  An accurate estimation of the Protein Space size, in light of the factors
that govern it, is a long-standing problem and of paramount importance in
evolutionary biology, since it determines the nature of protein evolvability. A
simple analysis will enable us to, firstly, reduce an unrealistic Protein Space
size of ~10^130 sequences, for a 100-residues polypeptide chain, to ~10^9
functional proteins and, secondly, estimate a robust average-mutation rate per
amino acid (x ~1.23) and infer from it, in light of the protein marginal
stability, that only a fraction of the sequence will be available at any one
time for a functional protein to evolve. Although this result does not solve
the Protein Space vastness problem, frames it in a more rational one.
