You would be better off using strrep instead of regexprep since your match expression is not a regular expression. Note that strrep uses regexprep internally.
If you want to use regexprep with an arbitrary match string that may contain characters that have meaning in regular expressions (such as the '\' and {' in your string), you need to escape these special characters with regexptranslate:
string2 = regexprep(string1, regexptranslate('escape', '\mathrm{Prmk}_{1}'), 'K_{i}')
strrep will do that for you (and for the replace expression as well). The above replace is not an issue, but if your replace string can be anything it could also be a poblem (e.g. if it contains '$')
string2 = strrep(string1 , '\mathrm{Prmk}_{1}', 'K_{i}')
