Ubiquitin reaction buffer contains the optimal formulation for the charging of ubiquitin(U-100) by E1 (E-305), the first reaction in the conjugation cascade. This E1-Ubiquitin thioester can be subsequently used for the initiation of any E1 mediated reaction including E2 ubiquitin thioester formation and/or the ubiquitination conjugation reaction of target substrate proteins with the addition of E2 / E3 enzymes. This buffer can supplement the fraction II HeLa conjugation (K-930) and fraction II reticulocyte conjugation (K-935), ubiquitin-protein conjugation (K-960), and ubiqiutin
thioester/conjugation (K-995) kits.