Protein degradation post-translational

C. Most membrane proteins undergo post-translational glycosylation to improve their interactions with the aqueous environment and to protect them from degradation by proteases. 

I. Gruic-Sovulj, N. Uter, T. Bullock, and J. J. Perona, Protein synthesis, post-translation modification, and degradation. J. Biol. Chem., 280 (2005), 23978—23986. 

There are at least two answers to question (i). First, abnormal proteins can arise in cells due to spontaneous denaturation, errors in protein synthesis, errors in post-translational processing, failure of the correct folding of the protein or damage by free radicals. They are then degraded and replaced by newly synthesised proteins. Secondly, turnover helps to maintain concentrations of free amino acids both within cells and in the blood. This is important to satisfy the requirements for synthesis of essential proteins and peptides (e.g. hormones) and some small nitrogen-containing compounds that play key roles in metabolism (see Table 8.4). 

Protein processing in the endoplasmic reticulum makes mistakes. All membrane-associated proteins and proteins that are secreted by the cell are synthesised on membrane-bound ribosomes and pass into the lumen of the reticulum, where they are modified by post-translational processes, so that much biochemical manipulation of the proteins takes place. Consequentially mistakes are often made. Such abnormal proteins are exported from the lumen into the cytosol for ubiquitination and degradation in the proteasome. 

Hydrophilic hormones and other water-soluble signaling substances have a variety of biosynthetic pathways. Amino acid derivatives arise in special metabolic pathways (see p. 352) or through post-translational modification (see p. 374). Proteohormones, like all proteins, result from translation in the ribosome (see p. 250). Small peptide hormones and neuropeptides, most of which only consist of 3-30 amino acids, are released from precursor proteins by proteolytic degradation. 

In contrast to ubiquitin, the covalent attachment of ubiquitin-like molecules (Ubls) to other proteins seems to be more important for post-translational protein modification than for protein degradation. There is not much known yet about the subcellular distribution of these proteins but since they are frequendy involved in targeting functions it can be assumed that they are also subjected to a spatial organization. 

Activator proteins themselves can be boimd in regulatory networks, as shown in the example of the cyclins (chapter 14). The function of an activator protein can be regulated at the level of gene expression, degradation, or post-translational modification (e.g. protein phosphorylation). 

The levels of particular enzymes (and indeed of specific proteins in general) is determined by the balance of protein degradation versus the specific expression of the protein (through the process of specific gene transcription, translation and post-translational processing of the protein). Genes can either be constitutively expressed (in which case they are normally always being transcribed) or are inducible, that is, specific transcription factors are activated... 

This review focuses upon the post-translational modification and chemical changes that occur in elastin. Outlined are the steps currently recognized as important in the assembly of pro-fibrillar elastin subunits into mature fibers. Descriptions of some of the proposed mechanisms that appear important to the process are also presented. It will be emphasized that from the standpoint of protein deterioration, elastin is a very novel protein. Under normal circumstances, the final product of elastin metabolism, the elastin fiber does not undergo degradation that is easily measured. Unlike the metabolism of many other proteins, deterioration or degradation is most evident biochemically in the initial stages of synthesis rather than as a consequence of maturation. Since the presence of crosslinks is an essential component of mature elastin, a section of this review also addresses important features of crosslink formation. 

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