Engineered protein kinases which can utilize modified nucleotide triphosphate substrates that are not as readily utilized by the wild-type forms of those enzymes, and methods of making and using them. Modified nucleotide triphosphate substrates and methods of making and using them. Methods for using such engineered kinases and such modified substrates to identify which protein substrates the kinases act upon, to measure the extent of such action, and to determine if test compounds can modulate such action. Also Engineered forms of multi-substrate enzymes which covalently attach part or all of at least one (donor) substrate to at least one other (recipient) substrate, which engineered forms will accept modified substrates that are not as readily utilized by the wild-type forms of those enzymes. Methods for making and using such engineered enzymes. Modified substrates and methods of making and using them. Methods for using such engineered enzymes and such modified substrates to identify the recipient substrates the enzymes act upon, to measure the extent of such action, and to measure whether test compounds modulate such action.