Learning Objectives
? Ingestion of Dietary Proteins
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? Digestion of Dietary Proteins? Absorption of Amino Acids
? Transport of Amino acids
Introduction
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Dietary Protein? Consists of long chains of amino acids (aa)
? In the digestive process, enzymes in stomach and small
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intestine break down complex protein into polypeptides andfurther into individual aa
? aa are absorbed through wall of small intestine, pass into blood
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and further to liver through portal vein
Importance of Dietary Proteins
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? Role of proteins in diet is important for structural component ofcells and tissues.
? Without adequate protein in diet, body cells and tissues not able
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to function.
? Proteins are large, complex molecules made up of smaller aa
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compounds.Cont--
? Some aa are made by body and are nonessential, but others
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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.
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Cont--
? Essential aa: Cannot be
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synthesize in the body so"essential" to eat them
from dietary food.
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? Non-essential: Body can
synthesize them from
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other proteins so notessential to eat them
Table 27.1. Harper's Illustrated Biochemistry 30th Edition
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Ingestion of Dietary Proteins? Ingested dietary proteins is hydrolysed to aa, which are source
of essential aa in blood
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? Absorbed from intestine and utilization of these aa for synthesis
of body proteins ex. Structural proteins, plasma proteins,
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enzymes, milk proteins, hormones? Also synthesis of necessary non-protein nitrogen compounds
includes urea, uric acid, creatine, creatinine, aa, polypeptides.
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Cont--
? Recommended Dietary allowance (RDA) for both men and
women: 0.8 g of protein/kg body weight/day
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? Dietary proteins: Dietary proteins in our diet are either from
animal source or vegetable source
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? Animal sources: Milk and dairy products, meat, fish, eggs? Vegetable sources: Cereals, pulses, peas, beans and nuts
Overview of the Digestion of Dietary Proteins
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Marks, Marks and Smith, Medical BiochemistryDigestion of Dietary Proteins
? Proteins are too large to be absorbed by the intestine, therefore,
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hydrolysed into di- and tripeptides as well as individual aa,
which can be absorbed
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? Proteolytic enzymes responsible for degrading proteins areproduced by three different organs: stomach, pancreas, and
small intestine
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Activation of Gastric and Pancreatic zymogens? Pepsinogen catalyzes its own
cleavage at the pH of the
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stomach
? Trypsinogen is cleaved by
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enteropeptidase? Active form of the enzyme
trypsin plays a key role by
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catalysing the cleavage of
other pancreatic zymogens
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Marks, Marks and Smith, Medical BiochemistryCont--
Digestion by gastric secretion: Digestion of proteins begins in
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the stomach, which secretes gastric juice, a unique solution
containing hydrochloric acid (HCl) and the proteolytic enzyme
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? Pepsin is a potent proteolytic enzyme and is present in gastricjuices
? It is secreted as inactive zymogen form, pepsinogen.
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Cont--? It is synthesised in "chief cells" of stomach
? HCl maintains gastric pH at about 1 to 2 and ensures maximum
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pepsin activity
? Optimum pH for pepsin is 1.6 to 2.5 and pepsin gets denatured
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if the pH is greater than 5.Cont--
? Pepsinogen is hydrolysed
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in stomach with help of
HCl or pepsin itself
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(autocatalytically) to formthe "active" pepsin
? In process of activation (i)
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An inactive peptide called
as "pepsin inhibitor and (ii)
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5 smaller peptides areliberated.
Cont--
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? Pepsin is a proteinase, a non-specific endopeptidase, and ithydrolyses peptide bonds inside protein molecule and produces
proteoses and peptones
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? It is particularly active on a peptide bond, which connects the ?
COOH group of an aromatic aa like Phe, Tyr, and Tryp with
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amino group of either a dicarboxylic acid or an aromatic a.aCont--
Pepsin also hydrolyse the peptide bonds of:
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COOH group of methionine and leucineLeucine and glutamic acid
Glutamic acid and asparagine
Leucine-valine
Valine and cysteine
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? Pepsin cannot act on proteins like keratins, Silkfibroins,
mucoproteins, mucoids and protamines
Digestion by pancreatic secretion
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? Optimum pH for activity of pancreatic enzymes (pH 8) providedby alkaline bile and pancreatic juice.
? Secretion of pancreatic juice is stimulated by peptide hormones,
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Cholecystokinin
Cont--
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? On entering the small intestine, large polypeptides produced instomach by action of pepsin are cleaved to oligopeptides and
amino acids
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? Catalyzed by a group of pancreatic proteases that include both
endopeptidases (Trypsin, Chymotrypsin, Elastase) and
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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-
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terminal residue from oligopeptides to produce smaller peptides
and free aa.
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? Requires Zn++, Mn++ and Mg++ as a cofactor which help information of a metal-enz-substrate coordination complex for
catalysis
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Cont--
? Can hydrolyse a terminal peptide bond connected to an end a.a
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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
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places
? In microvil us membrane of intestinal epithelial cells, or inside
epithelial cells after peptides absorbed inside cell
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Cont--? Tri-peptidase acts on a tri-peptide and produces a di-peptide
and free a.a
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? Di-peptidase hydrolyses a di-peptide to produce two molecules
of aa
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? They require Mn++, Co++ or Zn++ as cofactors for their activity.Cleavage of dietary protein in small intestine by pancreatic proteases
? Peptide bonds susceptible
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to hydrolysis for each of
the five major pancreatic
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proteases? First three are serine
endopeptidases, whereas
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last two are exopeptidases
Fig 19.5. Lippincott's Illustrated Reviews, Biochemistry, 6th Ed
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? Each is produced from aninactive zymogen
Absorption of Amino Acids
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? Free aa are taken into enterocytes by a sodium-linkedsecondary transport system of apical membrane.
? Di- and tripeptides, are taken up by a proton-linked transport
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system.
? Peptides are hydrolyzed in cytosol to aa that are released into
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portal system by facilitated diffusion.Cont--
? Therefore, only free aa are found in portal vein after a meal
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containing protein
? These aa are either metabolized by liver or released into
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general circulationAbsorption of products of protein digestion by carrier protein
transport system
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? AA are absorbed into epithelial cells byNa+-linked secondary transport via
symporter
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? Various aa are transported by carriers
specific for them
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? AA exit cell at basal membrane viavarious passive carriers by facilitated
transporter
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? AA enter the blood by simple diffusion
Marks, Marks and Smith, Medical Biochemistry
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Interaction with students? Distributed subtopics of class to students for participate in group
discussion in next class.
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Reference Books1) Text Book of Medical Biochemistry by Chatterjee & Rana
Shinde, 8th Ed
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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
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27
Thank you
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