Proteolysis, partial

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. 

Plasminogen can bind to fibrin and fibrinogen. This process is markedly increased by partial proteolysis of fibrin, by which more lysine residues are... 

Structural-Functional Investigations Based on Partial Proteolysis and Physical Measurements... 

Additionally, the cellular location at which the resultant polypeptide will function often cannot be predicted from RNA detection/sequences nor can detailed information regarding how the polypeptide product s functional activity will be regulated (e.g. via post-translational mechanisms such as phosphorylation, partial proteolysis, etc.). Therefore, protein-based drug leads/targets are often more successfully identified by direct examination of the expressed protein complement of the cell, i.e. its proteome. Like the transcriptome (total cellular RNA content) and in contrast to the genome, the proteome is not static with changes in cellular... 

This example illustrates nicely how extracellular signals can induce the ubiquitinyla-tion and degradation of specific proteins. As shown by the processing of the pl05 precmsor, ubiquitinylation can be also used for partial proteolysis and for specific activation of a regulatory protein. 

Partial Proteolysis Results in Irreversible Covalent Modifications... 

Table 9.1 lists some of the other enzymes activated by partial proteolysis. A common pattern is for proteolysis to occur in a loop that connects two different domains of the protein, relieving a constraint that interferes with the formation of the active site. 

The enzymes that participate in blood clotting also are activated by partial proteolysis, which again serves to keep them in check until they are needed. The blood coagulation system involves a cascade of at least seven serine proteases, each of which activates the subsequent enzyme in the series (fig. 9.2). Because each molecule of activated enzyme can, in turn, activate many molecules of the next enzyme, initiation of the process by factors that are exposed in damaged tissue leads explosively to the conversion of prothrombin to thrombin, the final serine protease in the series. Thrombin then cuts another protein, fibrin, into peptides that stick together to form a clot. 

A type of covalent modification used more widely f than partial proteolysis is phosphorylation of the side... 

Partial proteolysis, an irreversible process, is used to activate proteases and other digestive enzymes after their secretion and to switch on enzymes that cause blood coagulation. Common types of reversible covalent modification include phosphorylation, adenylyla-tion, and disulfide reduction. 

Based on the work done in our laboratory, we believe that multiple enzymes of the same type are derived from the same enzyme and potentially arise from partial proteolysis of such an enzyme (10). In previous studies, we have purified three distinct cellobiases from T. reesei which are chromatographically distinct yet kinetically similar. 

By) (24) and partial proteolysis by exposure to trypsin or chymo-trypsin destroys the combined activity to a greater extent than the glucosidase activity (38). These results also suggest that the transferase and the glucosidase activities are located at separate sites on the same enzyme molecule. 

The gentle deamidation of gluten proteins by acetic acid resulted in minor electrophoretic changes in gluten proteins, caused by partial proteolysis, especially when... 

Martin et al. [22] treated Se-enriched yeast with simulated gastric juice containing 1 percent m/m pepsin (pH = 1.8, in 0.9 percent m/m NaCl) in a one-step procedure for 4 h. As expected, pepsin itself could provide partial proteolysis. Of the total Se content 11 percent could be identified as SeMet and Se(I V). 

Partial proteolysis has been used by several researchers to improve functional properties, i.e. foaming, solubility of proteins (7,8,9). The significant problems associated with enzyme hydrolysis of proteins are excessive hydrolysis occurring under batch conditions, the generation of bitter flavors during hydrolysis and the cost of enzymes. Extensive information on factors affecting proteolysis of proteins and the problem of bitterness has been reviewed by Fujimaki et al. (7) in conjunction with studies of the plastein reaction. 

Enzyme hydrolysis is occasionally used to modify the functional properties of proteins and yeast autolyzates are used commercially as food flavorants (66,86). Partial proteolysis of... 

Thrombin, which catalyzes the proteolysis of fibrinogen, circulates as an inactive precursor, prothrombin, which in turn is activated by partial proteolysis to remove a peptide sequence that masks the catalytic site. There are two distinct pathways leading to the activation of prothrombin to thrombin (see Figure 5.3) ... 

In addition to the problem of partial proteolysis, the solubility of larger unprotected peptide segments may be crucial in the enzyme-assisted approaches, since solubilization with... 

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