BioInformatics Lecture Notes

 Amino Acid Substitution Matrices (continued)

Principles of Scoring Matrix Construction

  • The Dayhoff Matrix:  Proteins evolve through a succesion of independent point mutations, that are accepted in a population and subsequently can be observed in the sequence pool.
  • (Dayhoff, M.O. et al. (1978) Atlas of Protein Sequence and Structure. Vol. 5, Suppl. 3 National Biomedical Reserach Foundation, Washington D.C. U.S.A).

    • First step: Pair Exchange Frequencies

     A PAM (Percent Accepted Mutation) is one accepted point mutation on the path between two sequences, per 100 residues.

    Second step: Frequencies of Occurence

    fi = number of observations of i / number of observations of all amino acids
    Amino acid frequencies: 
             1978        1991
L       0.085       0.091
A       0.087       0.077
G       0.089       0.074
S       0.070       0.069
V       0.065       0.066
E       0.050       0.062
T       0.058       0.059
K       0.081       0.059
I       0.037       0.053
D       0.047       0.052
R       0.041       0.051
P       0.051       0.051
N       0.040       0.043
Q       0.038       0.041
F       0.040       0.040
Y       0.030       0.032
M       0.015       0.024
H       0.034       0.023
C       0.033       0.020
W       0.010       0.014

    Third step: Relative Mutabilities

    mi = number of times i is observed to change * fi
    Relative mutabilities of amino acids: 
             1978        1991
A         100         100
C          20          44
D         106          86
E         102          77
F          41          51
G          49          50
H          66          91
I          96         103
K          56          72
L          40          54
M          94          93
N         134         104
P          56          58
Q          93          84
R          65          83
S         120         117
T          97         107
V          74          98
W          18          25
Y          41          50
    All values are taken relative to alanine, which is arbitrarily set at 100.

    Fourth step: Mutation Probability Matrix

    The probability that an amino acid in row i of the matrix will replace the amino acid in column j : the mutability of amino acid j, multiplied by the pair exchange frequency for ij divided by the sum of all pair exchange frequencies for amino acid i:
    Mij = mj Aij/sum(i=1,20)(Aij)

    Last step: the log-odds matrix

    log to base 10: a value of +1 would mean that the corresponding pair has been observed 10 times more frequently than expected by chance.  The most commonly used matrix is the matrix from the 1978 edition of the Dayhoff atlas, at PAM 250: this is also frequently referred to as the MDM78 PAM250 matrix.