Expect value (E-value)
description
Transcript of Expect value (E-value)
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Expect value(E-value)
• Expected number of hits, of equivalent or better score, found by random chance in a database of the size searched.
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Conserved domains
Domain: sequence of amino acids that typically fold to a stable tertiary structure. Many proteins are multi-domain.
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Blast to Psi-Blast
• Blast makes use of Scoring Matrix derived from large number of proteins.
• What if you want to find homologs based upon a specific gene product?
• Develop a position specific scoring matrix (PSSM).
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PSSM
M
G
A
S
F
M F W Y G A P V I L C R K E N D Q S T H
5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0
1 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 0
0 4 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Determine frequency of substitution, and converts to LogOdd score.
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PSSM
M
G
A
S
F
M F W Y G A P V I L C R K E N D Q S T H
5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0
1 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 0
0 4 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Can include a score for permitting insertions and deletions. Perhaps this position is at a turn, where INDELs are common.
INDEL
Indel 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0
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PSSM
• In evaluating (scoring) alignments, PSSM approaches typically:– Reward matches to columns that have
conserved amino acids– Penalize mismatches to columns with
conserved amino acid more than mismatches in a variable column
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PSI-BLAST
• Input a single query sequence.
• Executes a BLAST run.
• Program takes significant hits, incorporates matches into a PSSM.
• Sequences >98% similar not included (avoid biasing the PSSM).
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Power of approach:
• PSI-BLAST is iterative.
• Takes best hits and improves the scoring matrix.
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Original Blast had 84 hits.
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The PSSM will skewtowards this region