E1 Enzymes

E1 enzymes, also known as Ubiquitin Activating Enzymes, serve as an essential enzyme within all cells. One of the functions of E1 enzymes is that it catalyzes the first step of the Ubiquitin Reaction, which leads to targeting a protein for degradation via a proteasome (1). The proteasome pathway of protein degradation (UPP) is a two step pathway which involves initially tagging a particular protein with ubiquitin by covalently attached multiple ubiquitin molecules (conjugation), and then the tagged protein becomes degraded by the proteasome (1). Ubiquitin Reaction plays a role within the cell as a recycling and defensive mechanism. Roles such as regulating DNA repair and immune responses, as well as controlling protein trafficking are a few

If E1 fails to work properly it can disrupt homeostasis and lead to disorders. E1 enzyme disruption is associated with cancer, diabetes, stroke, Alzheimer’s disease, asthma, stroke, etc (4).

In the first step of the UPP pathway, protein is being activated by attaching to an enzyme. This enzyme is known as E1. The E1 enzyme contains two Rossman-type folds, a catalytic cysteine domain, and an ubiquitin-folding domain (4). Rossman-type folds are involved in ATP binding (4), the catalytic cysteine serves as a nucleophile (4,6), and the ubiquitin-folding domain is involved in the recruitment of the E2 enzyme (4). The activation process is condensed into three major reactions: adenylation, thioesterfication, and transthioesterfication (4).

E1 enzymes begin ubiquination adenylation at the carboxyl terminus of ubiquitin. To increase the affinity for ubiquitin, ATP binds to the Rossman-type fold of the E1 subunit, resulting in a conformational change (4). The carboxyl ends of ubiquitin then attacks the alpha prostate of ATP (4). This reaction forms an ubiquitin-AMP molecule and stimulates the release of a pyrophosphate molecule (4). The E1 molecule has hydrophobic portions that are involved in Ubiquitin recognition and hydrophilic portions at the Rossman-type folds that allow the carboxyl end of the ubiquitin to interact with ATP and allow free release of pyrophosphate

This cysteine residue remains in an open conformation during adenylation, which decreases the affinity for the Rossman-type fold, and the cysteine domain is in a closed confirmation during thioesterfication, which has an increased affinity for the rossman-type fold (4). Because of the increased affinity between the Rossman-type fold with the adenyalted ubiquitin and the cysteine domain results in a thioesterfication reaction between both the adenylated ubiquitin molecule and the catalytic cysteine