Download Digestive Enzymes and their Action

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
Location of
digestion
digestion
SalivaryDextrins
Mouth
Gastric
Stomach
Intestinal
Small
intestine
Digestive juice
and enzymes
Saliva
Salivary amylase
(ptyalin)
Substrate products
Lingual
Milk
Gastric juice
PepsinoHydrochloric acid gen
protein
Fats
Intestine juice
TrypsinEnterokinase
ogen
Aminopeptides
Polypepti
Dipeptidase
des
Maltase Sucrose Maltose
Sucrose
Lactase
Lactase
Pancreatic juice
Trypsin
Chymotrypsin
Pancreatic
amylase
Pancreatic lipase
Amino acids
Starch
Protein
Fatty acids+
1,2diglycerides
Pepsin
polypeptides
+ Fatty acid
Glycose
Trypsin
Amino
Acids
Amino Acid
Glucose
Glucose+
Fructose
Glucose+
Galactose
Polypeptides
Protein
Polypeptides
Starch + Maltose
dextrins
Fats
Fatty acids
+ Glycerol
Carboxy Amino o Polypepti
Amino
peptidase
des
acid