Crystal structure of human APT1, PDB code 1fj2. Alpha helices are in red, beta strands in gold, catalytic site residues in black. The 2 different monomers of the dimer are shaded in green and brown.
Identifiers
Symbol	Acyl-protein thioesterases (APTs)
Pfam	PF02230
InterPro	IPR029058
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Acyl-protein thioesterases are enzymes that cleave off lipid modifications on proteins, located on the sulfur atom of cysteine residues linked via a thioester bond. ^[1] Acyl-protein thioesterases are part of the α/β hydrolase superfamily of proteins and have a conserved catalytic triad. ^[2] For that reason, acyl-protein thioesterases are also able to hydrolyze oxygen-linked ester bonds.

Function

Acyl-protein thioesterases are involved in the depalmitoylation of proteins, meaning they cleave off palmitoyl modifications on proteins' cysteine residues. Cellular targets include trimeric G-alpha proteins, ^[3] ion channels ^[4] and GAP-43. ^[5] Moreover, human acyl-protein thioesterases 1 and 2 have been identified as major components in controlling the palmitoylation cycle of the oncogene Ras. ^{[6] [7]} Depalmitoylation of Ras by acyl-protein thioesterases potentially reduces Ras' affinity to endomembranes, allowing it to be palmitoylated again at the Golgi apparatus and to be directed to the plasma membrane. Acyl-protein thioesterases, therefore, are thought to correct potential mislocalization of Ras.

Known enzymes

Currently fully validated human acyl-protein thioesterases are APT1 ^[8] and APT2 ^[9] which share 66% sequence homology. ^[10] Additionally there are a handful of putative acyl-protein thioesterases reported, including the ABHD17 enzyme family. ^{[11] [12]} In the lysosome, PPT1 of the palmitoyl protein thioesterase family has similar enzymatic activity as acyl-protein thioesterases.

Structure

Acyl-protein thioesterases feature 3 major structural components that determine protein function and substrate processing: 1. A conserved, classical catalytic triad to break ester and thioester bonds; ^[2] 2. A long hydrophobic substrate tunnel to accommodate the palmitoyl moiety, as identified in the crystal structures of human acyl-protein thioesterase 1, ^[2] human acyl-protein thioesterase 2 ^[13] and Zea mays acyl-protein thioesterase 2; ^[14] 3. A lid- loop that covers the catalytic site, is highly flexible and is a main factor determining the enzyme's product release rate. ^[14]

Inhibition

The involvement in controlling the localization of the oncogene Ras has made acyl-protein thioesterases potential cancer drug targets. ^[15] Inhibition of acyl-protein thioesterases is believed to increase mislocalization of Ras at the cell's membranes, eventually leading to a collapse of the Ras cycle. Inhibitors for acyl-protein thioesterases have been specifically targeting the hydrophobic substrate tunnel, ^{[16] [13]} the catalytic site serine ^[17] or both. ^[18]

Research

Current approaches to study the biological activity of Acyl-protein Thioesterases include proteomics, monitoring the trafficking of microinjected fluorescent substrates, ^{[19] [7]} the use of cell-permeable substrate mimetics, ^[20] and cell permeable small molecule fluorescent chemical tools. ^{[21] [22] [23] [24]}

References