Every element of BLOSUM matrix is computed by the formula (from Wikipedia):

is the probability of two amino acids and replacing each other in a homologous sequence, and and are the background probabilities of finding the amino acids and in any protein sequence. The factor is a scaling factor.

I would like to compute probability of replacing one amino acid by another . The BLOSUM matrix is implemented in biopython module, but unfortunately I have not found the probabilities and . Is there any easy way to obtain it or compute it?

Please upvote and "accept" if this answered your question.