The modal distribution of protein isoelectric points reflects amino acid properties rather than sequence evolution

Two-dimensional gel electrophoresis, a routine application in proteomics, separates proteins according to their molecular mass (M_r) and isoelectric point (p/). As the genomic sequences for more and more organisms are determined, the M_r and p/ of all their proteins can be estimated computationally. The examination of several of these theoretical proteome plots has revealed a multimodal p/ distribution, however, no conclusive explanation for this unusual distribution has so far been presented. We examined the p/ distribution of 115 fully sequenced genomes and observed that the modal distribution does not reflect phylogeny or sequence evolution, but rather the chemical properties of amino acids. We provide a statistical explanation of why the observed distributions of p/ values are multimodal.