may either proceed sequentially, starting from one end of the protein, or may involve
a ‘hairpin’ conformation of the protein entering the proteasome allowing limited prote-
olysis of an internal segment. Proteolysis is ATP dependent and involves an additional
19S regulatory complex unit that consists of approximately 20 subunits, six of which
have ATPase sites. These ATPases have similar structures but distinct functions that
include the capture of the protein to be degraded, unfolding its structure and injection
into the proteasome. This 19S complex unit combines with both ends of the 20S
proteasome cylinder to form a 26S proteasome that promotes the cleavage of the
peptide bonds with the concomitant hydrolysis of ATP. The recruitment and degrad-
ation of a protein relies on the presence of two subunits, Rpn1 and Rpn2, in the 19S
unit. Two other proteins, Hul5 and Ubp6, when bound to the proteasome also regulate
the degradation process (Fig. 15.16).
The balance between enzymede novosynthesis and proteolytic degradation coupled
with the regulation of enzyme activity enables the amount and activity of enzymes
present in a cell to be regulated to meet fluctuating cell and whole organism needs. There
is growing evidence to indicate that ubiquitination/deubiquitination is as important
as phosphorylation/dephosphorylation for cellular homeostasis and cell cycle control.
Dysfunction of the ubiquitin–proteasome pathway has been implicated in a number of
disease states. For example, there is evidence that the accumulation of abnormal or
damaged proteins due to impairment of the pathway contributes to a number of neuro-
degenerative diseases including Alzheimer’s. In contrast, deliberately blocking the path-
way in cancer cells could lead to a disruption of protein regulation that in turn could
cause the apoptosis of the malignant cells. Accordingly, proteasome inhibitors have been
developed for evaluation as anti-tumour agents against selected cancers. Bortezomib is
one such inhibitor that targets the 26S proteasome, and in combination with other
chemotherapeutic agents was shown to have therapeutic potential.
15.6 Suggestions for further reading
General texts
Frey, P. and Hegeman, A. (2007).Enzymatic Reaction Mechanisms. Oxford: Oxford University
Press. (Discusses over 100 case studies of enzyme mechanisms.)
Review articles
Barglow, K. T. and Cravatt, B. F. (2007). Activity-based protein profiling for the functional
annotation of enzymes.Nature Methods, 4 , 822–827.
English, B. P., Min, W., van Oijen, A. M., Lee, K. T., Luo, G., Sun, H., Cherayil, B. J., Kou, S. C. and
Xie, S. (2006). Ever-fluctuating single enzyme molecules: Michaelis–Menten equation revisited.
Nature Reviews Chemical Biology, 2 , 87–94.
Furnham, N., Garavelli, J. S., Apweiler, R. and Thornton, J. M. (2009). Missing in action: enzyme
functional annotations in biological databases.Nature Chemical Biology, 5 , 521–525.
Komander, D., Clague, M. J. and Urbe, S. (2009). Breaking the chains: structure and function of the
deubiquitinases.Nature Reviews Molecular Cell Biology, 10 , 550–563.
Kapure, S. and Khosia, C. (2008). Fit for an enzyme.Nature, 454 , 832–833.
Ravid, T. and Hochstrasser, M. (2008). Diversity of signals in the ubiquitin–proteasome system.
Nature Reviews Molecular Cell Biology, 9 , 679–689.
Ye, Y. and Rape, M. (2009). Building ubiquitin chains: Ezenzymes at work.Nature Reviews
Molecular Call Biology, 10 , 755–764.
Zalatan, J. G. and Herschlag, D. (2009). The far reaches of enzymology.Nature Chemical Biology,
5 , 516–520.
624 Enzymes
