Download MBBS (Bachelor of Medicine, Bachelor of Surgery) 1st year (First Year) Biochemistry ppt lectures Topic 62 Ingestion, Digestion And Absorption Of Dietary Notes. - biochemistry notes pdf, biochemistry mbbs 1st year notes pdf, biochemistry mbbs notes pdf, biochemistry lecture notes, paramedical biochemistry notes, medical biochemistry pdf, biochemistry lecture notes 2022 ppt, biochemistry pdf.
Ingestion, Digestion & Absorption of Dietary
Proteins
Specific Learning Objectives
Describe importance of dietary protein quality in maintenance of health, and consequences of protein-energy malnutrition (e.g.
Marasmus and Kwashiorkor)
Describe role of gastric hormone gastrin and paracrine hormone histamine in promoting secretion of HCl and pepsinogen, and
activation of pepsinogen to pepsin.
Describe pepsin action in digestion of peptides
Describe roles of secretin, bicarbonate and cholecystokinin in neutralization of duodenal pH, and release of bile and pancreatic
zymogens
Describe activation of pancreatic zymogens and roles of active enzymes in protein digestion.
Ingestion of Dietary Proteins
Describe digestion of dietary proteins in stomach, duodenum and smal intestine
Describe uptake of peptides and aa from gastrointestinal tract
Discuss absorption and transport of aa to liver
Identify defects in uptake of aa (e.g. Hartnup disease) or peptides that lead to clinical symptoms (failure to thrive, edema,
various vitamin deficiencies).
Introduction
Dietary Protein consists of long chains of aa
In digestive process, enzymes in stomach and small intestine break
down complex protein into polypeptides and further into individual aa.
aa are absorbed through wall of small intestine, pass into blood and
further to liver through portal vein.
Ingestion of Dietary Proteins
Ingested dietary proteins is hydrolysed to aa
Absorbed from intestine and utilization of these aa for synthesis of body
proteins ex. structural proteins, plasma proteins, enzymes, milk proteins,
and hormones
Also synthesis of necessary non-protein nitrogen compounds includes
urea, uric acid, creatine, creatinine, aa, and polypeptides
Cont-
Recommended Dietary allowance (RDA) for both men and women: 0.8 g
of protein/kg body weight/day
Dietary proteins: Dietary proteins in our diet are either from animal source
or vegetable source.
Animal sources: Milk and dairy products, meat, fish and eggs.
Vegetable sources: Cereals, pulses, peas, beans and nuts
Importance of Dietary Proteins
For structural component of cells and tissues
Without adequate protein in diet, body cells and tissues would not be able
to function.
Proteins are large, complex molecules made up of smaller aa
compounds.
Cont-
Some aa are made by body and are nonessential, but others are
essential, meaning that we need to get them from diet
Therefore, consume protein-rich foods each day, since body does not
have a way to store amino acids
Cont--
? Essential
aa:
Cannot
be
synthesize in body so "essential"
to eat them from dietary food.
? Non-essential:
Body
can
synthesize them from other
proteins so not essential to eat
them
Table 27.1. Harper's Illustrated Biochemistry 30th Edition
Overview of the Digestion of Dietary Proteins
Marks, Marks and Smith, Medical Biochemistry
Digestion of Dietary Proteins
Proteins are too large to be absorbed by intestine, therefore, must be
hydrolysed into di- and tripeptides as well as individual aa, which can
be absorbed
Proteolytic enzymes responsible for degrading proteins are produced
by three different organs: stomach, pancreas, and small intestine
Activation of Gastric and Pancreatic zymogens
? Pepsinogen catalyzes its own
cleavage at pH of stomach
? Trypsinogen is cleaved by
enteropeptidase
? Active form of enzyme trypsin
plays a key role by catalysing
cleavage of other pancreatic
Marks, Marks and Smith, Medical Biochemistry
zymogens
Cont--
Digestion by gastric secretion: Digestion of proteins begins in stomach,
which secretes gastric juice, contains hydrochloric acid (HCl) and
proteolytic enzyme
Pepsin is a potent proteolytic enzyme and is present in gastric juices.
It is secreted as inactive zymogen form, pepsinogen.
Cont--
It is synthesised in "chief cells" of stomach
HCl maintains gastric pH at about 1 to 2 and ensures maximum
pepsin activity.
Optimum pH for pepsin is 1.6 to 2.5 and pepsin gets denatured if pH
is greater than 5.
Cont--
? Pepsinogen is hydrolysed in stomach
with help of HCl or pepsin itself
(autocatalytically) to form "active"
pepsin
? In process of activation (i) inactive
peptide called as "pepsin inhibitor
and (ii) 5 smaller peptides (inactive)
are liberated.
Cont--
Pepsin is a proteinase, a non-specific endopeptidase, and it hydrolyses
peptide bonds well inside protein molecule and produces proteoses and
peptones
It is particularly active on a peptide bond, which connects ?COOH group
of an aromatic aa like Phe, Tyr, and Tryp with amino group of either a
dicarboxylic acid or an aromatic a.a
Cont--
Pepsin also hydrolyse peptide bonds of:
? COOH group of methionine and leucine
? Leucine and glutamic acid
? Glutamic acid and asparagine
? Leucine and valine
? Valine and cysteine
Pepsin cannot act on proteins like keratins, Silkfibroins,
mucoproteins, mucoids and protamines
Digestion by pancreatic secretion
Optimum pH for activity of pancreatic enzymes (pH 8) provided by
alkaline bile and pancreatic juice
Secretion of pancreatic juice is stimulated by peptide hormones,
Cholecystokinin
Cont--
On entering small intestine, large polypeptides produced in stomach
by action of pepsin are cleaved to oligopeptides and aa by a group of
pancreatic proteases
Includes both endopeptidases (Trypsin, Chymotrypsin, Elastase) and
exopeptidases (metalloenzyme, contains zinc).
Digestion by proteolytic enzymes in intestinal juice
Amino-peptidases: Luminal surface of intestinal epithelial cells contains
aminopeptidase, an exopeptidase that repeatedly cleaves N-terminal
residue from oligopeptides to produce even smaller peptides and free
aa.
Requires presence of Zn++, Mn++ and Mg++ which help in formation
of a metal-enz-substrate coordination complex for catalysis
Cont--
Hydrolyse a terminal peptide bond connected to an end a.a bearing a
free- NH2 group, splits off end a.a. from N-terminal end of a
peptide, changing latter gradually stepwise to a "tripeptide"
Tri and Di-peptidases: hydrolyse peptides at either of two places
In microvil us membrane of intestinal epithelial cells, or inside
epithelial cells after peptides absorbed inside cell
Cont--
Tri-peptidase acts on a tri-peptide and produces a di-peptide and free
a.a
Di-peptidase hydrolyses a di-peptide to produce two molecules of aa
They require presence of Mn++, Co++ or Zn++ as cofactors for their
activity.
Cleavage of dietary protein in small intestine by pancreatic proteases
Peptide
bonds susceptible to
hydrolysis for each of five major
pancreatic proteases
? First
three
are
serine
endopeptidases, whereas the last
two are exopeptidases
Fig 19.5. Lippincott's Illustrated Reviews, Biochemistry, 6th Ed
Each is produced from an inactive
zymogen
Absorption of Amino Acids
Free aa are taken into enterocytes by a sodium-linked secondary
transport system of apical membrane.
Di- and tripeptides, are taken up by a proton-linked transport system.
Peptides are hydrolyzed in cytosol to aa that are released into portal
system by facilitated diffusion.
Cont--
Therefore, only free aa are found in portal vein after a meal
containing protein.
These aa are either metabolized by liver or released into general
circulation
Absorption of products of protein digestion by carrier protein transport system
? AA are absorbed into epithelial cells by Na+-
linked secondary transport via symporter
? Various aa are transported by carriers specific
for them
? AA exit cell at basal membrane via various
passive carriers by facilitated transporter
? AA enter blood by simple diffusion
Marks, Marks and Smith, Medical Biochemistry
Role of Glutathione in Amino Acid Absorption
Meister proposed that glutathione participates in an active group
translocation of L-amino acids (except L-Pro) into cells of small intestine,
kidneys, seminal vesicles, and brain.
He proposed a "cyclic" pathway/-glutamyl cycle in which Glutathione is
regenerated again
-glutamyl transferases (GGT) plays a key role in this cycle, a pathway for
synthesis and degradation of glutathione and drug and xenobiotic
detoxification
-glutamyl/Meister cycle
Fig.26.1: Text Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8th Ed
Disorder related to aa transporter
Hartnup disease: Defect in SCL6A19 gene, it is a sodium-dependent and
chloride independent neutral aa transporter, expressed in kidneys and
intestine
This gene controls absorption of certain aa from intestine and
reabsorption of those aa in kidneys
Person with Hartnup disease cannot absorb aa properly from intestine
and cannot reabsorb them from tubules in kidneys
Cont--
Resulting excessive amount of aa such as Trp excreted in urine
Due to inadequate amount of Trp, body not able to make sufficient
amount of niacin (vit B3), which is necessary component of NAD+
Pellagra, a similar condition, is also caused by low nicotinamide; this
disorder results in dermatitis, diarrhea, and dementia
Cont--
Cystinuria: Dibasic (diamino aa) transporter Lys, Arg, Ornithine and
Cys. If there is any defect in this transport system, it leads to
Cystinuria
In medicine, Garrod's tetrad is a term named for British physician
Archibald Garrod, who introduced phrase "inborn errors of
metabolism" in a lecture in 1908
Tetrad comprises four inherited metabolic diseases: albinism,
alkaptonuria, Cystinuria and pentosuria
Intestinal aa transport systems and their disorders
Clinical-cases discussed
33
Reference Books
1) Biochemistry, Lippincott's Il ustrated Reviews, 6th Ed
2) Harper's Il ustrated Biochemistry-30th Ed
3) Lehninger Principles of Biochemistry, 6th Ed
4) Marks, Marks and Smith, Medical Biochemistry
5) Netter's Essential Biochemistry, 1st Ed
Thank you
This post was last modified on 05 April 2022