Pepsin denaturation

It is suggested that the process being investigated was the intramolecular, self-catalyzed activation of pepsinogen, since (A) the pepsinogen samples used in this study were all prepared in such a way that they could not contain active pepsin (denatured at pH 7 and above (B) activation was carried out at pH 2.5 (and 4.1, and the same result was obtained for the intermolecular catalyzed reaction) and (C) no active protein could ever be detected, except when the molar ratio of pepstatin/pepsinogen was less than 1 1. 

Table 2. Thermodynamic characteristics of pepsin denaturation at pH 2 in a presence of 10 mM A13+ ions, obtained by UV spectroscopy.

Solids were characterized by XRD, N2.BET surface area, and FT-IR. The antacid capacity of the synthesized zeolites was evaluated using the methodology reported by Rivera et al. [7] and Linares et al. [6]. The pepsin enzymatic activity was determined by the reaction between a specific mass of the solid and a denatured haemoglobin solution [8]. 

Piper and Fenton [10] indicated that extreme acidity or basicity of the gastric juice denaturalize the enzymatic activity of the pepsin, which shows has a higher activity at a pH = 2. At pH = 5 the enzyme starts to deactivate and at pH= 7, the enzyme irreversibly lose its activity. Fig. 3 shows the pepsin UV-visible spectra before and after interaction with the zeolites while Fig 4 shows the enzymatic activity of the denatured hemoglobin proteolysis versus reaction time. 

In Fig. 3, the pepsin dissolved in HC1, without interaction with any solid, showed a maximum at 272 nm. After interaction with the disordered cancrinite and the intermediate phase, a small decrease in the absorbance maximum of the pepsin spectrum was observed. This small decrease is due to the pepsin adsorption on the solid surfaces. The pepsin activity was also determined by the proteolysis reaction of a denatured haemoglobin solution at different times. Fig. 4 shows the obtained results. One can see, that the enzymatic activities (determined as absorbance), presented by the tested solids were very similar among them. These results show that pepsin enzymatic activity is not lost after the contact the pepsin with the tested solids. Therefore, the absorbance decrease observed in Fig. 4, is produced by the pepsin adsorption on the tectosilicate surface, and not by chemical reactions between pepsin and the tectosilicates... 

Kauzmann, W. and R.B. Simpson. 1953. Kinetics of protein denaturation. in. Optical rotations of serum albumin, fS-lactoglobulin, and pepsin in urea solutions. J Am Chem 75 5154-5157. 

The hydrochloric acid in gastric juice is important for digestion. It activates pepsinogen to form pepsin (see below) and creates an optimal pH level for it to take effect. It also denatures food proteins so that they are more easily attacked by proteinases, and it kills micro-organisms. 

Porcine pepsin is very sensitive to denaturation at pH 6.7 but becomes more stable as the pH is reduced. 

The pH optimum varies for different enzymes The pH at which maximal enzyme activity is achieved is different for different enzymes, and often reflects the [H+] at which the enzyme functions in the body. For example, pepsin, a digestive enzyme In the stomach, is maximally active at pH 2, whereas other enzymes, designed to work at neutral pH, are denatured by such an acidic environment (Figure 5.8). 

In the stomach, hydrochloric acid denatures dietary proteins, making them more susceptible to proteases. Pepsin, an enzyme secreted in zymogen form by the serous cells of the stomach, releases peptides and a few free amino acids from dietary proteins. 

Schlamowitz, M. and Peterson, L.U. 1959. Studies on the optimum pH for the action of pepsin on native and denatured bovine serum albumin and bovine hemoglobin. J. Biol. Chem. 234 3137-3145. 

Iodination of PIR (147) showed 1 residue buried, Tyr 25, and all others iodinated at least to the monoiodotyrosyl form. Pepsin-inactivated RNase also has only one abnormal tyrosyl by titration which is thus assumed to be 25. Iodination of RNase-S is very similar to RNase-A in the early stages (lift). Extensive iodination leads to dissociation of the protein and peptide components. Direct iodination of S-protein indicated that all 6 tyrosyl residues were accessible, in this sense comparable to urea-denatured RNase-A. Substantial structural changes must be involved for both S-protein and PIR if Tyr 97, in particular, is to become susceptible to attack (see Section IV,B,3). 

Koppelman, S.J., Nieuwenhuizen, W.F., Gaspari, M. et al. 2005. Reversible denaturation of Brazil nut 2S albumin (Ber el) and implication of structural destabilization on digestion by pepsin. JAgric Food Chem 53 123-131. 

Enzymatic gelation of partially heat-denatured whey proteins by trypsin, papain, pronase, pepsin, and a preparation of Streptomyces griseus has been studied (Sato et al., 1995). Only peptic hydrolysate did not form a gel. The strength of the gel depended on the enzyme used and increased with increasing DH. Hydrolysis of whey protein concentrate with a glutamic acid specific protease from Bacillus licheniformis at pH 8 and 8% protein concentration has been shown to produce plastein aggregates (Budtz and Nielsen, 1992). The viscosity of the solution increased dramatically during hydrolysis and reached a maximum at 6% DH. Incubation of sodium caseinate with pepsin or papain resulted in a 55-77% reduction in the apparent viscosity (Hooker et al., 1982). 

Figure 1. Optimum pH of pepsin partially purified from the stomach lining of arctic cod 30°C ( ) 5°C (A) A 280 = absorbancy of TCA solubles at 280 nm. Pepsin was assayed with acid-denatured hemoglobin (2%) at the indicated temperatures and activity was monitored by measurements of TCA-solu-ble products (53).

The stomach environment is acidic as a result of HC1 secretion by the parietal cells. The acidic pH serves to denature many proteins, thus making them susceptible to proteolysis. The chief cells of the stomach produce pepsinogen, which is activated to pepsin by the HC1 (see Table 20.3). The optimum pH of peptic activity is around 2, and pepsin is inactivated at neutrality. Another stomach enzyme is rennin or chymosin, which is present in infants but not in adults. It removes a glycopeptide from milk-K-casein, disrupting the casein micelle and promoting milk protein coagulation and digestion. 

These surface films of proteins are not infrequently formed on solutions in which the proteins are quite soluble, so that it may be argued that the protein has been altered or denatured by its unfoldment in the surface film, in such a way as to render it less soluble or completely insoluble. Whether the unfoldment and spreading of the molecule always results in the loss of solubility is not proven Gorter has been able to remove pepsin from a surface on which it has been spread by means of a fine net pulled up through the surface, and subsequently dissolved the material in water, finding that it retained its normal properties, including proteolytic activity. 

Protein and starch digestion, on the other hand, have potent nonpancreatic compensatory mechanisms. Due to the compensatory action of salivary amylase and brush border oligosaccharidases, a substantial proportion of starch digestion can be achieved without pancreatic amylase. Similarly, protein denaturation and hydrolysis is initiated by gastric proteolytic activity (acid and pepsin) and continued by intestinal brush border peptidases, and is thus partly maintained even in the absence of pancreatic proteolytic activity. 

The digestion of heated or unheated soybean proteins by various enzymes is schematically compared with the nutritive values in Figure 18. Pattern A is typical of pepsin where, because of low pH of the reaction, the protein does not have to be denatured prior to addition to the reaction. Pattern B is typical of enzymes such as papain, bacterial neutral protease etc. where prior de-naturation of the substrate protein is required but there are no inhibitors of the enzyme present. Pattern C is typical of trypsin where prior heat treatment of the substrate protein is required to destroy inhibitors of trypsin as well as to denature the protein for digestion. The decrease in digestibility with prolonged heating in all three cases is due to modification of the substrate protein as described above. 

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