The adoption of a circular economy model for biopolymers necessitates new technologies for valorizing keratin-rich wastes, particularly from wool. This study presents an approach to convert hard keratin proteins into flexible, moldable, and freestanding bioplastics. This method relies on reprogramming the keratin's structure by grafting building blocks on fully unfolded keratin via the thiol-based Michael-type addition between double bonds of the building blocks and thiols of keratin. The engineered protein showed new functionalities, resulting in: thermoplastic keratin after grafting with poly(ethylene glycol) methyl ether methacrylate; and increased toughness with poly(ethylene glycol) dimethacrylate, a fully bio-based, flexible, and tough material (that outperformed every other regenerated keratin material) from epoxidized soybean oil acrylate. This efficient reaction occurs at room temperature in the same aqueous solution used for the extraction, without the need for additional steps. This approach emphasizes the potential of proteins as sustainable plastic alternatives.