Hydrogels are polymeric networks, capable of absorbing large amounts of water and biological fluids. They are insoluble due to the presence of chemical or physical cross-links between the constituents. Hydrogels are promising materials for use as injectable biomaterials due to their high water content, tunable viscoelasticity, and biocompatibility. Peptides and proteins are important building blocks in the design of hydrogels, since they are easily degraded by the body and display a high biocompatibility. This review aims to give an overview of hydrogels in which peptides and proteins are structural elements of the polymer network. The review starts with hydrogels derived from naturally occurring structural proteins, followed by all-protein and peptide-based synthetic systems. Next, hybrid hydrogels composed of synthetic polymeric and peptide structural elements will be discussed. The potential of these hydrogels is illustrated with applications that are mainly derived from the field of tissue engineering.

Thiolysine‐mediated chemical ligation has generated fluorescence polarisation assay reagents based on isopeptide‐linked ubiquitin‐like protein conjugates. These have been used to monitor the activity of ubiquitin(‐like) proteases. Thus, it is now possible to generate assay reagents that contain substrate‐derived elements around the isopeptide linkage, with no practical limitation.

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A main challenge in the area of bioconjugation is to devise reactions that are both activatable and fast. Here, we introduce a temporally controlled reaction between cyclooctynes and 1,2-quinones, induced by facile oxidation of 1,2-catechols. This so-called strain-promoted oxidation-controlled cyclooctyne-1,2-quinone cycloaddition (SPOCQ) shows a remarkably high reaction rate when performed with bicyclononyne (BCN), outcompeting the well-known cycloaddition of azides and BCN by 3 orders of magnitude, thereby allowing a new level of orthogonality in protein conjugation.

A new class of potent proteasome inhibitors is described, of which the members contain an amino acid inspired sulfonyl fluoride as the electrophilic trap. In total, 24 peptido sulfonyl fluoride inhibitors have been designed and synthesized, which were inspired by the backbone sequences of the proteasome inhibitors bortezomib, epoxomicin, and Cbz-Leu(3)-aldehyde. Nine of them were very potent proteasome inhibitors, the best of which had an IC(50) of 7 nM. A number of the peptido sulfonyl fluoride inhibitors were found to be highly selective for the β5 proteasome subunit.