Digestive enzymes catalytic activity

For most enzymes, catalytic activity is temperature-dependent to a maximum level, above whioh they lose their activity. Also, by analogy with other proteins, enzymes are stable only within a limited range of pH. Outside this range, enzymes are denatured by ohanges in the charges of ionizable amino acid residues that alter the tertiary structure of proteins. Enzyme activity reaches a peak or a plateau at a specific pH, so enzymatic digestion is usually performed in a buffered medium. The process is also affected by the enzyme concentration, which must therefore be optimized as well. 

The allosteric model for AMP inhibition of FDPase is supported by the results of experiments in which the enzyme is desensitized to the inhibitor with little or no loss of catalytic activity. Mild digestion with papain causes almost complete loss of AMP inhibition under conditions where the catalytic activity is only slightly decreased (13). Indeed the catalytic activity measured at alkaline pH has been observed to increase (36). 

A different type of covalent regulation of enzyme activity is the enzyme-catalysed activation of inactive precursors of enzymes (zymogens) to give catalytically active forms. The best examples are the digestive enzymes, e.g. trypsin. Proteolytic enzymes would digest the inside of the cells that produce the enzyme, so they are produced in an inactive form which is activated to the true enzyme once they have entered the digestive system of the animal. 

Schematic diagrams of the amino acid sequences of chymotrypsin, trypsin, and elastase. Each circle represents one amino acid. Amino acid residues that are identical in all three proteins are in solid color. The three proteins are of different lengths but have been aligned to maximize the correspondence of the amino acid sequences. All of the sequences are numbered according to the sequence in chymotrypsin. Long connections between nonadjacent residues represent disulfide bonds. Locations of the catalytically important histidine, aspartate, and serine residues are marked. The links that are cleaved to transform the inactive zymogens to the active enzymes are indicated by parenthesis marks. After chymotrypsinogen is cut between residues 15 and 16 by trypsin and is thus transformed into an active protease, it proceeds to digest itself at the additional sites that are indicated these secondary cuts have only minor effects on the enzymes s catalytic activity. (Illustration copyright by Irving Geis. Reprinted by permission.)...

Interactions between serine proteases are common, and substrates of serine proteases are usually other serine proteases that are activated from an inactive precursor [66]. The involvement of serine proteases in cascade pathways is well documented. One important example is the blood coagulation cascade. Blood clots are formed by a series of zymogen activations. In this enzymatic cascade, the activated form of one factor catalyzes the activation of the next factor. Very small amounts of the initial factors are sufficient to trigger the cascade because of the catalytic nature of the process. These numerous steps yield a large amplification, thus ensuring a rapid and amplified response to trauma. A similar mechanism is involved in the dissolution of blood clots. A third important example of the coordinated action of serine proteases is the intestinal digestive enzymes. The apoptosis pathway is another important example of coordinated action of other types of proteases. 

The chemical reactions associated with the digestion of nutrients are catalyzed by many different enzymes. Some enzymes are secreted in their catalytically active form, whereas others are secreted as the corresponding zymogens (given in parentheses). Zymogens require chemical modification to be converted to their catal5 ti-cally active forms. 

Although the cytoplasm of the cell and the fluids that bathe the cells have a pH that is carefully controlled so that it remains at about pH 7, there are environments within the body in which enzymes must function at a pH far from 7. Protein sequences have evolved that can maintain the proper three-dimensional structure under extreme conditions of pH. For instance, the pH of the stomach is approximately 2 as a result of the secretion of hydrochloric acid by specialized cells of the stomach lining. The proteolytic digestive enzyme pepsin must effectively degrade proteins at this extreme pH. In the case of pepsin the enzyme has evolved in such a way that it can maintain a stable tertiary structure at a pH of 2 and is catalytically most active in the hydrolysis of peptides that have been denatured by very low pH. Thus pepsin has a pH optimum of 2. 

Carboxypeptidase A (EC 3.4.17.1) is a pancreatic digestive enzyme that consists of a single polypeptide chain of 307 amino acids with a total of 36,000. It catalyzes the cleavage of amino acid residues from C-termini of polypeptides. Importantly, for its mechanism of action, it contains one Zn +in its active site. The amino acid side chains that form its active site and the catalytic sequence are shown in Fig. 5-27. 

Many enzymes are not sufficiently stable under the operational conditions and they may lose catalytic activity due to auto-oxidation, self-digestion and/or denaturation by the solvent, the solutes or due to mechanical shear forces. 

---