Protein Synthesis and Protein Degradation

Question: Discuss about the Protein Synthesis and Protein Degradation. Answer: Introduction: Protein turnover refers to the process by which protein synthesis and protein degradation is balanced. Selective process of protein turnover mediated by ubiquitin therefore refers to protein regulatory process that control degradation of some proteins in cell process. Ubiquitin is 76 amino acid regulatory protein that helps in controlling degradation of selected proteins during protein turnover. According to Tymoczko, Stryer Berg (2002) ubiquitination refers to the addition of ubiquitin to substrate that can signal degradation of the substrate. The following paper describe selective protein turnover of ubiquitin-mediated degradation of keratin in the intermediate filament protein. Degradation of keratin in the intermediate filament occurs through a series of steps that include activation and conjugation, Transesterification, ligation by ubiquitin ligase and Transfer to target protein via E3 to lysine residue intermediate filament protein. Firstly, ubiquitin is activated by ubiquitin activation enzyme E1 that is followed by conjugation of ubiquitin. Secondly, transesterification that involve transfer of ubiquitin to ubiquitin-enzyme E2. Thirdly, the E2 complex is then delivered to ubiquitin-ligase E3 substrate site in the intermediate filament proteins. Fourthly, the keratin protein is delivered via E3 to lysine residue in the intermediate filament protein (Harper Schulman, 2009). In the last step, ubiquitin binds to the lysine residue on the keratin through isopeptide bond. Ubiquitin therefore bind to target protein at different sites and to amino group at N-terminus. The overall result is cleavage of keratins bonds at different amino group and high energy is required in this process. The degradation of ubiquitin- substrate is done by 26S proteasome complex that at the end release reusable ubiquitin and free ubiquitin (Philpott Itzhaki, 2012). Reference Harper, W. Schulman A.(2009) Ubiquitin protein activation. Nature review molecular cell biology 12(5): p. 23-31 Philpott, A Itzhaki, L. (2012) Regulation and control of Proteins Degradation. Biology Open 1(13): p. 136-72 Tymoczko, J. Stryer L Berg, M. (2002) Biochemistry: Protein Turnover. New York : W H Freeman and company.