Protein splicing is an autocatalytic reaction where an intervening element (intein) is excised and the remaining two flanking sequences (exteins) are joined. The reaction requires specific conserved residues, and activity may be affected by both the intein and the extein sequence. Predicting how sequence will affect activity is a challenging task. Based on first-principles density functional theory and multiscale quantum mechanics/molecular mechanics, we report C-terminal cleavage reaction rates for five mutations at the first residue of the C-extein (+1), and describe molecular properties that may be used as predictors for future mutations. Independently, we report on experimental characterization of the same set of mutations at the +1 residue resulting in a wide range of C-terminal cleavage activities. With some exceptions, there is general agreement between computational rates and experimental cleavage, giving molecular insight into previous claims that the +1 extein residue affects intein catalysis. These data suggest utilization of attenuating +1 mutants for intein-mediated protein manipulations because they facilitate precursor accumulation in vivo for standard purification schemes. A more detailed analysis of the "+1 effect" will also help to predict sequence-defined effects on insertion points of the intein into proteins of interest.