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Tripeptidyl-peptidase 2 (or TPP2) is a cytosolic protease found in most eukaryotic cells. TPP2 is characterized by a high molecular mass of ~4.5-5 MDa in human cells or ~6 MDa in Drosophila melanogaster (fruit fly) cells. The exact functions of TPP2 have not yet been identified, but increased activity during muscle wasting events suggest that it is a key factor in protein degradation and turnover. TPP2 has been reported to cleave tripeptides (usually longer than 15 amino acids ) located on the N-terminus of oligopeptides, permitting the tripeptides to then be broken down further. TPP2 has both exoproteolytic and endoproteolytic activity, however its endoproteolytic activity is poorly understood.

Enzyme Commission Number
Tripeptidyl-peptidase 2 is identified by its Enzyme Commission Number of 3.4.14.10. The numbers are interpreted to signal that TPP2 catalyzes a hydrolysis reaction on peptide bonds, and that the enzyme is either a dipeptidyl-peptidase or tripeptidyl-peptidase.

Reaction Pathway
As a peptidase, TPP2 catalyzes the hydrolysis reaction of a protein (usually a polypeptide) which results in the protein's breakdown into smaller polypeptides and/or amino acids. In the case of TPP2, the protein being hydrolyzed is usually a tripeptide, which is then broken into dipeptides and/or amino acids.

Structure
TPP2's crystal structure can be divided into 3 distinct parts: the sequence of ~200 amino acids between Asp and His, the central domain containing the β-sheets, and a c-terminus.

The opening in the structure of the TPP2 enzyme allow for the selection of certain proteins to be cleaved, permitting long peptides to pass through while proteins with folds are kept from approaching the active site of the enzyme. Furthermore, the substrate-binding cleft contains two negatively charged glutamic amino acids, which limits the types of molecules that can interact. This negative charge also orients substrates in order to ensure that peptides are removed only from the N-terminus.

In the Cell
TPP2 is thought to be involved in the formation of MHC class 1 peptides, due to its main function as a protease. Through this, TPP2 was thought to be instrumental in the development of CD8 T cells, however this was disproven through a study performed on mice. The TPP2 gene, which codes for the enzyme TPP2 was removed in the experimental mouse population, little to no effect on the formation and function of CD8 T cells was observed. However, when the amount of the TPP2 enzyme was decreased, embryonic lethality was detected along with the degradation of the immune system through a gradual loss of thymic output.

Active Sites
In TPP2 found in humans and rodents, Glu-305 and Glu-331 interact with the free N-terminus of the substrate. This interaction forms a 'molecular ruler', which guides the cleaving of tripeptides performed by the TPP2 enzyme.

In Drosophila melanogaster, Glu-312 and Glu-343 are essential for binding to the substrate, while Glu-312 and Glu-342 perform a similar function in humans.

Inhibitors
As of 2019, no natural inhibitors of TPP2 have been identified, but several synthetic ones have seen use in labs :


 * Diisopropyl flurophosphate (DFP)
 * Phenylmethylsulphonyl fluoride (PMSF)
 * N-ethylmaleimide
 * Butabindide
 * Ala-ala-phe-chloromethyl ketone (AAF-CMK)