Effect of Enzymatic Methylation of Proteins on Their Isoelectric Points

Enzymatic methylation of arginine and lysine residues of several cytochrome c and lysine residue of calmodulin always resulted in lowering of their respective isoelectric points (pI). Employing cytochromes c derived from various sources, we examined a possible relationship between the degree of amino acid sequence degeneracy and the magnitude of change in the pI values by enzymatic methylation, and found that there was no correlation between these two parameters. By constructing space-filling models of oligopeptide fragments adjacent to the potential methylation sites, we have noted that not all the methylatable residues are able to form hydrogen bonds prior to the methylation. Two preparations of yeast apocytochrome c, one chemically prepared by removing heme from holocytochrome c and the other by translating yeast iso-1-cytochrome c mRNA in vitro, exhibited slightly higher Stokes radii than the homologous holocytochrome c, indicating relatively 'relaxed or open' conformation of the protein. However, when the in vitro synthesized methylated apocytochrome c was compared with the unmethylated counter-part, the Stokes radius of the latter was found to be larger.