Proenzyme pepsinogen

The digestion of proteins begins in the stomach, which secretes gastric juice—a unique solution containing hydrochloric acid and the proenzyme, pepsinogen ... 

Pepsin This acid-stable endopeptidase is secreted by the serous cells of the stomach as an inactive zymogen (or proenzyme), pepsinogen. In general, zymogens contain extra amino acids in... 

Dietary proteins, with very few exceptions, are not absorbed rather they must be digested into amino acids, or di- and tripeptides. Protein digestion begins in the stomach, where proenzyme pepsinogen is autocatalytically converted to pepsin A. Most proteolysis takes place in the duodenum via enzymes secreted by the pancreas, including trypsinogen, chymotrypsinogen and pro-carboxypeptidase A. These serine and zinc proteases are produced in the form of their respective proenzymes they are both endopeptidase and exopeptidase, and their combined action leads to the production of amino acids, dipeptides and tripeptides. 

Proteolytic proenzymes pepsinogen 1 and 11 are released by chief cells which are located at the base of... 

Chief Cells Secrete proenzyme pepsinogen (pepsin)... 

Pepsin is secreted as the inactive pepsinogen, which is activated by H+ ions at a pH below 5. Determination of its crystal structure revealed that in the proenzyme the N-terminal 44-residue peptide segment lies across the active site, blocking it.384 At low pH the salt bridges that stabilize the proenzyme are disrupted and the active site is opened up to substrates. 

The answer is d. (Murray, pp 48-62. Scriver, pp 3 5. Sack, pp 1-3. Wilson, pp 101-120.) Pepsin is secreted in a proenzyme form in the stomach. Unlike the majority of proenzymes, it is not activated by protease hydrolysis. Instead, spontaneous acid hydrolysis at pH 2 or lower converts pepsinogen to pepsin. Hydrochloric acid secreted by the stomach lining creates the acid environment. All the enzymes secreted by the pancreas are activated at the same time upon entrance into the duodenum. This is accomplished by trypsin hydrolysis of the inactive proenzymes trypsino-gen, chymotrypsinogen, procarboxypeptidase, and proelastase. Primer... 

Other denaturing factors are scarcely employed at present. Denatura-tion by acids in polarographic examinations proceeds very slowly moreover, in acidified sera the present proenzymes or enzymes can be activated for example, at pH about 1.8 pepsinogen is activated to pepsin, causing proteolytic cleavage at pH about 3.8, cathepsin is activated or perhaps another pH value can be attained which happens to be most favorable for proteolysis caused by enzymes of catalytic nature (65). These facts could have led in the past to faulty results of polarographic examinations of protein denaturation by acids. 

An acid protease with an optimum pH of 2.5 was first described in human seminal plasma as pepsin and pepsinogen (1), but had not been purified or characterized. Recently, we have purified the acid protease and its proenzyme from human seminal plasma (2,3). In many respects, the properties of seminal plasma acid protease are similar to those of gastric pepsin. Since the proenzyme is more stable than the active enzyme in alkaline solution and can be converted into its active form in acidic solution, the acid protease is likely to exist in seminal plasma, at the physiological pH around 7.5 (4), in proenzyme form. 

Conversion of the proenzyme into its active form. We concluded that only the proenzyme form of acid protease can exist in human seminal plasma at the physiological pH of around 7.5 therefore, we investigated activation of proenzyme in acid medium. The purified proenzyme was incubated in 1 mM HCl, pH 3, for 1 hr, and then chromatographed on Sephadex G-50 column. The proenzyme was converted into an active form and some peptide of small molecular weight was released (Fig.2). As shown in Table II, when the amino acid analyses of the proenzyme, the active form, and activation peptide were carried out, the number of each amino acid residue of the proenzyme agreed well with the additive value between the number of that amino acid in the active form and activation peptide. This supported the conversion of the proenzyme to an active form. The amino acid composition of the active protease and of the proenzyme were comparable to those of bovine pepsin (10) and pepsinogen (11). However, definite differences are present. About forty residues which carried most of the basic amino acids, were released from pepsin, while sixty-nine residues, which carried about 30% of basic amino acid of the precursor were liberated from the proenzyme of seminal plasma acid protease. 

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