Proteolysis processing

The receptors are continuously subjected to a process of synthesis and destruction, which achieves a steady state. The concentration of receptors in the cell only reflects the situation of the steady state at that moment. As a consequence of hormone action, the number of receptors per cell drastically diminishes in the hours that follow. This observation has led to the postulation that the receptors undergo a process of destruction, or processing , induced by the hormone. Despite all efforts, receptor processing has not been deciphered. The destruction of receptors implies the existence of a proteolysis process. Nevertheless, signs of proteolysis, in the form of small peptides of degradation originating in the receptor, have not been detected in the cell. Therefore, if there is a process of receptor proteolysis, it has to be very fast and complete (Beato 1989 Edwards et al. 2002 Kassis et al. 1983). 

The previous section dealt with the mechanisms behind the bioactivation of 1,2,4-trioxanes and endoperoxides. In this section we will examine briefly the suggested targets of the artemisinins. Since the original proposal by Meshnick and coworkers it is still believed by many researchers in the field that heme liberated from the haemoglobin proteolysis process is the species responsible for the bioactivation of the endoperoxide bridge to potentially toxic free radicals in the food vacuole of the parasite (see above). 

The presence of fluorine atoms does not only protect a molecule from oxidative metabolism but can also protect from proteolysis, by disfavouring the formation of cationic intermediates involved in proteolysis processes. This is particularly important for oral administration of drugs sensitive to acidic media, because of... 

In addition to the destructive proteolysis processes in the proteasome and lysosome, many constructive proteolysis processes occur in cells. In both prokaryotes and eukaryotes, secreted proteins contain a signal peptide at the N-terminus that directs them to the secretary pathway. This signal peptide must be cleaved later by signal peptidases (typically serine proteases)... 

It can be observed that with time the proteasome can be damaged or inactivated by oxidants [62]. Furthermore, the subunit composition of the proteasome changes with age. Alterations for LMP2, LMP7, subunit Z, Ub Thiolesterase and 26 S components have been found [63-66]. Mitochondrial mutations accumulating with aging [67] often decrease ATP production. This also affects the ATP-dependent proteolysis process. 

The shell of all picomaviruses is built up from 60 copies each of four polypeptide chains, called VPl to VP4. These are translated from the viral RNA into a single polypeptide, which is posttranslationally processed by stepwise proteolysis involving viraily encoded enzymes. First, the polypeptide chain is cleaved into three proteins VPO (which is the precursor for VP2 and VP4), VPl and VP3. These proteins begin the assembly process. The last step of the processing cascade occurs during completion of the virion assembly the precursor protein VPO is cleaved into VP2 and VP4 by a mechanism that is probably autocatalytic but may also involve the viral RNA. VPl, VP2, and VP3 have molecular masses of around 30,000 daltons, whereas VP4 is small, being 7000 daltons, and is completely buried inside the virion. 

APP undergoes proteolytic processing by several secretases. First, the bulk of the ectodomain needs to be removed by membrane-bound a- or (3-secretases leading to secreted forms of APP and membrane-bound C-terminal fragments a-CTF or (3-CTF, respectively. Regulated intramembrane proteolysis (RIP) of the (3-CTF by y-secretase occurs only after ectodomain shedding and releases the A(3 pqrtide from the membrane (Fig. 2). 

Bone Resorption The removal of mineralised bone by osteoclasts. Bone resorption, which is part of the bone remodelling process, includes the release of mineral (mostly calcium and phosphate) and subsequent proteolysis of organic matter (mostly collagen). 

The human genome contains more than 90 different DUBs. Besides cleaving ubiquitin from distinct substrates, DUBs are also responsible for the recycling of free ubiquitin from ubiquitin chains and processing of ubiquitin- or ubiquitin like precursor proteins. Certain DUBs are also associated with the proteasome in order to detach ubiquitin chains before proteolysis. 

In mammalian cells, the two most common forms of covalent modification are partial proteolysis and ph osphorylation. Because cells lack the ability to reunite the two portions of a protein produced by hydrolysis of a peptide bond, proteolysis constitutes an irreversible modification. By contrast, phosphorylation is a reversible modification process. The phosphorylation of proteins on seryl, threonyl, or tyrosyl residues, catalyzed by protein kinases, is thermodynamically spontaneous. Equally spontaneous is the hydrolytic removal of these phosphoryl groups by enzymes called protein phosphatases. 

The biogenesis of membranes is thus a complex process about which much remains to be learned. One indication of the complexity involved is to consider the number of posttranslational modifications that membrane proteins may be subjected to prior to attaining their mamre state. These include proteolysis, assembly... 

Biosynthesis of the polypeptide chain is realised by a complicated process called translation. The basic polypeptide chain is subsequently chemically modified by the so-called posttranslational modifications. During this sequence of events the peptide chain can be cleaved by directed proteolysis, some of the amino acids can be covalently modified (hydroxylated, dehydrogenated, amidated, etc.) or different so-called prosthetic groups such as haem (haemoproteins), phosphate residues (phosphoproteins), metal ions (metal-loproteins) or (oligo)saccharide chains (glycoproteins) can be attached to the molecule by covalent bonds. Naturally, one protein molecule can be modified by more means. 

In some instances it may be necessary to demonstrate that all traces of specific contaminants have been removed prior to final product filling. This would be true, for example, of many proteolytic inhibitors added during the initial stages of downstream processing to prevent proteolysis by endogenous proteases. Some such inhibitors may be inherently toxic, and many could (inappropriately) inhibit endogenous proteases of the recipient patient. 

As secretory vesicles mature, many secretory polypeptides undergo post-translational modifications. Many hormones and neuropeptides as well as hydrolytic enzymes are synthesized as inactive polypeptide precursors that need to undergo proteolysis to become active. This maturation process usually starts in the TGN and continues in the secretory vesicles, but may be completed in the extracellular space soon after exocytosis takes place in some cases. The maturation process for neuropeptides is described in Chapter 18. 

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