Digestive Enzymes and their Action - University of Babylon



Digestive Enzymes and their Action

All foodstuffs except water , inorganic salts , vitamins and nonsaccharides are hydrolysed into smaller molecules in the digestive tract before absorption from the intestines . The hydrolysis is accomplished by the enzymes of digestive fluids namely saliva , gastric juice, pancreatic juice and the intestinal juices .

Salivary Digestion

Salivea provides α – amylase . The flow is stimulated by the sight , smell , taste and even thought of food . Besides water (99.5 %) , saliva includes a food Iubricant called mucin ( a glycoprotein ) and an enzyme , α – amylase . This enzyme catalyses the partial and hydrolysis of starch to dextrins and maltose and it works best at the pH of saliva in between 5.8 to 7.1 . Proteins and lipids pass through the mouth unchanged .

Gastric Juice

Gastric juice starts the digestion of proteins with pepsin . When food reached the stomach , the cells of the glands are stimulated by hormones to release the fluids that combine to give gastric juice . One kind of gastric gland secretes mucin , which coats the stomach to protect it against its own digestive enzymes and its acid . Mucin is continuously produced and only slowly digested , When the protection of the stomach is hindered m part of the stomach itself could be digested leading to ulcers . Another gastric gland secretes hydrochloric acid ( pH 1 to 2) about a million times more acidic than blood . The acid coagulates proteins and activates the enzyme protease . Protein coagulation retains the protein in the stomach longer for exposure to the protease . The third gastric gland secretes the zymogen pepsinogen . Pepsinogen is changed into pepsin , protease , by the action of hydrochloric acid and traces of pepsin . The optimum pH is 1.0 to 1.5 , which is found in the stomach fluid . Pepsin catalyses the only important digestive work in the stomach , namely the hydrolysis of some of the peptide bonds of proteins to make short polypeptides . Adult gastric juice also has a lipase , but it dose not start its work until it arrives in the higher pH medium of upper intestinal tract . The gastric juice of infants is less acidic than adults . To compensate for the protein coagulation work normally done by the acid , infant gastric juice contains rennin , a powerful protein that in adult , its lipase gets an early start on lipid digestion .The churning and digesting activities in the stomach produce a liquid mixture called chyme . This is released in portions through the pyloric valve into the duodenum , the first 12 inches of the upper intestinal tract .

Pancreatic Juice

The pancreatic juice furnishes several zymogenes and enzymes . As soon as the chyme appears in the duodenum m hormones are released that circulate to the pancreas and induce this organ to release two juice . One is dilute sodium bicarbonate , which neutralizes the acid in the chyme . The other is usually called pancreatic juice and it carries enzymes or zymogens involved in the digestion of practically everything in food . It contributes an α – amylase similar to that present in saliva , a lipase , nucleases and zymogens for proteins digestion enzymes . The conversion of the proteolytic zymogens to active enzymes being with the enzyme called enteropeptadase released from cells that line the duodenum when chyme arrives . It catalyses from cells that line the duodenum when chyme arrives . It catalyses the formation of trypsin from its zymogen , trypsinogen .

enteropeptidase

Trypsinogen trypsin

Trypsin then catalyses the change of the other zymogen into active enzymes .

trypsin

Procharboxypeptidase carboxypeptidase

Trypsin

Chymotrypsinogen chymotrypsin

trypsin

Proelastase elastase

Trypsin , chymotrypsin and elastase catalyse the hydrolysis of large polypeptides to smaller ones . Carboxypeptidaes working form C- terminal ends of small peptides, carries the action further to amino acids and dipeptides or tripeptides .

Intestinal Juice

The intestinal juice contains the following enzymes

1. Disaccharidases

2. Peptidases

3. polynucleotidases

4. Nucleosidases

5. Enterokinases

6. Phosphatases

7. Lecithinase

Disaccharidases

They are enzymes by hydrolyse disaccharides . They attack the glucosidic linkage of the disaccharides to convert them to the corresponding monosaccharides . For example

1. Maltase spilts maltase into2molecule of glucose

2. Sucrase splits sucrase into glucose and fructose

3. Lactase splits lactase into glucose and galactose

Peptidases

These are enzymes hydrolyzing peptide chains . There are two main types of peptidases :

a. Aminopeptidases act on the peptide linkage of terminal amino acids , possessing a free amino group .

b. Tripeptidases and dipeptidases split tripeptides and dipeptides into their individual amino acids .

polynucleotidases

It hydrolyses and splite the nucleotide into phosphoric acid and nucleoside .

Nucleosidase

It splits nucleosides into their nitrogenous bases and sugars

Enterokinase

This enzyme is secreted by the duodenal and mucosal cells . It converts trypsinogen to trypsin . It dose not have any direct digestive action .

Phosphatase

Enzyme phosphatase present in the intestinal juice splite the phosphate from organic phosphate derivatives , such as glycerophosphate and hexose phosphate. Lecithinase Intestinal juice contains the lecithinase which splite lecithins to yield fatty acid , glycerol , phosphoric acid and choline .

SUMMARY OF DIGESTION OF FOODSTUFFS

Carbohydrates

Digestion of polysaccharides ( starch and glycogen ) being in the mouth by ptyalin . The action of ptyalin may continue for a short period in the stomach till the gastric acidity inactivates the enzymes . The polysaccharides are partially hydrolysed to dextrins and maltose by ptyaline . In the intestines , the unhydrolysed polysaccharides and dextrin are further split to maltose by pancreatic amylase . Maltose along with other disaccharides such as lactase and sucrose are by maltase , lactase and sucrase of intestinal juice to their constituents , glucose , galactose and fructose .

Proteins

Both native and denatured proteins are hydrolysed step –by –step by pepsin of gastric juice , and trypsin and chymotrypsin of pancreatic juice to from polypeptides and small amounts of amino acids . Casein forms insoluble paracasein by the action of rennin in infants and the actions of pepsin and chymotrypsin in adults .

Lipids

Very little digestion of fat takes place in the stomach . The main digestion of fat begins in the intestines where they are first emulsified by bile salts . Further emulsification is carried out by the products of fat digestion. The emulsification fats are hydrolysed by pancreatic lipase which is the most important lipolytic enzyme of pancreatic juice and by the lipase of intestinal secretion incompletely to fatty acids , glycerol , momoglycerides and diglycerides . Some amount of triglycerides escape hydrolysis and are present among the products of digestion .

Nucleoproteins

Dietary nucleoproteins are split by proteolytic enzymes to nucleic acids and proteins . The later undergo digestion like any other protein . The nucleic acid are split ribonuclease and deoxyribonuclease pf pancreatic juice to form oligonucleotides and mononucleotides . Phosphodiesterases of intestinal secretion split further into nucleosides and inorganic phosphate by non-specific phosphatases .

Bile

rsecreted by liver . It is slightly viscus and tastes bitter . It has a pH 7.5 to 8.5 . About 500 to 700 ml of bile are secreted daily by the liver . A part of it , about 500 ml , is stored in the gallbladder , where it is concentrated and periodically discharged into small intestines . The bile in the gallbladder is more viscus and has a greenish tinge because of the presence of bile pigments . During digestion , the gallbladder contracts to supple bile to the intestines m via the common bile duct . the bile mixes with the pancreatic juice and helps to emulsify the water insoluble fatty materials and thus greatly increase the exposure to water and lipase . Triacyl glycerols are hydrolysed to fatty acids , glycerol and some monoacyl glycerols

Bile Salts

Sodium glycolate and sodium taurocholate are both bile salts derived from cholic acid. They have the ability to lower the surface tension and increase surface area, thus aiding in the emulsification of fats. They also increase the effectiveness of pancreatic lipase in its digestive action on emulsified fats,

In addition, bile salts aid the absorption of fatty acids through the walls of the intestine. After absorption of these fatty acids, the bile salts are removed and carried back by portal circulation to the liver, where they are again returned to the bile. Bile salts also stimulate intestinal motility.

The bile salts also assist in the absorption of fat soluble vitamins (A,D,E and K) from the absorption of digestion tract into blood. This work reabsorbs some bile pigments some bile pigments some of which eventually leave the body through urine. Bile pigments are responsible for the colour of both faeces and urine. If bile duct is blocked, the bile pigments remain in the blood stream producing jaundice.

|Type of digestion |Location of digestion |Digestive juice and enzymes |Substrate |products |

|Salivary |Mouth |Saliva |Starch |Dextrins |

| | |Salivary amylase (ptyalin) | | |

| | |Lingual |Milk |Fatty acids+ |

| | | | |1,2diglycer-ides |

|Gastric |Stomach |Gastric juice |Pepsino-gen protein |Pepsin polypeptides |

| | |Hydrochloric acid |Fats |+ Fatty acid Glycose |

|Intestinal |Small intestine |Intestine juice |Trypsin-ogen |Trypsin Amino Acids Amino |

| | |Enterokinase Aminopeptides Dipeptidase |Polypeptides Maltose |Acid |

| | |Maltase Sucrose |Sucrose |Glucose |

| | | | |Glucose+ |

| | |Lactase |Lactase |Fructose |

| | | | |Glucose+ Galactose |

| | |Pancreatic juice Trypsin Chymotrypsin |Protein |Polypeptides |

| | | | | |

| | | |Protein |Polypeptides |

| | |Pancreatic amylase |Starch + dextrins |Maltose |

| | |Pancreatic lipase |Fats |Fatty acids + Glycerol |

| | |Carboxy peptidase |Polypeptides |Amino acids |

| | | | |Amino o Amino |

| | | | |acid |

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