This study presents a method for synthesizing protein-branched polymer bioconjugates using controlled radical branching polymerization (CRBP) with a water-soluble inibramer, sodium 2-bromocrylate. The green-light-induced atom transfer radical polymerization (ATRP) technique allows for the direct grafting of branched polymers from the protein surface in open air. The resulting bioconjugates exhibit a predetermined molecular weight, well-defined architecture, and high branching density. Conformational analysis and enzymatic assays validate the controlled grafting of branched polymers and demonstrate that densely grafted polymers prevent protein inhibitor penetration while retaining up to 90% of their enzymatic activity. This approach offers a promising strategy for designing protein-polymer bioconjugates with tunable sieving behavior, which could be applied in drug delivery and biotechnology.This study presents a method for synthesizing protein-branched polymer bioconjugates using controlled radical branching polymerization (CRBP) with a water-soluble inibramer, sodium 2-bromocrylate. The green-light-induced atom transfer radical polymerization (ATRP) technique allows for the direct grafting of branched polymers from the protein surface in open air. The resulting bioconjugates exhibit a predetermined molecular weight, well-defined architecture, and high branching density. Conformational analysis and enzymatic assays validate the controlled grafting of branched polymers and demonstrate that densely grafted polymers prevent protein inhibitor penetration while retaining up to 90% of their enzymatic activity. This approach offers a promising strategy for designing protein-polymer bioconjugates with tunable sieving behavior, which could be applied in drug delivery and biotechnology.