PHD Rost B, Sander, C. (1993) Prediction of protein secondary structure at better than 70% accuracy. J. Mol. Biol. 232, PSIPRED Jones, D. T. (1999) Protein secondary structure prediction based on position-specific scoring matrices. J. Mol. Biol. 292:

24 3rd Generation Methods Exploit evolutionary information Based on conservation analysis of multiple sequence alignments (HMMs or profiles) Extract some long-range information via accessibility patterns Conserved Hydrophobic Residues -> BURIED Variable Polar Residues -> EXPOSED

25 Variability and Hydrophobity of Amino Acids Variable & Polar Conserved & Hydrophobic Water soluble Variable & Very Hydrophobic Conserved & Hydrophobic Transmembrane

26 Pros & Cons of 3rd Generation Methods PROs High residue accuracy Less underprediction of strands Good quality segment predictions CONs Provides prediction for FAMILY CONSENSUS structure NOT THE STRUCTURE OF THE TARGET SEQUENCE

27 PSIPRED Works directly on PSI-BLAST profiles (PSSMs) Uses 2 separate stages of neural networks First network predicts secondary structure Second network cleans outputs from 1st net Trained on profile from ~3000 different proteins of known structure (taken from PDB)

28 4 x 1st Networks PSIPRED Neural Networks 15x21 Input units 75 hidden units 3 output units (H/E/C) 2nd Network 15x3 Input units 60 hidden units 3 output units (H/E/C)

29 PSIPRED Method Raw profile from PSI-BLAST Log File Position-based scoring matrix used A R N D C Q E G H I L K M F P S T W Y V Window of 15 rows A R N D C Q E G H I L K M F P S T W Y V x 20 scaled inputs to 1st network 1st Network 315 inputs 75 hidden units 3 outputs Window of 15 x 3 outputs fed to 2nd network 2nd Network 60 inputs 60 hidden units 3 outputs Final 3-state Prediction

30 2 nd Network: Filtering Raw Predictions from 1 st Network Stage 1 output Stage 2 output Actual 49 T E L E V E R E V E P C G C S C W E E H I H P H V H A H C H E 49 T E L E V E R E V E P C G C S H W H E H I H P H V H A H C H E 49 T E 50 L E 51 V E 52 R E 53 V E 54 P C 55 G C 56 S H 57 W H 58 E H 59 I H 60 P H 61 V H 62 A H

31 Common Measures of Secondary Structure Prediction Accuracy Q 3 scores give the percentage of correctly predicted residues across 3 states (H,E,C) This is the most commonly used measure Other scores such as Matthew s Correlation Coefficient try to identify accuracy for individual states (Coil, Strand, Helix) and are more sensitive to over-prediction e.g. if you predict all residues to be random coil you will get a Q 3 score of around 50% just because around 50% of residues in proteins are in random coils. However, the MCC scores will be close to zero!

32 Matthews Correlation Coefficient

33 Number PSIPRED Benchmark Results Mean Q 3 score: 77.8% (80.6% now) Q3 Score (%)

34 OBSOLETE!! First use of neural network Current best method Comparison of Generations by Average Q3 Scores GEN 1 GEN 2 GEN Chou & Fasman GOR I Qian & Sejnowski PHD (1994) PSIPRED 10 0

35 Protein Fold Recognition

36 Sequence Comparison > 30% Identity between two protein sequences implies probable common structure and possibly common function However, there are many exceptions to this rule of thumb 1PLC 1NIN IDVLLGADDGSLAFVPSEFSISPGEKIVFKNNAGFPHNIVFDEDSIPS-GVDASKISM : X:.: : :.: :...:.::.. : :: :::.::: :...:.: :. ETYTVKLGSDKGLLVFEPAKLTIKPGDTVEFLNNKVPPHNVVFDAALNPAKSADLAK-SL PLC 1NIN SEEDLLNAKGETFEVAL---SNKGEYSFYCSPHQGAGMVGKVTVN- :...:X. : :::.::: ::.:::::::.:: SHKQLLMSPGQSTSTTFPADAPAGEYTFYCEPHRGAGMVGKITVAG

37 Similar Sequence Similar 3-D Structure (RMSD = 2.1 A, Seq. ID = 30%) Ribonuclease MC1 Ribonuclease Rh

38 Structure similarity

39 Increasing accuracy/reliability Prediction Methods Comparative modelling Requires: Known fold + clear homology Fold recognition Requires: Known fold Ab initio / new fold methods Requires: only target sequence Increasing Difficulty

40 Tertiary Structure Prediction BASIS: native fold is expected to be the conformation of lowest energy True for small molecules IMPLICATION: native fold can be found by defining a potential energy function and searching all conformations for the one with lowest energy

41 The Levinthal Paradox For a protein sequence of length l, the total number of possible chain conformations N is given by: N 10 l >> Even if a protein was able to rearrange itself at the speed of light it would take ~10 75 years to locate the global energy minimum for a 100 residue protein.

42 Fold recognition a short cut to predicting protein tertiary structure Although there are vast number of possible protein structures, only a few have been observed in Nature The chance of a newly solved structure having a previously unknown fold is only ~20% >> We might be able to predict protein structure by selecting from already observed folds (a Multiple Choice version of the protein folding problem)

43 Protein Structure Prediction by Threading

44 An objective function for protein folding As yet there is no accurate physical model for protein folding Physics based force fields are not able to properly handle entropic solvent effects We cannot rely on classical physics Can we define a statistical model? Can we estimate Prob(Structure Sequence)?

45 Original structure (fragment) T G P A S K Native threading opposite charges stabilise the fold. D I Q

46 Threading alignment 1 - W R T M E D Y Ouch! Equal sign charges repel!! S

47 Threading alignment 2 - W - T M E R Y Much better - opposite charges again! S

48 A Scoring Function for Threading We want a want of assessing the energy of a model i.e. a model based on an alignment of a sequence with a structure Energy functions based on physics do not work Let s use a KNOWLEDGE-BASED APPROACH APPROACH - Look at known structures and see how often particular features (e.g. contacts between amino acids) occur. In other words we calculate probabilities.

49 Converting Probabilities to Potentials Probabilities are inconvenient for computer algorithms due to the required multiplications Additive quantities (e.g. energies) are easier to handle >> For many applications it is common to transform probabilities into energy-like quantities

50 The Inverse Boltzmann Principle The basic assumption in generating empirical potentials from probabilities is the so-called Inverse Boltzmann principle. According to the Boltzmann principle, the probability of occurrence of a given conformational state of energy E scales with the Boltzmann factor e -E /RT, where R is the gas constant (1.987 x 10-3 kcal.mol -1 K -1 ) and T is the absolute temperature (e.g. room temperature).

51 Potentials of Mean Force Count interactions of given type (e.g. alanine->serine betacarbon to alpha-carbon) in real protein structures Count interactions of same type in randomly generated protein structures or randomly selected sites (the reference state) Ratio of probabilities provides an estimate of the free energy change according to the inverse Boltzmann equation: E RT log p( interactio n in real proteins ) p( interactio n in decoy structures ) E High Energy State Low Energy State

52 Potentials of Mean Force k = 4, s = 14 Angstroms A S How common is this configuration in real proteins? What about in randomly folded protein chains?

53 Fold Recognition Potentials SR terms (n <11) MR terms (10 < n < 30) LR terms (n >= 30) Solvation terms (Rel. Acc.)

54 Potential Short-range C Pair Potential Separation 4) Distance (A)

55 We can estimate the stability of a given protein fold by summing potentials of mean force for all residue pairs Loops are sometimes ignored Single residue (solvation) terms usually included Can scale terms according to protein size

56 Partially correct Immunoglobulin Heavy Chain Fab Fragment Model Coloured by Threading Potential HIGH LOW

57 Can statistical potentials find the correct fold amongst a large set of incorrect decoy structures?

58 Search Problem: Threading Searching for the optimum mapping of sequence to structure while optimizing the sum of pair interactions is NP-complete (proven by R. Lathrop in 1994) i.e. an exhaustive search is needed to guarantee an optimal solution This search process is called THREADING i.e. what is the best way of threading this sequence through this structure In practice due to short range of pair potentials, heuristic solutions work fairly well: Exhaustive search (not practical) Dynamic programming. Double dynamic programming. Branch and bound. Monte Carlo, simulated annealing, Gibbs sampling, genetic algorithm.

59 Threading Methods in Practice Compared to comparative modelling, threading methods can produce models where there is no detectable homologue to be found in PDB The simplifying assumption that the backbone of the structure does not change when the sequence changes also results in poor recognition of folds (~30% reliability) and inaccurate models (i.e. inaccurate alignments) For distant homologues, however, there are weak clues in the sequences that can help

60 Learning to recognise protein folds Ideally we would like to define a single value which denotes the compatibility of a structure with a sequence i.e. P(Structure Seq) In practice this is not straightforward as each feature is estimated differently and are not independent >> We need to combine different features to decide whether or not a given fold recognition match is correct

61 GenTHREADER Neural Net Pair Energy Solv. Energy Alignment score Proteins related Alignment Length Proteins unrelated Nres (Struct) Nres (seq)

62 FSSP Z-score GenTHREADER4 Network Output Correlation with Structural Similarity Network Output

63 Calibrating the scores In theory, the network outputs should be good estimates of posterior probabilities In practice, they are not accurate estimates of p-values when benchmarked However, we can empirically estimate p- values by fitting to a suitable distribution

64 Frequency Estimating GenTHREADER p-values % 90.00% 80.00% 70.00% 60.00% 50.00% 40.00% 30.00% 20.00% 10.00%.00% Frequency Cumulative % Network Score p 26537e 25.33x

65 P-value p-value dependence on Pair Energy and Solvation Energy Solv = 10 Solv = Pair Energy

66 P-value p-value dependence on Pair Energy and Profile Alignment Score E = +250 E = -250 Profile Alignment Score

67 Comparison of Search Methods Run time to search SWISSPROT Release 37 (77977 sequences) for matches to Adenylate Kinase Method Hits Time Taken BLAST 61 8 sec PSIBLAST min GenTHREADER min Adenylate Kinase Perfect threading method 2000? days

68 Error Rate (%) Ratio Error Rate/Coverage Plot for Sequence-based Profiles (i.e. HMMs) vs GenTHREADER (1% error rate thresholds marked) Profile GenTHR % 52% Coverage (Sensitivity)

69 An Example of Fold Recognition

70 An Example ORF HI0073 from H. influenzae 114 a.a. long Function UNKNOWN: E64000 hypothetical protein HI Haemophilus influenzae (str e-46 A71149 hypothetical protein PH Pyrococcus horikoshii 101 7e-22 H70345 conserved hypothetical protein aq_507 - Aquifex aeolicus 99 2e-21 F72600 hypothetical protein APE Aeropyrum pernix (strain K1) 50 1e-06 C75046 hypothetical protein PAB Pyrococcus abyssi (strain e-04 C64354 hypothetical protein MJ Methanococcus jannaschii D64375 hypothetical protein MJ Methanococcus jannaschii H90346 conserved hypothetical protein [imported] - Sulfolobus so S62544 hypothetical protein SPAC12G12.13c - fission yeast (Schiz C90279 conserved hypothetical protein [imported] - Sulfolobus so H64462 hypothetical protein MJ Methanococcus jannaschii C69282 conserved hypothetical protein AF Archaeoglobus ful

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75 Conclusions HI0073 is a probable nucleotidyl transferase (now confirmed) Consistent fold recognition results Match to 1FA0 Chain B Secondary structure in reasonable agreement Functionally Important Residues are CONSERVED