Site-specific Modification
While naturally occurring proteins often inspire drug discovery, modifications are necessary for proteins to become actual drug candidates.
Proteins are becoming increasingly more interesting as pharmaceuticals. While proteins found in nature often serve as inspiration in drug discovery they can normally not be used directly for various practical reasons. The need to identify novel reactions and develop protein modification protocols is therefore growing. Moreover, there is a need for such modifications to be site-specific. If the goal is related to a change of the protein’s working mechanism, the modification should be at the active site of the molecule. If, however, the modification seeks to alter the pharmacological properties of the protein without hampering the working mechanism, it is desirable to modify an area of the surface far away from the active site. The project deals with novel synthetic pathways which are able to modify proteins in a site-specific manner.
A series of peptide based ligands was synthesized and their usability was confirmed via photo induced conjugation to the protein human serum albumin.
Simultaneously, a novel hypothesis of neighboring group assisted acylation reagents was tested. While testing a small series of acylation reagents on the peptide hormone GLP-1 (37) it was observed that some specific aldehydes reacted selectively with the N-terminal histidine of the peptide. Screening revealed that the aldehydes required specific properties to perform the reaction. It was shown that complete conversion was only obtained with electron deficient aldehydes bearing a hydrogen bonding substituent in one ortho position and often a halogen in the second.
The reactivity of histidine versus tryptophan was tested. The reaction displayed a large degree of chemo-selectivity towards histidine in a competitive environment a very mild conditions at pH 8. The pattern changes in acidic buffer, where the reactivity of histidine is almost completely absent.
In a final experiment, a 10 kDa PEG Pictet- Spengler reagent was synthesized and used to modify the N-terminal of human antithrombin III, a glycosylated 57 kDa protein from the coagulation cascade.
The Pictet-Spengler reaction with N-terminal histidine adds a very useful reaction to the bio-conjugate toolbox. The method does not require any preceding modification of the native peptide or protein. It involves a canonical amino acid and the optimal conditions are very close to the physiologic N-terminal histidine containing bio-molecules. The method will hopefully be a novel reliable tool for protein modification.