Protein Digestion and Absorption

7/23/2019 Protein Digestion and Absorption

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Protein Digestion and Absorption

Dietary proteins, with few exceptions, are

not absorbed.


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Dietary proteins, with few exceptions, are

not absorbed.

They must be digested first into amino acids

or di- and tri-peptides.


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Dietary proteins, with few exceptions, arenot absorbed.

They must be digested first into amino acidsor di- and tri-peptides.

Through the action of gastric and pancreatic

proteases, proteins are digested within thelumen into medium and small peptides

(oligopeptides.


4/98

Digestion of protein - hydrolysis

R CH C

O

COOHCH

R

N

HNH2

R CH

NH2

C

O

OHCOOHCH

R

HHN

+H2O+ proteases


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Protein digestion begins in stomach

Pepsin - inactive precursor pepsinogen

ctive ! pH 2-"# inactive pH$%

&ecretion stimulated by acetylcholine or

acid

Only protease 'hich can brea( do'n

collagen

ction terminated by neutralisation by

bicarbonate in duodenum)

N)*) ll proteases stomach , pancreatic secreted as

inactive precursors) .ost protein digestion occurs in the

duodenum/0e0unum


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ctivation of pancreatic proteases

1rypsinogen 1rypsin

ntero(inase

1rypsinogen

Chymotrypsinogen

ProelastaseProcarbo3ypeptidase

1rypsin

Chymotrypsin

lastaseCarbo3ypeptidase

ctive proteases inactivated by trypsin


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peptidases

aminopolypeptidasetransporters

amino acidsDi/tripeptides

Cytoplasmic peptidase

transporters

mino acids

Protein

peptides


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Pancreatic en4ymes

ssential for digestion essential for life

Proteases

5nactive form

ctivated in gut

cinar cells

6ipases mylases

ctive en4ymes


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Pancreatic !n"ymes

The bul# of protein

digestion occurs within

the intestine due to theaction of pancreatic

proteases.


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Pancreatic Proteases

The two primary pancreatic proteases are

trypsin and chymotrypsin.


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The two primary pancreatic proteases are trypsinand chymotrypsin.

They are synthesi"ed and pac#aged withinsecretory $esicles as inacti$e proen"ymes%

trypsinogen chymotrypsin


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The two primary pancreatic proteases are trypsin andchymotrypsin.

They are synthesi"ed and pac#aged within secretory $esiclesas inacti$e proen"ymes%

trypsinogen chymotrypsin

The secretory $esicles also contain a trypsin inhibitor to ser$eas a safeguard against trypsinogen con$erted to trypsin.


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&ther Pancreatic Proteases

Procarboxypeptidasecarboxypeptidase

Proelastase elastase


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Trypsin

Trypsinogen is con$erted to trypsin by the

en"yme entero#inase (enteropeptidase

secreted by cells lining duodenum.


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Trypsin

Trypsinogen is con$erted to trypsin by the

en"yme entero#inase (enteropeptidase

secreted by cells lining duodenum. Trypsin then acti$ates the con$ersion of

other "ymogens from their inacti$e to acti$e

forms.


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Trypsin

Trypsinogen is con$erted to trypsin by theen"yme entero#inase (enteropeptidase

secreted by cells lining duodenum. Trypsin then acti$ates the con$ersion of

other "ymogens from their inacti$e to acti$eforms.

'nhibition of trypsin will slow acti$ation ofother proteases.


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Trypsin contd

Trypsin cataly"es the splitting of peptide

bonds on the carboxyl side of lysine and

arginine residues. 't has a p) optimum of *.+ to . (al#aline.

lassified as a serine protease (serine and

histidine at the acti$e site.


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Trypsin, hymotrypsin

/imilar chemical compositions

hief differences are specificity of action%

trypsin 0 lysine, arginine

chymotrypsin 0 tyrosine, phenylalanine, tryptophan,methionine,leucine

(aromatic or large hydrophobic side chains


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1oc# and 2ey 3odel of

!n"yme Acti$ity


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4isuali"ation of the 1oc# and

2ey 3odel of !n"yme 5unction


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1oc# 2ey !n"yme atalysis

The 6Acti$e /ite7 contains%

A shape that fits a specific substrate(s

/ide chains that attract (chemically thesubstrate

/ide chains that are positioned to speed thereaction


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!n"yme atalysis

-&) of serine 89:

attac#s ;& of peptide

bond. )istidine :*donates a proton to the

< of the peptide bond

leading to clea$age

and acylation of theen"yme. Asp-8= is

also in$ol$ed.


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arboxypeptidase

&&- terminal peptide bond )ydrolysis occurs most readily if the &&-

terminal residue has an aromatic or bul#y

aliphatic side chain. >inding of a typical substrate results in a

rearrangement of the acti$e site (induce fit.

?lutamate-=*, Arginine-8@:, Arginine-8=*,Tyrosine-=@ are important at the acti$e site.


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arboxypeptidase

) ) & ) &

' ' '' ' ''

< - - - < - -

' ' ' '

B ) )= &

' AB&3AT'

/'D! )A'owman->ir# inhibitor

>oth are inacti$ated during moist heat

treatment.

Protease inhibitors are proteins which bind

to the en"yme, rendering them inacti$e.

/ymptoms include pancreatic hypertrophy

due to stimulated secretory acti$ity.


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bsorption of peptides and amino acids

1ransport at the brush border7) ctive transport by carrier)

2) .ostly dependent on Na+ gradient - co-transport

similar to that for glucose

") &ome amino acids basic# and neutral 'ith

hydrophobic side chains are absorbed by facilitated

diffusion

Protein assimilation affected by -

Pancreatitis# congenital protease deficiencies#

deficiencies of specific transporters


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Absorption of Amino Acids

The transporters bind amino acids only after

binding sodium.

The fully loaded transporter undergoes aconformational change that dumps


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Absorption of Amino Acids

Absorption of amino acids is dependent on

the electrochemical gradient of


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Absorption of Peptides

There is $irtually no absorption of peptides

longer than three amino acids but there is

abundant absorption of di- and tri-peptides,probably by a single transport molecule.

The $ast bul# of di- and tri-peptides are

digested into amino acids by cytoplasmicpeptidases.


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Absorption of 'ntact Proteins

Absorption of intact proteins occurs rarely.

4ery few proteins can get through the

gauntlet of soluble (lumen and membrane-bound proteases intact.

6


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/hortly after birth, neonates can absorb

intact proteins.


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/hortly after birth, neonates can absorb

intact proteins.

3ost of these intact proteins areimmunoglobulins which can be absorbed

from the $ery first mil# (colostrum and this

imparts early neonatal passi$e immunity.


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/hortly after birth, neonates can absorbintact proteins.

3ost of these intact proteins areimmunoglobulins which can be absorbedfrom the $ery first mil# (colostrum and thisimparts early neonatal passi$e immunity.

6losure7 is when the small intestine losesthe capacity to absorb intact proteins.


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Protein BeFuirements

3aintenance ; nutritional reFuirements to

stay ali$e (does not reFuire positi$e >G

gain


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3aintenance ; nutritional reFuirements to

stay ali$e (does not reFuire positi$e >G

gain

?rowth ; positi$e tissue accretion


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3aintenance ; nutritional reFuirements to stay

ali$e (does not reFuire positi$e >G gain

?rowth ; positi$e tissue accretion

Beproduction ; tissue specific growth related toreproduction, reproducti$e function (mil#, eggs,

reproducti$e tissue


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Protein Re8uirements

0

10

20

30

40

50

60

1 3 5 7 911

13

15

17

19

21

23

25

27

ge

1issue9ro'th

Intestine

Muscle

Reproduction

Maintenance


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)ow do you express a protein

reFuirement H Protein percent of the diet


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Amino acid percent of the diet


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?rowth Gill Dictate 5eed 'nta#e

Body Wt

0

24

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7 8

ge

Body Wt


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'nta#e Gill Dictate Actual BeFuirement

0

5

10

15

20

25

1 2 3 4 5 6 7 8

ge

Dietary: % rotein

% !"ino !cid

Body Wt


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Amino acid percent of total protein


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Digestible protein percent of the diet


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Digestible Protein !stimates

Digestible protein ; inta#e 0 output

(amino acids


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(amino acids

Diet formulated to .9I T/AA

(methionine cystine


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(amino acids

Diet formulated to .9I T/AA

(methionine cystine

5eathermeal ; *I digestible methionine

5ishmeal ; 9I digestible methionine


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(amino acids

'ntestinal microbes will modify amino acid

composition of digesta


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(amino acids

'ntestinal microbes will modify amino acid compositionof digesta

!xcreta will reflect microbial as well as de no$o dietaryamino acid a$ailability


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Digestible Amino Acid !stimates

eacectomi"ed roosters 0 precision feeding

- total excreta collection, amino acid

determination


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Digestible Amino Acid !stimates

eacectomi"ed roosters 0 precision feeding,

ollect all excreta,amino acid determination

'leal Digesta 0 collect digesta from terminal

small intestine, non-digestible dietary

mar#er (cannula for pigs, terminalcollection for poultry


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Digestible Amino Acid Determination


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I Digestibility ;

AA J A'A (feed 0 AAJA'A (digesta

KKKKKKKKKKKKKKKKKKKKKKKKKKKKKK

AA J A'A (feed


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I Digestibility ;

A'A ; 8.:I feed, @.I digesta

methionine ; .:I feed, .=:I digesta

I A$. 3eth ; .:J8.: - .=:J@.

.:J8.:


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I Digestibility ;



I A$. 3eth ; .:J8.: - .=:J@. ; .LL - .+=:

.:J8.: .LL


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I Digestibility ;



I A$. 3eth ; .:J8.: - .=:J@. ; .LL - .+=:

.:J8.: .LL

8I Digestible 3ethionine


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'deal Protein ratios (relationships among

amino acids


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'deal Protein ratios (relationships among

amino acids

Protein or amino acid inta#eJday (gms


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9ro'th Response to 6ysine

00#2

0#4

0#6

0#81

1#2

1#4

1 2 3 4 5 6 7 8 9 10 11

*ody ;t

6ysine

0

50

100

150

200

9ro'th

$ysine

Body Wt


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0

20

40

60

80

100

120

140

160

180

1 2 3 4 5 6 7 8 9 10 11

0

5

10

15

20

25

Body Wt

Breast Wt


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Carcass Development

0

20

40

60

80

100

120

140

1 3 5 7 911

13

15

17

19

21

23

9e

Relative9ro'th s%eleton

"uscle

&at



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0

10

20

30

40

50

60

1 3 5 7 911

13

15

17

19

21

23

25

27

ge

1issue

9ro'th

Intestine

Muscle

Reproduction

Maintenance


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!conomics of Protein


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!conomics of Protein


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aloric cost of protein deposition

There is a genetic limit to protein accretion.


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The goal is to maximi"e muscle accretion

without feeding protein as an energy source.


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The goal is to maximi"e muscle accretion

without feeding protein as an energy source.

arbohydrate ; ,),& (@ calories

5at ; ,) (9 calories

Protein ; ,),< (@ calories


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>alancing diets, research and real world

5or a gi$en set of growing conditions, there

is an energetic mix of caloric sources that is

optimum


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5or a gi$en number of calories consumed%

on$erting protein to energy is

energetically inefficient and results inmetabolic heat production

(en$ironmental considerations


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5or a gi$en number of calories consumed%

!xcess calories relati$e to the animals

genetic capacity to synthesi"e protein will

increase carcass fat deposition (%P ratio


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oncept of caloric density%

Ghat is the proportion of total calories

coming from protein, fat, carbohydrate.

'socaloric diets - high fat, low fat


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Amino Acid >alance

)ow would an optimum balance of amino

acids be defined H


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Amino Acid >alance

)ow would an optimum balance of amino

acids be defined.

This Fuestion is outcome dependent.


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Amino Acid >alance

)ow would an amino

acid optimum be

defined H

&rder of limitation

influencing growth.

Diets selected amino

acids. 0

20

40

60

80

100

120

140

160

180

1 2 3 4 5 6 7 8 9 10 11

0

5

10

15

20

25

Body Wt

Breast Wt


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Amino Acid >alance

)ow would an aminoacid optimum bedefined H

&rder of limitationinfluencing growth.

omposition ofcarcass protein depots.

0

20

40

60

80

100

120

140

160

180

1 2 3 4 5 6 7 8 9 10 11

0

5

10

15

20

25

Body Wt

Breast Wt


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Amino Acid >alance

3inimi"ing ammonia production (alance

3inimi"ing ammonia production (G loss by P treatment ; + gms


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ommercial Application of P!B

/ome companies will use the P!B assay in

Fuality control assays for incoming

ingredients.


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ommercial Application of P!B

/ome companies will use the P!B assay inFuality control assays for incomingingredients.

These assays will often incorporate chic#sand utili"e one le$el of P, usually +I or9I.

'A3/ uses this assay to monitor theirincoming poultry byproduct meal.

Protein Digestion and Absorption

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