Aims: Disulfide-rich domains (DRDs) are small proteins whose native structure is stabilized by the presence of covalent disulfide bonds. These domains are versatile and can perform a wide range of functions. Many of these domains readily unfold on disulfide bond reduction, suggesting that in the absence of covalent bonding they might display significant disorder. Results: Here, we analyzed the degree of disorder in 97 domains representative of the different DRDs families and demonstrate that, in terms of sequence, many of them can be classified as intrinsically disordered proteins (IDPs) or contain predicted disordered regions. The analysis of the aggregation propensity of these domains indicates that, similar to IDPs, their sequences are more soluble and have less aggregating regions than those of other globular domains, suggesting that they might have evolved to avoid aggregation after protein synthesis and before they can attain its compact and covalently linked native structure. Innovation and Conclusion: DRDs, which resemble IDPs in the reduced state and become globular when their disulfide bonds are formed, illustrate the link between protein folding and aggregation propensities and how these two properties cannot be easily dissociated, determining the main traits of the folding routes followed by these small proteins to attain their native oxidized states. Antioxid. Redox Signal. 21, 368–383.

NvCI is a novel exogenous proteinaceous inhibitor of metallocarboxypeptidases from the marine snail Nerita versicolor. The complex between human carboxypeptidase A4 and NvCI has been crystallized and determined at 1.7 Å resolution. The NvCI structure defines a distinctive protein fold basically composed of a two-stranded antiparallel β-sheet connected by three loops and the inhibitory C-terminal tail and stabilized by three disulfide bridges. NvCI is a tight-binding inhibitor that interacts with the active site of the enzyme in a substrate-like manner. NvCI displays an extended and novel interface with human carboxypeptidase A4, responsible for inhibitory constants in the picomolar range for some members of the M14A subfamily of carboxypeptidases. This makes NvCI the strongest inhibitor reported so far for this family. The structural homology displayed by the C-terminal tails of different carboxypeptidase inhibitors represents a relevant example of convergent evolution.

Abstract: Metallocarboxypeptidases (MCPs) are zinc-dependent exoproteases that have been for long considered benchmark enzymes, perform a wide range of physiological roles and have been regarded as interesting drug targets. Several crystal structures of MCPs in complex with protein and small molecular weight inhibitors have recently been obtained providing a framework for understanding the binding properties of these ligands. Much of the latest research focused on carboxypeptidase U or thrombin-activable fibrinolysis inhibitor (CPU/TAFI) which has fueled new designs in the field of cardiovascular drugs. Further, new details on the catalytic mechanism of MCPs have emerged from recent crystal structures of covalently modified forms and the pace of investigations on inhibitors has been steadily fastening in the last years. This paper will focus on the latest research carried on metallocarboxypeptidase small molecular weight inhibitors as drug candidates and will give an update of protein inhibitors to emphasize the growing interest for products isolated from natural sources.