Ubiquitin conjugation cascade

From a chemical point of view, the El/ubiquitin thiol ester should be competent to donate ubiquitin to a substrate amino group. In fact, aminoacyl-errzyme thiol esters are used in exactly this way in non-ribosomal polypeptide synthesis, a process that was discovered around the same time as ubiquitin-protein conjugation [5]. In spite of the attractive simplicity of this model, however, biochemical reconstitution studies showed that besides El two additional fractions were required to conjugate ubiquitin to a model substrate. They were called ubiquitin carrier protein (E2) and ubiquitin-protein ligase (E3), respectively, since the respective factors seemed to act sequentially [6]. Interestingly, the E2 factor apparently formed a thiol ester with ubiquitin. Based on these results, Hershko and co-workers proposed the ubiquitin conjugation cascade (Figure 5.1). 

An E2 needs to associate with several different proteins in the course of the ubiquitin conjugation cascade, with the first being the El. Mutational studies con-... 

Ghemical catalytic mechanisms in the ubiquitin conjugation cascade have proved difficult to decipher. The reactions leading to E2/ubiquitin thiol ester and isopep-... 

The first step in the ubiquitin conjugation cascade is the activation by the El enzyme. El enzymes and specific y their active site cysteine residue are essential for cell viability (Finley et al. 1984 Ciechanover et al. 1984). 

There are now several striking examples of disease-related defects in ubiquitin conjugation, but most of them involve E3s rather than E2s. This is not surprising given the paramount role of E3s in substrate selection and the corresponding intensity of research effort that has been focused on E3s. Still, there are several hints that defects at the E2 level of the conjugation cascade can also contribute to disease. 

The reader should consult earlier reviews [70, 94, 95] and other chapters in this volume for a detailed discussion of UbL biology and biochemistry. There are two important points for the current discussion. First, the conjugation cascades of UbLs differ from that of ubiquitin chiefly in terms of complexity - there is one conjugating enzyme per UbL, and many fewer E3s. Second, because modifier proteins (including ubiquitin) do not interact strongly with their dedicated E2s (Section 5.6.1), it is believed that El enzymes play the major role in matching E2s with the correct modifier protein (see Ref [96]). 

Increasing evidence indicates that accumulation of aberrant or misfolded proteins, protofibril formation, ubiquitin-proteasome system dysfunction, and the direct or indirect consequences of abnormal protein aggregation and accumulation represent deleterious events linked to neurodegeneration (255,256). Ubiquitination is an essential cellular process affected by a multienzyme cascade involving Els (ubiquitin-activating enzymes), E2s (ubiquitin-conjugation enzymes or UBCs), and E3s (ubiquitin-protein Ugases) (12,257) (see Fig. 10.4). 

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