Biological Overview of Ub and SUMO
The mechanism for the formation of the isopeptide bond between these two PTMs and their respective target protein is an ATP-dependent biochemically assisted process predominantly occurring via a triple enzyme cascade. There are three types of enzymes involved: an E1-activating enzyme, an E2-conjugat- ing enzyme, and an E3-ligating enzyme. The process begins with the activation of one of these two Ub/Ubl proteins, which occurs by the formation of a thioester bond between the thiol group of a cysteine residue on the active site of the E1 enzyme and the glycine residue at C-terminus of the Ub/Ubl protein. This forms the so-called E1-Ub/Ubl complex. The E2 enzyme is involved in a transesterification reaction with the E1-Ub/Ubl complex and covalently bonds itself to the C-terminal of the Ubl via the formation of a thioester bond, forming the E2-Ub/Ubl complex. The E3 ligase enzyme catalyzes the formation of the isopeptide bond between the e-amine group of the acceptor lysine on the target protein and the glycine residue at the C-terminus of the Ub/Ubl protein. This process results in the formation of a Ub/Ubl-target protein complex. It is also possible that the E2 enzyme can act independently of the E3 enzyme and specifically facilitate protein SUMOylation. This occurs through the E2 enzyme being able to recognize and establish noncovalent stabilizing interactions with the presence of specific types of amino acids surrounding the acceptor lysine of the target protein within a region known as SUMOylation consensus sites. A typical example of a consensus site would be VKXD/E, where у represents a hydrophobic amino acid, X is any amino acid, and D/E are acid residues [5, 6]. It is important to state that the process of SUMOylation in general also occurs on target proteins where no known consensus motif is present [7, 8]. There are a number of different types and classes of E3 ligases that have been identified for both ubiquitination and SUMOylation [9, 10]. E3 ligases govern the specificity of the type of proteins that will be targeted for Ub/Ubl modification. The formation of covalent isopeptide bonds is reversible and hydrolysis of these isopeptide bonds occurs biochemically through deu- biquitinating enzymes and deSUMOylating enzymes. [11, 12]. The functions of these types of enzymes are essential and they are central to the function and regulation of these two PTMs [13, 14].