Figure 1:Three dimensional structure of Palmitoyl-Protein Thioesterase 1. The blue color represents the α-helices and the purple represents the β-sheets. The pink signifies random coil.
Palmitoyl-Protein Thioesterase 1 (PPT1) is a lysosomal enzyme that plays a role in the degradation of lipid-modified proteins[1]. PPT1 derives its catalytic power from its catalytic triad, α/β hydrolase fold, and hydrophobic groove in order to remove fatty acid acyl groups, typically palmitate from cysteine residues in proteins[1]. PPT1 is able to be modified by cofactor enzymes, which can induce biological changes[1]. Misregulation of PPT1 modifications can cause various diseases, including infantile neuronal ceroid lipofuscinosis[2], kufs disease[2], and late-infantile neuronal ceroid lipofuscinosis[2]. Within these diseases, the production of PPT1 is decreased or eliminated completely, which leads to fatty acid buildup primarily in neuronal cells, leading to slowed developmental progress[2].
Structure
The secondary structure of PPT1 contains several α-helices and few β-sheets (Figure 1). PPT1 includes residues 28-306, after the 27-residue signal peptide has been removed [3]. An insertion is found between β6 and β7, residues 140-223, and that forms a , shown in blue, that is compromised almost entirely of the fatty acid binding site. This second domain region contains six helices, α2-α7[3].
The α/β hydrolase fold is common to many other hydrolases [3]. The α/β hydrolase fold has a central 6 stranded parallel β-sheet consisting of and α-helices [3]. A catalytic triad and an oxyanion hole are also common features to the PPT1 protein family. None of the enzymes within the α/β hydrolase fold family require a cofactor for catalytic activity.
Catalytic Triad
The is composed of Ser115, His289, and Asp233, which is the same as the catalytic triad in chymotrypsin [1]. A water molecule is occupying the and it is hydrogen bonded to Ser115 [4]. The purpose of the oxyanion hole is to stabilize the oxyanion that is formed after the nucleophilic attack of the transition state. Ser115 acts as a nucleophile, while His289 and Asp233 are coordinated to Ser115 to lower its pKa value so it can undergo catalytic activity[4]. The pKa of the nucleophile in the catalytic triad is lowered to allow the nucleophilic attack[4].
Hydrophobic Groove
The is located in the second domain of PPT1, where palmitate mainly binds. The fact that palmitate has to to fit into the binding pocket suggests that this pocket is designed to bind an unsaturated fatty acid, with a possible cis-double bond between C4 and C5[3]. The top portion of the groove is formed by the residues from α2 to α3. Several residues that are present near the active site create the rest of the groove, including [3].
Function
Biological
PPT1 is biologically involved in sensory transduction and the vision process[1]. Within sensory tranduction, PPT1 is involved in the process of converting extracellular signals, such as light, taste, touch, or smell, into electric signals that are then sent throughout the body[1]. With vision, PPT1 is involved in the process of seeing images and then processing them into information that is then interpreted by the brain[1].
Molecular
The main molecular function of PPT1 is to breakdown lipid-modified proteins and act as a hydrolase of thioester bonds[3]. In the main catalytic reaction for PPT1, a cysteine residue is removed from the palmitoylated protein by PPT, resulting in a free cysteine residue and palmitoyl-CoA[3]. A water molecule was suggested to come in and stabilize the transition state, along with protonating the cysteine residue on the palmitoylated protein, allowing the palmitoyl-CoA to break free.
Medical Relevance
PPT1 mutations and a decrease or depletion of PPT1 are the root cause of several diseases. Infantile neuronal ceroid lipofuscinosis (INCF) is characterized by impaired mental and motor development, including difficulty with walking, speaking, and intellectual function, beginning around the first or second year of life[2]. The PPT1 mutation involved in INCF replaces an arginine with a stop signal in the instructions to make the enzyme. This mutation leads to a vast reduction in the production of PPT1, which impairs the removal of fatty acids from proteins. This impaired removal leads to fatty acid accumulations throughout the body, particularly in neuronal cells in the brain[2]. Late-infantile neuronal ceroid lipofuscinosis has the same characteristics as INCF, but the mutation varies slightly. This leads to a slight reduction in the activity of PPT1 instead of completely wiping it out[2].
Mutations can also cause premature stop signals to be added to the instructions to create PPT1, resulting in Kufs disease[2]. This is characterized by seizures, problems with movement, and a decline of intellectual function, usually beginning in early adulthood[2]. Although premature stop signals are added, these mutations allow enough PPT1 to be produced so that the onset is later on in life and the life expectancy is higher.
