Cited by 251Protein Structure and DynamicsMachine Learning in BioinformaticsEnzyme Structure and Function

AlphaFold 2: Why It Works and Its Implications for Understanding the Relationships of Protein Sequence, Structure, and Function

Open access
Skolnick, Jeffrey, Gao, Mu, Zhou, Hongyi, Singh, Suresh·2021·Journal of Chemical Information and Modeling

Abstract

AlphaFold 2 (AF2) was the star of CASP14, the last biannual structure prediction experiment. Using novel deep learning, AF2 predicted the structures of many difficult protein targets at or near experimental resolution. Here, we present our perspective of why AF2 works and show that it is a very sophisticated fold recognition algorithm that exploits the completeness of the library of single domain PDB structures. It has also learned local side chain packing rearrangements that enable it to refine proteins to high resolution. The benefits and limitations of its ability to predict the structures of many more proteins at or close to atomic detail are discussed.

AI snapshot

Written from the paper’s full text.

TL;DR

This Viewpoint argues that AlphaFold 2 (AF2) succeeds mainly as a sophisticated fold-recognition algorithm that exploits the near-completeness of the single-domain PDB structure library, plus learned local side-chain packing refinement. It also assesses AF2’s human-exome coverage and discusses implications and limits for inferring protein function, ligand binding, and phenotype.

Key findings

  • AF2 often produced models with TM-score > 0.9, while MSA-based methods plateau around TM-score 0.85; TM-score > 0.85 indicates both global fold and details are correct.
  • In human exome predictions, AF2 confidence score correlated with TM-score at 0.67. For low-quality models, only 10% had a closest template with TM-score ≥ 0.55; for good models, 53%; for high-accuracy models, 86%. These trends held with a current PDB library of almost 90,000 structures clustered at 70% sequence identity.
  • Comparing 26,996 consensus human exome sequences, TASSER-VMT predicted at least one domain with TM-score > 0.4 for 89% of sequences; AF2 had confidence >70 for 67.4% and >60 for 86.9%. AF2 added 8.7% of human sequences with confidence >60, and the union provided confident models for at least one domain for 97.5% of human sequences.
  • For ligand homology modeling, low-resolution structures are adequate, with at most a 20% improvement in enrichment factor going from lower resolution (TM-score > 0.4) to crystal structures, because binding sites are generally the most accurately predicted parts of a protein’s structure.

Methods

  • This is a Viewpoint/perspective, not a new AF2 training study. It interprets AF2’s mechanism using prior deep-learning work and structural analyses.
  • The authors used TM-align to generate structural alignments of AF2-predicted human proteins to a 41,000-member PDB library from 2016, and also examined results with a current PDB library of almost 90,000 structures clustered at 70% sequence identity.
  • They analyzed correlations between AF2 confidence scores and TM-score/contact order, binned confidence as <70, 70–90, and >90, and compared AF2 coverage with TASSER-VMT on 26,996 consensus human exome sequences.

Limitations

  • The paper states that structure alone does not provide biochemical function and cannot predict phenotypical function, which often depends on complex interactions among proteins, RNA, DNA, and metabolites.
  • The paper states that AF2 predicts a particular conformational state; if an active conformation is predicted, it may not work for targeting the inactive state. It also notes that AF2 was trained on proteins with particular ions and might not accurately represent binding pockets under other conditions.
  • The paper states that much work remains to establish AF2’s strengths and limitations, including whether it can predict effects of missense mutations, splice variants, chameleon sequences, cryptic pockets, and allosteric transitions. Evident from the text: the analysis is a perspective focused on the human exome and single-domain PDB completeness, and multimeric/multi-domain prediction is discussed as likely but not fully established.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1021/acs.jcim.1c01114

Cites

61 of 61 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 33 cited sources checked · 0 supported · 0 flagged

  • Findingnone given

    For some multiple domain CASP14 targets, AF2 made high quality structure predictions.

    “For some multiple domain CASP14 targets, AF2 made high quality structure predictions.”
    Quote in the paperSubstantial claim
  • Findingnone given

    The Pearson correlation coefficient of the average confidence score per protein with contact order is 0.28.

    “Interestingly, the Pearson correlation coefficient of the average confidence score per protein with contact order is 0.28—a weak correlation of model quality with whether the fold has a sequential or nonsequential arrangement of secondary structural elements.”
    Quote in the paperSubstantial claim
  • Findingnone given

    AF2 sometimes refines the structure to experimental accuracy, with TM-scores greater than 0.9.

    “Having selected an approximate fold, AF2 sometimes refines the structure to experimental accuracy, with TM-scores greater than 0.9.”
    Quote in the paperNot a substantial claim
  • Findingown data

    These results are preserved when the current PDB library of almost 90,000 structures clustered at a sequence identity cutoff of 70% is used, with a shift toward higher TM-scores for the various prediction confidence levels.

    “These results are preserved when the current PDB library of almost 90,000 structures clustered at a sequence identity cutoff of 70% is used, with a shift toward higher TM-scores for the various prediction confidence levels.”
    Quote in the paperSubstantial claim
  • Findingown data

    The correlation coefficient of AF2's confidence score with TM-score is 0.67.

    “The correlation coefficient of AF2’s confidence score with TM-score is 0.67.”
    Quote in the paperSubstantial claim
  • May combine two claims. 11
  • Classified as probably not a substantial claim. 1
  • May combine two claims.

    Findingown data

    53% of proteins with good models and 86% with high accuracy models have a TM-score ≥ 0.55.

    “In contrast, 53% of proteins with good models and 86% with high accuracy models have a TM-score ≥ 0.55.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    AF2 has a confidence score greater than 70 for 67.4% of sequences, while 86.9% have a confidence score greater than 60.

    “AF2 has a confidence score greater than 70 for 67.4% of sequences, while 86.9% have a confidence score greater than 60.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Premisecitation

    The structural space of protein-protein interfaces is small (<1000 structurally distinct interfaces), with nothing special about protein-protein interfaces.

    “In practice, the structural space of protein–protein interfaces is small (<1000 structurally distinct interfaces),41,42 with nothing special about protein–protein interfaces.43”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [41] source

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    Premisecitation

    DESTINI CNN deep learning learned contact map patterns of secondary structure packing and eliminated nonphysical, isolated residue pairs.

    “The DESTI-NI convolutional neural network (CNN) deep learning approach to structure prediction23 demonstrated that deep learning learned the contact map patterns of secondary structure packing and eliminated nonphysical, isolated residue pairs.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [23] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    AF2 is a very sophisticated fold recognition algorithm that exploits the completeness of the library of single domain PDB structures.

    “Here, we present our perspective of why AF2 works and show that it is a very sophisticated fold recognition algorithm that exploits the completeness of the library of single domain PDB structures.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Premisecitation

    The reason SAdLSA trained on helical proteins works for β-proteins is that the global topology of β-proteins is in the training set; the chain contour of helical proteins provides a globally correct fold for β-proteins.

    “The reason that SAdLSA trained on helical proteins works for β-proteins is that the global topology of β-proteins is in the training set;2,12,13 the chain contour of helical proteins provides a globally correct fold for β-proteins.13”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [2] source

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    • cites [13] source

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  • May combine two claims.

    Interpretationreasoning

    Structure alone does not provide the biochemical function of the protein nor can AF2 predict phenotypical function.

    “First, structure alone does not provide the biochemical function of the protein nor can AF2 predict phenotypical function which is often the result of the complex interplay of proteins, RNA, DNA, and metabolites.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Premisereasoning

    Using ligand homology modeling, low resolution structures are adequate for virtual ligand screening, with at most a 20% improvement in enrichment factor on going from lower resolution (TM-score > 0.4) to crystal structures.

    “Contrary to popular belief that high resolution structures are required, using ligand homology modeling,45–48 low resolution structures are adequate, with at most a 20% improvement in enrichment factor on going from lower resolution (TM-score > 0.4) to crystal structures.”
    Quote in the paperSubstantial claim0 of 4 sources checked
    • cites [45] source

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    • cites [46] source

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    • cites [47] source

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    • cites [48] source

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    • Rests on an assumption: Binding sites are generally the most accurately predicted parts of a protein's structure.
  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationcitation

    How inclusion of higher quality structures improves the ability to predict drug side effects, efficacy and the role of missense variations in non-Mendelian diseases depends on accurately characterizing the interplay of many molecular players.

    “How inclusion of higher quality structures improves the ability to predict drug side effects, efficacy64 and the role of missense variations in non-Mendelian diseases depends on accurately characterizing the interplay of many molecular players.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [64] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    AF2 can accurately predict backbone and side chain conformations, but it is for a particular conformational state.

    “Even though AF2 can accurately predict backbone and side chain conformations, it is for a particular conformational state.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Interpretationcitation

    Knowledge of a protein's structure alone does not provide its biochemical, much less phenotypical effects, but structure can be a component.

    “Importantly, while knowledge of a protein’s structure alone does not provide its biochemical, much less, phenotypical effects, structure can be a component.64”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [64] source

      Not checked in the free run.

11 of 11
  • The paper relies on this without support.

    Binding sites are generally the most accurately predicted parts of a protein's structure.

    “The reason is simple; binding sites are generally the most accurately predicted parts of a protein’s structure.”
  • The paper relies on this without support.

    AF2 was trained on proteins having particular ions.

    “Since AF2 was trained on proteins having particular ions, for example, it might not provide an accurate representation of the binding pocket under other conditions.”
2 of 2

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Two key factors contributing to AF2's success are the completeness of the structural space of single-domain proteins and an abundance of sequences in sequence databases.

    “two key factors contributing to the success of AF2 are the completeness of the structural space of single-domain proteins and an abundance of sequences in sequence databases3.”
  • It is very likely that neural network models trained for single-chain structure prediction have already learned the representations necessary to model protein complexes made of multiple single-chain proteins.

    “Taken together, it is very likely that the neural network models trained for single-chain structure prediction have already learned the representations necessary to model protein complexes made of multiple single-chain proteins3.”

Cites

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Authors
Skolnick, J., Gao, M., Zhou, H., & Singh, S.
Affiliations
  • Georgia Institute of Technology (US)
  • Twilight Design, 4 Adams Road, Kendall Park, New Jersey 08824, United States
Corresponding author
Skolnick, Jeffrey
Venue
Journal of Chemical Information and Modeling
Published
2021-09-29
Volume
61
Issue
10
Pages
4827-4831
ISSN
  • 1549-9596 (ISSN-L)
  • 1549-960X
OpenAlex
W3201869122
Cited by
251
References
62
Citations by year
2026: 28 · 2025: 58 · 2024: 64 · 2023: 66 · 2022: 31 · 2021: 2 · 2012: 1
FWCI
12.86
Type
journal article
Language
en
Open access
Locations
  • Journal of Chemical Information and Modeling (publishedVersion)
  • PubMed (publishedVersion)
  • PubMed Central (acceptedVersion)
Funders
U.S. Department of Health and Human Services (GM-118039)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Protein Structure and Dynamics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Machine Learning in Bioinformatics
  • Physical Sciences › Materials Science › Materials Chemistry › Enzyme Structure and Function
Keywords
AlphaFold2, protein structure prediction, CASP14, deep learning, protein fold recognition, side-chain packing, sequence-structure-function relationships
MeSH terms
Algorithms, Amino Acid Sequence, Proteins, Protein Folding

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