Download MBBS (Bachelor of Medicine, Bachelor of Surgery) 1st year (First Year) Biochemistry ppt lectures Topic 61 Ingestion Digestion And Absorption Of Dietary Proteins 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
Department of Biochemistry
Learning Objectives
? Ingestion of Dietary Proteins
? Digestion of Dietary Proteins
? Absorption of Amino Acids
? Transport of Amino acids
Introduction
Dietary Protein
? Consists of long chains of amino acids (aa)
? In the 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
Importance of Dietary Proteins
? Role of proteins in diet is important for structural component of
cells and tissues.
? Without adequate protein in diet, body cells and tissues not 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, so we need to get them from diet.
? Therefore, consume protein-rich foods each day, since body
does not have a way to store protein.
Cont--
? Essential aa: Cannot be
synthesize in the 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
Ingestion of Dietary Proteins
? Ingested dietary proteins is hydrolysed to aa, which are source
of essential aa in blood
? Absorbed from intestine and utilization of these aa for synthesis
of body proteins ex. Structural proteins, plasma proteins,
enzymes, milk proteins, hormones
? Also synthesis of necessary non-protein nitrogen compounds
includes urea, uric acid, creatine, creatinine, aa, 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, eggs
? Vegetable sources: Cereals, pulses, peas, beans and nuts
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 the intestine, therefore,
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 the pH of the
stomach
? Trypsinogen is cleaved by
enteropeptidase
? Active form of the enzyme
trypsin plays a key role by
catalysing the cleavage of
other pancreatic zymogens
Marks, Marks and Smith, Medical Biochemistry
Cont--
Digestion by gastric secretion: Digestion of proteins begins in
the stomach, which secretes gastric juice, a unique solution
containing hydrochloric acid (HCl) and the 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 the pH is greater than 5.
Cont--
? Pepsinogen is hydrolysed
in stomach with help of
HCl or pepsin itself
(autocatalytically) to form
the "active" pepsin
? In process of activation (i)
An inactive peptide called
as "pepsin inhibitor and (ii)
5 smaller peptides are
liberated.
Cont--
? Pepsin is a proteinase, a non-specific endopeptidase, and it
hydrolyses peptide bonds inside protein molecule and produces
proteoses and peptones
? It is particularly active on a peptide bond, which connects the ?
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 the peptide bonds of:
COOH group of methionine and leucine
Leucine and glutamic acid
Glutamic acid and asparagine
Leucine-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 the small intestine, large polypeptides produced in
stomach by action of pepsin are cleaved to oligopeptides and
amino acids
? Catalyzed by a group of pancreatic proteases that include 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 repeatedly cleaves N-
terminal residue from oligopeptides to produce smaller peptides
and free aa.
? Requires Zn++, Mn++ and Mg++ as a cofactor which help in
formation of a metal-enz-substrate coordination complex for
catalysis
Cont--
? Can hydrolyse a terminal peptide bond connected to an end a.a
bearing a free- NH2 group and splits off the end a.a. from N-
terminal end of a peptide, changing latter to a "tripeptide"
Tri and Di-peptidases: hydrolyse the 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 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
the five major pancreatic
proteases
? First three are serine
endopeptidases, whereas
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 the blood by simple diffusion
Marks, Marks and Smith, Medical Biochemistry
Interaction with students
? Distributed subtopics of class to students for participate in group
discussion in next class.
Reference Books
1) Text Book of Medical Biochemistry by Chatterjee & Rana
Shinde, 8th Ed
2) Biochemistry, Lippincott's Il ustrated Reviews, 6th Ed
3) Harper's Il ustrated Biochemistry-30th edition
4) Lehninger Principles of Biochemistry
5) Marks, Marks and Smith, Medical Biochemistry
27
Thank you
This post was last modified on 05 April 2022