SUMMARY Hemoglobin digestion is an essential process for blood-feeding parasites. Using chemical tools, we deconvoluted the intracellular hemoglobinolytic cascade in the tick Ixodes ricinus, a vector of Lyme disease and tick-borne encephalitis. In tick gut tissue, a network of peptidases was demonstrated through imaging with specific activity-based probes and activity profiling with peptidic substrates/inhibitors. This peptidase network is induced upon blood feeding and degrades hemoglobin at acidic pH. Selective inhibitors were applied to dissect the roles of the individual peptidases and determine the peptidase-specific cleavage map of the hemoglobin molecule. The degradation pathway is initiated by endopeptidases of aspartic and cysteine class (cathepsin D supported by cathepsin L and legumain) and continued by cysteine amino- and carboxy-dipeptidases (cathepsins C and B). The identified enzymes are potential targets to developing novel anti-tick vaccines.

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Activity-based proteomics employing active-site-directed chemical probes for enzymatic activity profiling in complex proteomes has greatly accelerated the functional annotation of proteins. [1,2] In protease research, new generations of activitybased probes have led to tremendous progress in our understanding of the biochemistry and physiology of cysteine peptidases over the last decade. [3,4] These probes were designed for in vitro applications as well as for in vivo monitoring in living cells and whole animals, and has directed research towards biomarker discovery and drug screening. However, the current availability of analogous chemical tools selective for other classes of peptidases is limited. The situation is especially critical in the case of aspartic peptidases, despite the fact that these enzymes play an important role in a number of devastating human pathologies, such as AIDS, Alzheimer's disease, and cancer. A major reason for the limited number of available probes for aspartic peptidases is the lack of suitable covalent inhibitors that can be used as starting templates for probe derivation. There are only a few examples of recent advances in this field, including probes for human g-secretase and malaria plasmepsins that were constructed based on the concept of photoaffinity labeling. [5,6] In this work, we focus on cathepsin D, an aspartic peptidase that is critically associated with cancer development and progression through its complex proteolytic and proliferative action and is a prognostic marker in breast cancer (for a recent review, see ref. [7]). There is still insufficient knowledge regarding the interaction partners and activity regulation mechanisms of this therapeutic target, and the introduction of novel imaging tools is required for these studies.Here, we present a convenient synthesis of novel proteomicactivity-based probes for cathepsin D that are suitable for applications in activity profiling and analysis of post-translational activation processing.The design of three probes, sBAP-09, dBAP-09, and FAP-09, is shown in Figure 1. Their structures consist of three functionalities: 1) a peptidic binding core, 2) a photoreactive group as a probe "warhead", and 3) a fluorescent group or biotin as a reporter tag. The binding core was constructed by using pepstatin, a reversible peptidomimetic inhibitor of aspartic peptidases, as a template. The pepstatin molecule was structurally minimized and optimized for cathepsin D inhibition [8] to yield the binding core sequence shown in Figure 1. The statin residue in the peptidic core interacts with the enzyme's catalytic residues and acts as a transition-state mimic. The binding core scaffold was modified at its N and/or C terminus to incorporate functionalities 2) and 3). The crystal structure of human cathepsin D in complex with pepstatin (PDB ID: 1LYB) suggests that bulky substituents in the terminal positions will protrude from the enzyme active site and should not interfere with the interaction of ligand and enzyme. In the sBAP-09 and dBAP-09 p...