Fragments, proteolytic

Figure 14.15 Stmcture of the SI fragment of chicken myosin as a Richardson diagram (a) and a space-filling model (b). The two light chains are shown in magenta and yellow. The heavy chain is colored according to three proteolytic fragments produced by trypsin a 25-kDa N-terminal domain (green) a central 50-kDa fragment (red) divided by a cleft into a 50K upper and a 50K lower domain and a 20-kDa C-terminal domain (blue) that links the myosin head to the coiled-coil tail. The 50-kDa and 20-kDa domains both bind actin, while the 25-kDa domain binds ATP. [(b) Courtesy of 1. Rayment.]...

FIGURE 5.20 Trypsin is a proteolytic enzyme, or protease, that specifically cleaves only those peptide bonds in which arginine or lysine contributes the carbonyl function. The products of the reaction are a mixture of peptide fragments with C-terminal Arg or Lys residues and a single peptide derived from the polypeptide s C-terminal end. 

Krieger, N., Njus, D., and Hastings, J. W. (1974). An active proteolytic fragment of Gonyaulax polyedra. Biochemistry 13 2871-2877. 

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). 

The proteolytic digestion of j6-lactoglobulin was carried out with trypsin which, as indicated in Table 5.4 above, is expected to cleave the polypeptide backbone at the carboxy-terminus side of lysine (K) and arginine (R). On this basis, and from the known sequence of the protein, nineteen peptide fragments would be expected, as shown in Table 5.7. Only 13 components were detected after HPLC separation and, of these, ten were chosen for further study, as shown in Table 5.8. 

There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, fi"om the ends of polypeptides. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from rhe free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. 

IgG consists of four polypeptide subunits held together by disulphide bonds. Native immunoglobulins are rather resistant to proteolytic digestion but certain enzymes have been usefiil in elucidating their structure. Papain cleaves the molecule into three fragments of similar size ... 

Fig. 3. (A) Disposition of afi unit in the membrane, based on sequence information [14,15], selective proteolytic digestion of the a subunit [5,6] and hydrophobic labelling (Table 1). The model for the (S subunit is based on sequencing of surface peptides and identification of S-S bridges [64,65]. T, T2 and C3 show location of proteolytic splits. CHO are glycosylated asparagines in the P subunit. (B) Peptide fragments remaining in the membrane after extensive tryptic digestion of membrane-bound Na,K-ATPase from outer medulla of pig kidney as described by Karlish et al. [7,58].

The fragmentation of Ca -ATPase by proteolytic enzymes [42,85,235,236] and by vanadate-catalyzed photocleavage [104,105] occurs at well defined and conforma-tionally sensitive cleavage sites that delineate functional domains within the Ca -ATPase. The functional changes that follow the cleavage of the polypeptide chain provide useful hints about the role of various domains in the mechanism of Ca transport. 

Subsequently, proteolytic fragments of the rabbit renal 25-kDa amiloride-binding protein were micro-sequenced and found to have high sequence homology with rat and human NAD(P)H quinone oxidoreductase. Indeed, enzymatic assays revealed that renal brush border membrane vesicles contain significant NADPH quinone oxidoreductase activity. Presumably NAD(P)H quinone oxidoreductase coincidentally binds amiloride analogs with the same rank order as the Na /H exchanger [39]. 

Kirschbaum, N. E. and Budzynski, A. Z., A unique proteolytic fragment of human fibrinogen containing the Aa COOH-terminal domain of the native molecule, J. Biol. Chem., 265, 13669, 1990. 

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