Chemical modification of the carboxyl groups of protein substrates enhances their thrombin susceptibility

Native or denatured protein substrates which are hardly digested by thrombin can become much more efficiently cleaved by the enzyme after chemical modification of their carboxyl groups. Five antibody κ‐chains were used to demonstrate this effect. The selective cleavage sites were determined by quantitative N‐terminal analysis and N‐terminal sequencing. All five κ‐chains share the same cleavage sites at Arg‐Thr (residues 108–109), Arg‐Glu (residues 142–143, Glu side chain modified with glycine amide), Lys‐Ser (residues 207–208) and Arg‐Gly (residues 211–212). One of the major cleavage sites (Arg‐Thr) is located at the joint of the variable/constant region. The amino acids adjacent to these cleavage sites underline the proposed structural requirements for a potential thrombin substrate [(1985) Eur. J. Biochem. 151, 217–224]. This approach can facilitate the application of thrombin in generating large polypeptide fragments of proteins.