Calculate Position Specific Blosum Score

BLOSSUM (Blocks Substitution Matrix) scores are used in bioinformatics to measure the similarity between amino acids in protein sequences. Position-specific scoring takes into account the specific location of amino acids in the sequence, providing more accurate alignment results.

What is BLOSSUM?

BLOSSUM (Blocks Substitution Matrix) is a family of matrices used for protein sequence alignment. These matrices contain log-odds scores representing the likelihood of amino acid substitutions occurring in nature. The scores are derived from multiple sequence alignments of protein blocks.

Key Points

BLOSSUM matrices are based on observed amino acid substitution patterns
Different BLOSSUM matrices (e.g., BLOSSUM62, BLOSSUM80) have different substitution frequencies
Higher BLOSSUM numbers indicate more distant evolutionary relationships

The most commonly used BLOSSUM matrix is BLOSSUM62, which is based on a multiple sequence alignment of 62 proteins. Each position in the matrix represents the log-odds score for the substitution of one amino acid for another.

Position-Specific Scoring

Position-specific scoring takes into account the specific location of amino acids in a protein sequence. This approach is more sophisticated than simple pairwise comparison because it considers the context in which each amino acid appears.

Position-Specific Score Formula

For a given alignment of two sequences, the position-specific score S is calculated as:

S = Σ (BLOSSUM(a_i, b_i)) for all aligned positions i

Where a_i and b_i are the amino acids at position i in sequences A and B, respectively.

Position-specific scoring is particularly useful for identifying conserved regions in protein sequences and for predicting functional similarities between proteins.

How to Calculate

To calculate the position-specific BLOSSUM score for two protein sequences:

Align the two protein sequences using a sequence alignment algorithm
For each aligned position, look up the BLOSSUM score for the substitution of one amino acid for another
Sum all the individual BLOSSUM scores to get the total position-specific score

The resulting score provides a measure of the similarity between the two protein sequences, taking into account the specific positions of amino acids in the alignment.

Example Calculation

Consider the following aligned protein sequences:

Position	Sequence A	Sequence B	BLOSSUM62 Score
1	M	M	5
2	K	L	-2
3	L	L	5
4	E	E	5
5	E	D	2

The total position-specific BLOSSUM score is calculated as:

5 (M→M) + (-2) (K→L) + 5 (L→L) + 5 (E→E) + 2 (E→D) = 15

Interpretation

A score of 15 indicates a relatively high similarity between the two sequences, with several conserved positions and only one moderate substitution.

Applications

Position-specific BLOSSUM scoring is used in various bioinformatics applications, including:

Protein sequence alignment and comparison
Identification of conserved protein domains
Prediction of protein function based on sequence similarity
Detection of homologous proteins in different species
Analysis of protein evolution and divergence

By considering the specific positions of amino acids, position-specific scoring provides more accurate and biologically meaningful results than simple pairwise comparison methods.

FAQ

What is the difference between BLOSSUM and PAM matrices?

BLOSSUM matrices are based on observed amino acid substitution patterns in real protein sequences, while PAM matrices are based on evolutionary models. BLOSSUM matrices tend to be more accurate for real-world sequence comparisons.

How do I choose the right BLOSSUM matrix?

The choice of BLOSSUM matrix depends on the evolutionary distance between the sequences being compared. Higher BLOSSUM numbers (e.g., BLOSSUM80) are used for more distantly related sequences.

Can I use BLOSSUM scores for DNA sequences?

No, BLOSSUM matrices are specifically designed for protein sequences and cannot be directly applied to DNA sequences. For DNA sequences, you would use a different type of substitution matrix.