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The is a human poly (ADP-ribose) glycohydrolase.It is the major enzyme responsible for the catabolism of poly (ADP-ribose), a reversible covalent-modifier of chromosomal proteins. The protein is found in many tissues and may be subject to proteolysis generating smaller, active products.

This protein is only present when the DNA is damaged. It influences the damaged chromatin through a derepression of a gene promoter. Consequently this protein is quite interesting for biotechnological applications. Indeed, knowing the different pathways and protein interactions leading to DNA damage repair is a meaningful goal in research especially in new cancer therapies.

Catabolism of poly (ADP-ribose)

This protein has four principal domains on a : an A-domain, a catalytic domain and two substrate binding domains[1]. The first 456 amino acids of the peptide chain form the . Then, from the 610th to the 795th amino acids the can be found. This catalytic domain can bind to other proteins with (the 726th and 727th amino acids). Next, is located from the 869th to the 874th amino acid. [2] As such, most of the amino acids form the A-domain and the catalytic domain and only a few amino acids (8 a.a) make links with other proteins. Moreover, the ligand 7JB can bind the protein on the . These amino acids are located on a helix and on a beta sheet. [3]. There is the amino acid distribution.

The protein folds into an ADP-ribose-binding macro domain with an N-terminal extension. It also consists of a diphosphate-binding loop on one side of an ADP-ribose binding cavity. On the other side there are several amino acids matching to the specific signature sequence. In the macro domain fold, a loop is inserted to welcome the Glu115 side chain protecting the active site of the protein. This loop gives the ability to hydrolyze PAR. The . (PAR is represented here in pink).

Post-translational modifications

The protein is a complex composed of the poly (ADP-ribose) glycohydrolase (PARG) and the anthraquinone PDD00013907. The Poly(ADP-ribose)glycohydrolase can interact with either PCNA or NUDT5, this gives various possible functions to the protein. When this protein binds with NUDT5 it can remodel chromatin for example [4]. The post-translational modifications of the PAR protein (poly ADP-ribose) are important for DNA stability. PDD00013907 is, as already stated, an anthraquinone which is a polycyclic aromatic hydrocarbon usually used in biopesticides as a pest repellant. Here it is considered as a free (of identification number on PDB: 7JB) that can bind to the creating the.

There are several possible post-translational modifications to stabilize DNA. Most commonly there would be phosphorylation, acetylation or methylation ^[1].

Another post-translational modification concerning the 6HMM protein is made on the poly(ADP-ribose) protein (PAR). PAR is composed of a repetition of ADP-ribose units linked through glycosidic ribose-ribose bonds ^[2]. This allows the repair of single-strand breaks on DNA ^[3].

, a constituent of the will degrade PAR to allow the to free itself from the damaged site, that is now repaired, and completes as such reparation ^[4].

The anthraquinone PDD00013907 is a weakly active and cytotoxic anthraquinone 8a acting as a free ligand binding in the ADP-ribose binding site of the . This PDD00013907 should lead to the inhibition of , which is of interest in the search of novel cancer therapies ^[4].

The mechanism

The Poly (ADP-ribose) polymerase (PARP), more specifically the subtype PARP1, will recognize and will bind to the single-stranded break on the DNA. It will then autophosphorylate and form PAR chains. These will recruit other repair proteins to the site.

The role of PARG is the hydrolyzation of the specific ribose-ribose bonds present in PAR which leads to its degradation and as such the reparation cycle will be finished^[3]

Diseases and Treatment

Due to the function of the protein poly (ADP-ribose) glycohydrolase to be part of post-translational processes of DNA damage repair it could be used for new treatments in cancer therapy or for ther diseases. In cancer cells the rate of DNA damage is most probably higher than in normal cells. This could result from the considerably raised stress levels. A deficiency of results in the cessing of the cell cycle and the following cell death^[3][4]. Consequently, the inhibition of might be a solution on how to destroy tumor cells, for example.

The PAR protein is a potential target in drug discovery as there are no close homologues of . That’s why there has already been a lot of research concerning novel treatments.

The goal of searched therapeutics is to find a way to get the protein into a complex that is acting similar to the .Indeed the protein in complex with the anthraquinone does not work properly anymore [5], the described complex shows how an inhibited might act in the cell. For now, the research for anthraquinone as inhibitor has stopped as it is cytotoxic for the cell^[4].