Polymer–protein conjugates: an enzymatic activity perspective

Proteins have been modified with polymers in diverse manners over the past 30 years. However, while proteins have been used to prepare many functional constructs, they are sensitive biomolecules and their bioactivity can be either positively or negatively influenced by many different aspects of polymer modification. The primary focus of this review article is to highlight the opportunities offered by new trends in protein modification, and specifically how they influence the overall biological activity of the conjugate, including its dependence on temperature and pH. We survey the effect of polymer molecular weight, number of conjugated polymer chains, polymer coupling strategy (including random versus site-specific coupling, "grafting from", and multi-point covalent attachment), polymer architecture (including branched and comb-type), polymer interactions with the protein (including electrostatic and host-guest interactions), polymer interactions with enzyme substrate, and polymer biodegradability. We have selected six enzymes, which have been extensively modified with polymers in diverse fashions in the literature, as basis for this discussion. These proteins are L-asparaginase, alpha-chymotrypsin, trypsin, lysozyme, bovine serum albumin, and papain. This review includes polymers such as poly(ethylene glycol) (PEG), polysaccharides, polypeptides, and other synthetic (vinyl) polymers. From the discussed literature we attempt to extract tentative general trends observed between state-of-the-art methods of preparing protein-polymer conjugates and the activity of the conjugate.