## 1 Introduction  
Tyrosine degradation is a crucial five step pathway in animals that breaks down the aromatic amino acids obtained from diet and protein breakdown [1]. In this, the last enzyme used is fumarylacetoacetate hydrolase (fumarylacetoacetate; FAH, E.C. 3.7.1.2), which catalyzes the hydrolysis of fumarylacetoacetic acid into fumarate and acetoacetic acids. FAH is an enzyme encoded by the _FAH_ gene located on chromosome 15q25.1 containing 14 exons, spanning over 35 kb of DNA [2; 3; 4]. It is a cytosolic homodimer with two 46-kDa subunits conformed by a 120-residue N-terminal domain (N-term) and a 300-residue C-terminal domain (C-term). The N-term has an SH3-like fold and plays a regulatory role [5; 6]. In contrast, the C-term is defined by a new \(\beta\)-sandwich roll structure that is implicated in metal ion binding and catalysis, participating in intermolecular interactions at the dimer interface [7]. The loss of FAH activity, which is responsible for phenylalanine and tyrosine degradation, would be lethal to humans. Several compounds that accumulate upstream of FAH are toxic alkylating agents that can induce cell apoptosis [8]. Mutations in the _FAH_ gene cause type I tyrosineinemia (HT1, McKusick; OMIM: 276700), a hereditary metabolic condition characterized by increased tyrosine levels in the blood and urine patients [9]. In the absence of FAH, metabolites such as maleylactoacetate (MAA),fumarylacetoacetate (FAA) and succinylacetone (SUAC) accumulate during tyrosine degradation. In a mammalian cell assay, FAA has been shown to have mutagenic activity to induce cell cycle arrest at G2/M and apoptosis [8]. Tyrosinemia is characterized by hepatic failure, cirrhosis, renal dysfunction, hepatocarcinoma (HCC) and neurologic crises, in which the liver is the most severely affected organ [10]. Two distinct forms of the disease have been described according to the symptoms and age of onset [10]. The acute condition of tyrosinemia is diagnosed in the first months of life and results in the rapid deterioration of hepatic and renal functions leading to early death due to hepatic failure. In the chronic form, symptoms appear later in childhood and often culminate with the development of hepatocellular carcinoma (HCC). Thus, investigating the regulatory properties of FAH protein is necessary to understand the pathophysiological mechanisms in tyrosinenia and to delineate novel therapeutic strategies. More than 100 mutations were reported in the _FAH_ gene causing HT1; among them, most of the mutations lead to the destabilization of FAH protein, resulting in aggregation of the protein [11]. FAH is also linked with ubiquitin at numerous lysine residues as per the information in the PhosphoSitePlus (www.phosphoite.org) database. However, there are no reports to our knowledge about FAH protein ubiquitination and degradation or types of ubiquitin linkages associated with FAH protein and its cellular activities. Considering the reduced activity and deficiency of FAH observed in HT1 [12], research focusing on FAH protein ubiquitination and factors affecting its protein turnover is essential. Post-translational modifications (PTMs), such as ubiquitination and deubiquitin, play a significant role in many cellular processes, including proliferation and differentiation, by modulating the activities of target proteins and their related enzyme activities in cells [13; 14]. Ubiquitination is a post-translational modification that regulates the protein turnover and stabilization of any substrate by marking it for degradation by the 26S proteasome pathway. The ubiquitin proteasomal system (UPS) is regulated mainly by ubiquitin-activating enzyme (E1), ubiquitin conjugases (E2) and ubiquitin ligases (E3) [15; 16]. E3 ligases are a component of UPS that works in conjunction with an E1 ubiquitin-activating enzyme and an E2 ubiquitin-conjugating enzyme to transfer ubiquitin to a specific substrate protein. Ubiquitin is a highly conserved, small protein having 76 amino acids that determine the fate of its substrate proteins.