III. Hydrolases

These enzymes catalyze hydrolysis – addition of water molecule to the substrate and simultaneously decompose it, they catalyze all reactions with participation water. They have many subclasses (9).

1. protein hydrolyzing enzymes (proteinases or proteases or proteolitic enzymes. This subclass is divided into the 2 sub-subclasses: endopeptidases and exopeptidases. Endopeptidases catalyze the peptide bondswithin the protein (from word “endo” which means inside) – proteins à (under influence of endopeptidases)à n (polypeptide). The endopeptidases include a few types of enzymes: pepsin, trypsin, chymotrypsins, catepsins. Different endopeptidases affect hydrolysis at particular aminoacid residues. Pepsin is produced by the chief cells of the gastric mucosa as pepsinogen. Under influence of hydrochloric acid it turns into pepsin: pepsinogen (proenzyme) à pepsin (enzyme) + polypeptide. Pepsin is acid proteinase, so its active center are ions of carboxyl groups. Pepsin is very active enzyme: 1g of pepsin hydrolyzes 50 kg of ovalbumine during 2 hours. Besides pepsin splits caseinogen of milk:

caseinogen – 100 % (under influence of pepsin in adult; of rennin in young; of chymozyn in animal) à casein – 90 % + polypeptide (serum albumose) – 10 %. 1 g of pepsin turn into the curds 10000 liters of milk. Pepsin acts on the amino side of tyrosine and phenylalanine and the carboxyl side of dicarboxylic amino acids.

Trypsin is produced by the pancreatic cells as trypsinogen which in the intestineturns into trypsin under influence of enteropeptidase (enterokinase): trypsinogen à trypsin + hexopeptide. Trypsin is simple protein, its active center includes serine and hystidine; it acts on the carboxyl side of lysine and arginine. Chymotrypsin is produced by pancreatic cells as chymotrypsinogenwhich turns into chymotrypsin under the action of trypsin: chymotrypsinogen à chymotrypsin + polypeptides (from 2-10 residues). Its active center includes 2 2 amino acid residues – serine and hystidine. Chymotrypsin acts on the carboxyl side of tyrosine and phenylalanine. The ability of pepsin, trypsin and chymotrypsin act in different part of protein molecules to accelerate the hydrolysis of proteins. Catepsins are cellular endopeptidase. There are 6 types of these enzymes: A, B, C, D, E, Q. These are proteinase and have the enzymatic activity at pH 3-5 only. They are in lyposomes, and become active at the acidosis (inflammation, death, hypoxia) and hydrolyze a different proteins. This can to autolysis of cells and tissues. Exopeptidases catalyze the hydrolysis of a terminal peptide bonds. These are further subdivided into the following types: 1) polypeptidases – there are of two types aminopeptidases and carboxypeptidases: aminopeptidases are simple proteins (alfa-2 globulins). Ions of Zn, Mg, Mn activate these enzyme. These are in intestinal juice, cells and attack the protein molecule from the side containing a free aminogroup, yielding an amino acid and peptide smaller in size by one amino acid residue. The process may be repeated again and again.

Carboxypeptidase are conjugated proteins, coenzyme is Zn. These are in intestinal juice, pancreatic cells (procarboxypeptidase, trypsin is activator for this enzyme), different cells. Carboxypolypeptidase acts in the same way as aminopolypeptidase but attacks a polypeptide molecule from the side containing a free carboxyl group. Tripeptidases act on tripeptides. Dipeptidases act on dipeptide. These enzymes contain Zn and are in the cells, intestinal juice.

2. next subclass of hydrolyses is carbohydrases. These catalyze the hydrolysis of the glycosidic bonds: -C-O-C-. all these enzymes are simple proteins (see table).

<== попередня сторінка	\|	наступна сторінка ==>
Non-competitive inhibition	\|	Table 5. Glycosidases

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