Digestion, enzyme-catalysed

This digestive enzyme catalyses the hydrolysis of peptides, i.e. the reaction ... 

Hydrolases such as the digestive enzymes amylase and lactase catalyse hydrolysis of glycosides, esters, anhydrides and amides. 

Most cells have a superhcial layer of polysaccharide, known as the glycocalyx, which is attached to the cell surface. This is particularly well developed in enterocytes. Some of the digestive enzymes from the lumen are adsorbed onto the glycocalyx. The bulk of digestion occurs in the lumen of the intestine, but the enzymes on the glycocalyx catalyse the final stages of some processes. 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

More specific hydrolysis may be achieved by the use of enzymes. Thus, the enzyme a-amylase in saliva and in the gut is able to catalyse hydrolysis of al 4 bonds throughout the starch molecule to give mainly maltose, with some glucose and maltotriose, the trisaccharide of glucose. Amylose is hydrolysed completely by this enzyme, but the al 6 bonds of amylopectin are not affected. Another digestive enzyme, a-l,6-glucosidase, is required for this reaction. Finally, pancreatic maltase completes the hydrolysis by hydrolysing maltose and maltotriose. 

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. 

The most studied member of zinc proteases is the digestive enzyme bovine pancreatic carboxypeptidase A (CPA) which is a metalloenzyme containing one atom of zinc bound to its single polypeptide side chain of 307 amino acids with a molecular weight of 34 kD. It is an exopeptidase, which catalyses the hydrolysis of C-terminal amino... 

In most cases enzymes are able to catalyse reactions in more than one group of substrates. In such cases the specificity is said to be relative. Such group specificity may be of a low order, such as in the case of the digestive enzymes trypsin and pepsin, which catalyse the rupture of peptide bonds. In other cases it may be much higher. Chymotrypsin, for example, catalyses the hydrolytic cleavage only of peptide bonds in which the carboxyl residue is derived from an aromatic amino acid. 

Because of their secondary and tertiary structures, most proteins are resistant to digestive enzymes — few bonds are accessible to the proteolytic enzymes that catalyse hydrolysis of peptide bonds. However, apart from covalent links formed by reaction between the side-chains of lysine and aspartate or glutamate, and disulphide bridges, the native structure of proteins is maintained by relatively weak non-covalent forces ionic interactions, hydrogen bonding and van der Waals forces. 

Protein digestion occurs in two stages endopeptidases catalyse the hydrolysis of peptide bonds within the protein molecule to form peptides, and the peptides are hydrolysed to form the amino acids by exopeptidases and dipeptidases. Enteropeptidase initiates pro-enzyme activation in the small intestine by catalysing the conversion of trypsinogen into trypsin. Trypsin is able to achieve further activation of trypsinogen, i.e. an autocatalytic process, and also activates chymotrypsinogen and pro-elastase, by the selective hydro-... 

The role of the fibrinolytic system is to dissolve any clots that are formed within the intact vascular system and so restrict clot formation to the site of injury. The digestion of the fibrin and hence its lysis is catalysed by the proteolytic enzyme, plasmin, another serine proteinase. Plasmin is formed from the inactive precursor, plasminogen, by the activity of yet other proteolytic enzymes, urokinase, streptokinase and tissue plasminogen activator (tPA) which are also serine proteinases. These enzymes only hydrolyse plasminogen that is bound to the fibrin. Any plasmin that escapes into the general circulation is inactivated by binding to a serpin (Box 17.2). 

Enzymes can be isolated from micro-organisms and used to catalyse reactions in other processes. For example, proteases are used in biological detergents to digest protein stains such as blood and food. Also, catalase is used in the rubber industry to help convert latex into foam rubber. 

The International Union of Biochemistry has recommended that enzymes have three names, namely a systematic name, which shows the reaction being catalysed and the type of reaction based on the classification in Table A7.1, a recommended trivial name and a four figure Enzyme Commission code (EC code). Nearly all systematic and trivial enzyme names have the suffix -ase. Systematic names show, often in semi-chemical equation form, the conversion the enzyme promotes and the class of the enzyme. Trivial names are usually based on the function of the enzyme but may also include or be based on the name of the substrate. However, some trivial names in current use are historical and bear no relationship to the action of the enzyme or its substrate, for example, pepsin and trypsin are the names commonly used for two enzymes that catalyse the breakdown of proteins during digestion. The Enzyme Commission s code is unique for each enzyme. It is based on the classification in Table A7.1 but further subdivides each class of enzyme according to how it functions. The full code is... 

As indicated, one of the alternatives to preparing a bioactive structure immobilising an enzyme is to use the polymer structure considering the bioartificial matrix as a substrate for the enzyme. In this case, it is possible for example to drive drug release using erosion control devices, where the enzyme plays the catalytic role, and one of the components of the matrix is digested (in a controlled way) by the catalysed reaction. 

The structure and stability of proteins are dependent on the presence of water, and enzymes are associated with catalysis in water. Indeed, this is one of their advantages, since they catalyse reactions over the range of temperatures at which water is liquid at normal atmospheric pressures. Nevertheless, there are exceptions, for example in the high-temperature digestion of starch (see Section 6.6). Nor does the aqueous environment in which enzymes usually act preclude their use as catalysts in organic solvents (see Sections 6.3, 6.4.2, 6.9.2, and 6.9.4). 

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