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E1 Enzymes

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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 …show more content…

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 …show more content…

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

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The study also found that an increase in γ-H2AX (a marker for DNA double-strand breaks) and a decrease in RAD1 (a marker of HR directed DNA repair) focus-positive cells was associated with a depletion of MEN1 expression as predicted. The study also predicted that NHEJ could function to repair double strand breaks, and would increase with a loss of MEN1 functionality. The study found this prediction to be true. The study also investigated the role of ATM and ATR DNA damage kinases in relation to MEN1, and found that MEN1 is protected from ubiquitin mediated degradation through phosphorylation by ATM and ATR protein kinases. One of the most significant results from this study involved determining the functional relationship between the expression of HR target genes and MEN1, as well as investigating the mechanism of action involved in the cellular process.

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