Effect of Enzyme Concentration of the Morphology and Properties of Enzymatically Triggered Peptide Hydrogels

We have recently shown that thermolysine, a protease enzyme obtained from Bacillus thermoproteolyticus rokko, can be used to trigger the gelation of FEFK (F, phenylalanine; E, glutamic acid; K, lysine) tetrapeptides through reverse hydrolysis and formation of longer peptide sequences, mainly octapeptides, that self-assemble readily. In this article we investigate the effect of enzyme concentration on the morphology and properties of enzymatically triggered peptide hydrogels using HPLC, FTIR, real-time SAXS, TEM, and shear rheology. We have shown that the enzyme concentration, C-enz, does not affect the final composition of the samples. Instead, this is dictated by the initial tetrapeptide concentration, C-0, suggesting the existence of a chemical equilibrium. We went on to show that C-enz does not affect the self-assembly of these peptides at a molecular level either nor the structure of the fibrillar network formed at the nanometer scale. Interestingly, the mechanical properties were found to be affected by C-enz, where the shear moduli of the hydrogels were found to increase with increasing C-enz. These results suggest that morphological differences between the hydrogels at the microscale are at the origin of their difference in mechanical properties. In this paper, we propose a morphological model in which denser network regions are found around the enzymes, resulting in the creation of heterogeneous networks. These were confirmed by TEM measurements. The existence of these denser network regions will result in the reinforcement of the hydrogels, thus, explaining the high shear moduli obtained increasing C-enz.