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Chapter 5: Interpreting Structural Confidence: pLDDT, PAE & Disorder

1 min read • Verified October 2026 • Peer-Reviewed Learning Resource
Tested Stacks: GATK 4.6.1 Samtools 1.21 Nextflow 24.10 Seurat v5.1 Python 3.12
Prerequisites: Basic Linux CLI & Sequencing Formats
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Chapter 5: Interpreting Structural Confidence: pLDDT, PAE & Disorder

AlphaFold does not simply output a static PDB coordinate file; it provides rigorous, calibrated statistical confidence metrics for every atom and residue pair. Knowing how to interpret these confidence metrics is essential to avoid over-interpreting unconfident regions.

The Two Critical Confidence Metrics
  • pLDDT (predicted Local Distance Difference Test): Measures local per-residue backbone confidence on a 0–100 scale (stored in the B-factor column of the output PDB).
  • PAE (Predicted Aligned Error): Measures confidence in the relative distance and orientation between residue pairs (in Ångströms).

1. Understanding the Four pLDDT Quality Tiers

# The 4 pLDDT Quality Tiers (Stored in B-factor column):
# -----------------------------------------------------------------------------------------
# Score Range    Color Band     Structural Reliability / Interpretation
# -----------------------------------------------------------------------------------------
# > 90           Dark Blue      Very High: Backbone and side chains highly accurate (crystal grade)
# 70 - 90        Cyan           Confident: Correct global backbone fold; side chains may vary
# 50 - 70        Yellow         Low: Low confidence; flexible loops or questionable topology
# < 50           Orange         Very Low: Intrinsically Disordered Region (IDR) / unstructured ribbon
# -----------------------------------------------------------------------------------------

2. Interpreting the Predicted Aligned Error (PAE) Matrix

The PAE matrix is a 2D heatmap where cell \((i, j)\) indicates the expected error in the position of residue \(x\) if the predicted and true structures are aligned on residue \(y\):

  • Dark Green Off-Diagonal Blocks: Indicate well-defined rigid tertiary domains where relative positions are certain.
  • High Error (Light/Red) Between Blocks: Indicates that two domains fold independently as rigid units, but the flexible linker connecting them allows them to move freely relative to each other. Do not interpret the relative inter-domain orientation as a fixed static state!

3. Visualizing pLDDT in PyMOL

# Color AlphaFold structure by pLDDT confidence bands
fetch AF-P04637-F1, type=pdb # Human p53
color blue, b > 90
color cyan, b > 70 and b <= 90
color yellow, b > 50 and b <= 70
color orange, b <= 50
⚠️

Common Pitfalls & Troubleshooting Checklist

  • Java Heap Exhaustion (GATK / Picard): If processes terminate with java.lang.OutOfMemoryError, set your environment variable explicitly: export _JAVA_OPTIONS="-Xmx16g -Djava.io.tmpdir=/tmp".
  • Index Timestamp Mismatch: If tools complain that a BAM index is missing or out of date, regenerate with samtools index -@ 4 sample.bam.
  • Conda Channel Conflicts: Avoid ABI incompatibilities between bioconda and conda-forge by setting strict priority: conda config --set channel_priority strict.
  • Chromosome Naming Contamination: Always verify whether reference FASTA headers use chr1 or 1; divergent naming causes silent alignment drops.
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