If you look at the process above, it demonstrates the process whereby ubiquinin is transferred from E1 to E2 to E3 (all of which are ubiquinin ligases), which eventually transfers it onto a target protein. The ubiquinin is added to lysin residues on the target protein, and the more ubiquinin molecules that a target protein/enzyme has on it, the more likely it is to get degraded. In this case, it is important to see that the E3 ubiquinin ligase is SPECIFIC for each target protein. That is, each target protein has its own E3, whereas E1 and E2 are much more non-specific. Based on this information, which of the following do you think represents the relative concentrations of each in the cell? 

a) E1 > E2 > E3 
b) E2 > E3 > E1
c) E3 > E2 > E1
d) E3 < E1 < E2

And the answer is…

C. In this case, the question mentions that each protein has its OWN E3, and from previous knowledge, we know that there are COUNTLESS numbers of proteins. Since each protein has its own E3, there are bound to be a LARGE number of E3 ubiquinin ligases. On the other hand, E1 and E2 are both nonspecific, so they do not need to be present in as large numbers because they can literally bind to their substrates without being as picky. Looking at the answer choices, C is the only one that has E3 present in the largest quantity :).