computing Chapter 4 - ajums.ac.ir• detects any reducing sugar Glucose oxidase • can be quantitative •specific for glucose • false negative with vitamin C Branched structure

Chapter 4

computing

Presentation copyright © 2002 David A Bender and some images copyright © 2002 Taylor & Francis Ltd

Introduction to Nutrition and Metabolism, 3rd edition

David A Bender Taylor & Francis Ltd, London 2002

Chapter 4: Digestion and Absorption

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The gastro-intestinal tract - 1

computing


oesophagus

ileum

rectum

duodenum

pancreas

caecum

appendix

gall bladder

liver

The gastro-intestinal tract - 2

computing


salivary amylase

lingual lipase

gastric acid

pepsin

gastric lipase

alcohol absorption

bacterial fermentation

absorption of water

pancreatic amylase

lipase, phospholipase

trypsin, chymotrypsin, elastin

dipeptidases

disaccharidases

absorption of:

monosaccharides

amino acids

fatty acids, glycerol, fats

water

The intestinal mucosa - 1

computing


muscle

villi


computing


muscle

villi

Intestinal villi

computing


muscle

villi

arterial blood supply venous drainage lymphatic drainage

lacteal

cell proliferation in crypt

cells shed at tip of villus

absorptive

enterocyte

mucus secreting

goblet cell


computing


muscle

villi

mucus secreting

goblet cell

Carbohydrates

computing


Carbohydrates

No requirement since can be made in the body

average diets provide 45% energy from carbohydrate

ideally this should be increased to 55%

composed of carbon, hydrogen and oxygen only

in the ratio C6H12O6

Nutritional classification of carbohydrates

computing


Nutritional classification of carbohydrates

sugars

monosaccharides

disaccharides

polysaccharides oligosaccharides sugar

alcohols

non-starch

polysaccharides

dextrins

trisaccharides

tetrasaccharides

intrinsic

sugars

extrinsic sugars

in free solution

lactose

in milk

non-milk

extrinsic

starch

Nutritionally important monosaccharides - 1

computing


Nutritionally important monosaccharides

O

OH

OH

CH2OH

OH

H

H

H

HC O

C

CH

OH

C

HO

C

OH

CH2OH

OH

glucose

1

2

3

4

5

6

OH

galactose

H O

OH

OH

OH

O

CH2OH

H

H

HC O

C

CH

OH

CH

HO

C

CH2OH

OH

HO

fructose

HOCH2 O CH2OH

OH

O

OH

H

H

CH2OH

C

CH

O

C

HO

C

OH

CH2OH

OH

H

1

2

3

5

6

4

Nutritionally important monosaccharides - 2

computing


Nutritionally important monosaccharides

H

H

H

HC O

C

CH

OH

C

HO

C

OH

CH2OH

OH

O

OH

OH

CH2OH

OH

glucose

1

2

3

4

5

6

OHribose deoxyribose

HC O

C

C

OH

C

OH

CH2OH

OH

H

H

H

O OH

OHOH

HOCH2

HC O

CH

C

C

OH

CH2OH

OH

H

H

H

O OH

OH

HOCH2

Nutritionally important disaccharides - 1

computing


Nutritionally important disaccharides

maltose (glucosyl-glucose)lactose (galactosyl-glucose)

sucrose (glucosyl-fructose)

O

OH

OH

O

CH2OH

H

HOCH2 O CH2OH

OH

O

OHO

O

OH

OH

OH

CH2OH

O

OH

OH

O

CH2OH

H

O

O

OH

OH

OH

CH2OHCH2OH

O

OH

OH

O

O

H

O

OH

OH

OH

CH2OH

CH2OH

O

OH

OH

O

OH

trehalose (glucosyl-glucoside)

H

Nutritionally important disaccharides - 2

computing


Nutritionally important disaccharides

isomaltosemaltose (glucosyl-glucose)

O

OH

OH

OH

CH2OHCH2OH

O

OH

OH

O

O H

H

CH2OH

O

OH

OH

O

O

CH2

O OH

OH

OH

O

H

Measurement of plasma or urine glucose

computing


Measurement of plasma or urine glucose

HC O

C

CH

C

C

CH2OH

OH

OH

OH

HO

H

H

H

COOH

C

CH

C

C

CH2OH

OH

OH

OH

HO

H

H

H

glucose gluconate

alkaline copper reagent

Cu++

Cu2O

(red-brown precipitate)

O2

H2O2

ABTS (colourless)

oxidised ABTS (blue)

H2O

glucose oxidase

peroxidase

Alkaline copper reagent

• only semiquantitative

• detects any reducing sugar

Glucose oxidase

• can be quantitative

• specific for glucose

• false negative with vitamin C

Branched structure of starch and glycogen

computing


The branched structure of starch and glycogen

16 links: branch points inamylopectin and glycogen O

OH

OH

CH2OH

O

OH

OH

CH2OH

O O

O

OH

OH

CH2OH

O

OH

OH

CH2

O O O

O

O

O

OH

OH

CH2OH

O

O

OH

OH

CH2OH

Hydrolysis of starch by amylase

computing


Hydrolysis of starch by amylase

in saliva and pancreatic juice results in formation of dextrins,

then: glucose

maltose

isomaltose

16 links: branch points inamylopectin and glycogen O

OH

OH

CH2OH

O

OH

OH

CH2OH

O O

O

OH

OH

CH2OH

O

OH

OH

CH2

O O O

O

O

O

OH

OH

CH2OH

O

O

OH

OH

CH2OH

Major types of non-starch polysaccharides

computing


cellulose - glucose polymer linked 14

O

OH

OH

CH2OH

O

O

OH

OH

CH2OH

O

OH

OH

CH2OH

O

OH

OH

CH2OH

O O O O

chitin - N-acetylglucosamine polymer linked 14

O

OH

CH2OH

O

O

OH

CH2OH

O

OH

CH2OH

O

OH

CH2OH

O O O O

HN C CH3

O

HN C CH3

O

HN C CH3

O

HN C CH3

O

pectin - galacturonic acid polymer linked 14, partially methylated; some glactose and/or arabinose branches

O

OH

OH

COOH

O

OH

OH

COOH

O

OH

OH

CO CH3

O

OH

OH

COOH

O O O OO

O CH2

OH

OH

O

HOCH2

O CH2

OH

OH

HOCH2

O CH2

OH

OH

HOCH2

O CH2

OH

OH

HOCH2

O

O

O

O

inulin - fructose polymer linked 21

The major types of non-starch polysaccharide

Glycaemic and non-glycaemic

carbohydrates - 1

computing


Dietary starches can be classified as:

rapidly digested (hence high glycaemic index)

slowly digested (lower glycaemic index)

not all digested in small intestine

amylose is hydrolysed more slowly than amylopectin

resistant starch (low glycaemic index)

only hydrolysed to a limited extent in small intestine

starch may be resistant because:

it is crystalline and resistant to amylases

it is enclosed in plant cell walls that are not digested

some resistant starch is fermented by bacteria in the large intestine

Glycaemic and non-glycaemic carbohydrates

Glycaemic index

The extent to which a test dose of a carbohydrate increases blood glucose

compared with an equivalent amount of glucose

Glycaemic and non-glycaemic

carbohydrates - 2

computing


sugars

monosaccharides

disaccharides

polysaccharides oligosaccharides sugar

alcohols

non-starch

polysaccharides

dextrins

trisaccharides

tetrasaccharides

intrinsic

sugars

extrinsic sugars

in free solution

lactose

in milk

non-milk

extrinsic

starch

Glycaemic and non-glycaemic carbohydrates

Disaccharidases and monosaccharide

absorption

computing


inside mucosal cell mucosal cell membrane intestinal lumen

Na+

Na+

glucosegalactose

glucosegalactose

glucose transporter

maltose

2 x glucose

maltase

glucose + galactose

lactose

lactase

trehalose

2 x glucose

trehalase

sucrase fructose + glucose

sucrose

Na+ pump & Na+/K+ ATPase Na+ Na+

Monosaccharide absorption

computing


inside mucosal cell mucosal cell membrane intestinal lumen

Passive diffusion

(carrier mediated)

fructose fructose

monosaccharides monosaccharides

sugar alcohols sugar alcohols

The importance of dietary fat

computing


There is no absolute requirement for fat,

apart from small amounts of two polyunsaturated fatty acids

Fat provides 39 kJ /gram (cf protein 17 and carbohydrate 16 kJ /g)

at relatively high levels of intake it is easy to overeat

at very low levels of intake it is difficult to meet energy needs

the requirement is probably 10-15% of energy intake

Fat lubricates food in the mouth,

making chewing and swallowing easier

Much of the flavour in foods (especially meat) is in the fat

Vitamins A, D, E and K are present in dietary fat

and require fat for absorption

The importance of dietary fat

Triacylglycerols

computing


H2C

CH

C

O

O

O

C

C

C

(CH2)n

O

(CH2)n

(CH2)n

O

O CH3

CH3

CH3H2

Triacylglycerols

3 x fatty acids esterified to glycerol

CH

CH2

CH2

O

O

O

O

O

O

Saturated and unsaturated fatty acids

computing


polyunsaturated fatty acid (linoleic acid, C18:2 6)

mono-unsaturated fatty acid (oleic acid, C18:1 9)

saturated fatty acid (stearic acid, C18:0)

O

OH

O

OH

O

OH


Methylene interrupted double bonds

computing



Methylene interrupted double bonds

CH2

CH

CH

CH2

CH

CH

CH2

as opposed to conjugated double bonds

Cis- and trans-double bonds

computing


cis

trans

Most unsaturated fatty acids are in the cis-configuration

trans-fatty acids occur as a result of:

• rumen bacterial fermentation – hence in beef, lamb, dairy produce

• isomerization during catalytic hydrogenation of oils


cis- and trans-double bonds

Fatty acid nomenclature

computing


OH

O

C18:2 6 (linoleic acid)

OH

O

C18:3 3 (-linolenic acid)

6

O

OH9

C18:1 9 (oleic acid)

Fatty acid nomenclature

Saturated fatty acids

computing


Saturated fatty acids

no of C double bonds first C=C shorthand

butyric 4 0 - C4:0

caproic 6 0 - C6:0

caprylic 8 0 - C8:0

capric 10 0 - C10:0

lauric 12 0 - C12:0

myristic 14 0 - C14:0

palmitic 16 0 - C16:0

stearic 18 0 - C18:0

arachidic 20 0 - C20:0

behenic 22 0 - C22:0

lignoceric 24 0 - C24:0

Mono-unsaturated fatty acids

computing



palmitoleic 16 1 6 C16:1 6

oleic 18 1 9 C18:1 9

cetolic 22 1 11 C22:1 11

nervonic 24 1 9 C24:1 9

Mono-unsaturated fatty acids

Polyunsaturated fatty acids

computing



linoleic 18 2 6 C18:2 6

-linolenic 18 3 3 C18:3 3

g-linolenic 18 3 6 C18:3 6

arachidonic 20 4 6 C20:4 6

eicosapentaenoic 20 5 3 C20:5 3

docosatetraenoic 22 4 6 C22:4 6

docosapentaenoic 22 5 3 C22:5 3

docosapentaenoic 22 5 6 C22:5 6

docosahexaenoic 22 6 3 C22:6 3

Polyunsaturated fatty acids

Phospholipids

computing


OH

OHHO

HO OH

phosphatidylserine phosphatidylethanolamine phosphatidylcholine(lecithin)

phosphatidylinositol

CH2 CH

COO-

NH3+ CH2 CH2 NH3

+ CH2 CH2 N+

CH3

CH3

CH3

Major water-soluble groups in phospholipids

P O-O

O

O

R

O

O

CHO

CH2

CH2

O

Phospholipids

Phospholipids in cell membranes

computing


Phospholipids in cell membranes

outside

inside

cell surface protein

intracelluar

membrane protein

transmembrane

receptor protein

transmembrane protein

forming a transport pore

Cholesterol and steroid hormones

computing


O

O

HO OH

O

O

HOCH2

cortisol

OH

O

testosterone

progesterone

OH

HO

oestradiol

HO HO HO

cholesterol -sitosterol cholestanol

Cholesterol and some steroid hormones

Triacylglycerol digestion - 1

computing


O

O

O

CH

CH2

CH2

O

O

O

triacylglycerol – insoluble in water

glycerolHO CH

CH2

CH2

OH

OH

O

OH

O

OH

O

OH

free fatty acids

Triacylglycerol digestion

Cholesterol and bile salt metabolism - 1

computing


cholesterol secretion 2g /daycholesterol

cholesterol synthesis

conjugates

primary bile salts

cholesterol

bile salt conjugates 30 g /day

bacterial deconjugation

cholate + chenodeoxycholate

bacterial metabolism

deoxycholate + lithocholate

dietary cholesterol

0.5 g/day

faecal steroids

+ bile salts

1 – 2 g /day

Cholesterol and bile salt metabolism

Cholesterol and bile salt metabolism - 2

computing


glycochenodeoxycholic acidtaurochenodeoxycholic acid

glycocholic acidtaurocholic acid

conjugation with glycine or taurine

HO OH

OHCOOH

chenodeoxycholic acid cholic acid

cholesterol

HO OH

COOHHO

HO

COOH

HO

OHCOOH

deoxycholic acidlithocholic acid

intestinal bacterial deconjugation and metabolism

C

R

N

O

C

H

COO-

H

H

C

R

N

O

C

H

C

H

H

SO3-

H

H

glycine conjugates

taurine conjugates

Cholesterol and bile salt metabolism

Incidence of gallstones with obesity

computing


0

1

2

3

4

5

6

7

8

rela

tiv

e r

isk

<24 24.5 25.5 26.5 28 29.5 32.5 37.5 42.5 >45

body mass index

The incidence of gallstones with obesity

Triacylglycerol digestion - 2

computing


H2C

CH

C

O

O

O

C

C

C

(CH2)n

O

(CH2)n

(CH2)n

O

O CH3

CH3

CH3H2

H2O

triacylglycerol

diacylglycerol

lipase

H2C

CH

C

O

OH

O C

C(CH2)n

(CH2)n

O

O

CH3

CH3H2

CH3 (CH2)n COOH

free fatty acid

free fatty acidCH3 (CH2)n COOH

H2C

CH

C

O

OH

OH

C(CH2)n

O

CH3

H2

lipase

monoacylglycerol

H2O

H2C OH

CH

C OH

OH

H2

H2O

pancreatic esterasesand intracellular lipase

glycerol

CH3 (CH2)n COOH

free fatty acid

lipases:

hydrolyse ester bonds in triacylglycerol

• lingual lipase – secreted by tongue

• gastric lipase – secreted in stomach

• pancreatic lipase – secreted by pancreas

Formation of lipid micelles

computing


Due to the emulsifying action of:

free fatty acids

diacylglycerols

monoacylglycerols

phospholipids

bile salts

hydrophilic group

hy

dro

ph

ob

ic t

ail

hydrophobic core contains:

cholesterol

fat-soluble vitamins (A, D, E and K)

carotenes

other lipids

The formation of lipid micelles small enough to be absorbed

Re-esterification in the intestinal mucosa

computing


CoASH

CH3 (CH2)n COOH

ATP

AMP + pyrophosphate

CH3 (CH2)n C SCoA

O

fatty acid

fatty acyl CoA

the reaction of acyl CoA synthase

NAD+NADH

glycerol 3-phosphate dehydrogenase

dihydroxy-acetonephosphate

H2C OH

C

C O P

O

H2

glycerol phosphate

H2C OH

CH

C O P

HO

H2

CoASH

fatty acyl CoA

monoacylglycerol phosphate

H2C O

CH

C O P

C (CH2)n

O

CH3

HO

H2

diacylglycerol phosphate

CoASH

fatty acyl CoA

H2C O

CH

C O P

C (CH2)n

O

CH3

OCO

O

(CH2)nCH3

H2

H3PO4

triacylglycerol

CoASH

fatty acyl CoA

H2C O

CH

C O C

C (CH2)n

O

CH3

O

(CH2)n

O

CH3

CO

O

(CH2)nCH3

H2

H2O

Re-esterification in the intestinal mucosa

Assembly of chylomicrons

computing


Re-esterified triacylglycerols are packaged with:

proteins

phospholipids

cholesterol and cholesterol esters

carotenes and fat-soluble vitamins

to form chylomicrons

85 – 90% triacylglycerol

75 – 1200 nm diameter

Chylomicrons are absorbed into the lymphatic system

enter the bloodstream at the thoracic duct

Tissues take up fatty acids from triacylglycerol

by action of lipoprotein lipase at cell surface

Chylomicron remnants are cleared by the liver

Assembly of chylomicrons

Protein is 13 – 15% of body mass

computing


water

64%

essential fat

3%storage fat

12%protein

15%

minerals

6%

water

54%essential fat

9%

storage fat

19%

protein

13%

minerals

5%

Protein is 13 – 15% of body mass

The amino acids

computing


N COO-

glycine (Gly, G) alanine (Ala, A) proline (Pro, P)

small neutral amino acids

large neutral amino acids

*methionine (Met, M)

*leucine (Leu, L) *valine (Val, V)

*isoleucine (Ile, I)

branched-chain amino acids

aromatic amino acids

*phenylalanine (Phe, F)

tyrosine (Tyr, Y)

hydrophobic amino acids

basic amino acids

*lysine (Lys, K)

arginine (Arg, R)

*histidine (His, H)

amino acid amides

asparagine (Asn, N) glutamine (Gln, Q)

acidic amino acids

aspartate (Asp, D) glutamate (Glu, E)

neutral hydrophilic amino acids

hydrophilic amino acids

serine (Ser, S) *threonine (Thr, T) cysteine (Cys, C)

*tryptophan (Trp, W)

H

HCH

NH3+

COO-

CH

NH3+

COO-

CH2CHH3C

CH3

CH

NH3+

COO-

H3C

CH

NH3+

COO-

CH2CH2CH2CH2+H3N

CH

NH3+

COO-

CHH3C

CH3

CH

NH3+

COO-

CHCH2

CH3

H3CCH

NH3+

COO-

CH2CH2SH3C

CH

NH3+

COO-

CH2

CH

NH3+

COO-

CH2HO

N

CH

NH3+

COO-

CH2

CH

NH3+

COO-

CH2HO CH

NH3+

COO-

CH2HSCH

NH3+

COO-

CHH3C

OH

CH

NH3+

COO-

CH2CH2-OOCCH

NH3+

COO-

CH2-OOC

CH

NH3+

COO-

CH2CH2N

O

CH

NH3+

COO-

CH2CH2CH2N

O

CH

NH3+

COO-

CH2

N N

CH

NH3+

COO-

CH2CH2CH2NHCHN

NH2

HH

The amino acids (* indicates dietary essential amino acids)

Formation of the peptide bond

computing


H2O

C CN N C C

H

COOHH2N

H O R2

R1 H H O

C CN

H

H2N OH

H O

R1

N C C COOH

H R2

H H O

+

H2Ocondensation

hydrolysis

Formation of the peptide bond

Hydrogen bonds between peptide bonds in a peptide chain

computing


C CN N C C

H

COOHH2N

H O R2

R1 H H O

CC NNCC

H

COOH NH2

HOR2

RHHO

CC NNCC

H

COOH NH2

HOR2

RHHO

Hydrogen bonds between peptide bonds in a peptide chain

Side-chain links between peptide chains

computing


CH2

CH2

NH2

CH2

CH2

CHHN CO

CHHN

CH2

CO

CH2

COOH

H2O

CH2

CH2

NH

CH2

CH2

CHHN CO

C

CH2

O

CH2

CH COHN

glutamate

lysine

condensation

CHHN

CH2

CO

SH

CHHN

CH2

CO

SH

cysteine

cysteine

XXH2

oxidation

CHHN

CH2

CO

S

S

CH2

CHHN CO

cystine

Side-chain links between peptide chains

Native proteins are relatively resistant to enzymic hydrolysis

computing


Native proteins are relatively resistant to enzymic hydrolysis

the first step in digestion is disruption of H-bonds:

heat

gastric acid

so as to leave a straight peptide chain

that is accessible to digestive enzymes

C CN N C C

H

COOHH2N

H O R2

R1 H H O

CC NNCC

H

COOH NH2

HOR2

RHHO

CC NNCC

H

COOH NH2

HOR2

RHHO

C CN N C C

H

COOHH2N

H O R2

R1 H H O

CC NNCC

H

COOH NH2

HOR2

RHHO

CC NNCC

H

COOH NH2

HOR2

RHHO

Hydrolysis of the peptide bond

computing


H2O

C CN

H

R1

O

N C

H H

R2

C

O

H

COOHH2N

C CN

H

R1

OH

H2N OH N C

H H

R2

C

O

COOH

H

In vitro 10 – 12 hours in 12 mol /L HCl at 105ºC, random hydrolysis of peptide bonds

In vivo 1 – 2 hours at 37ºC, specific bonds hydrolysed

Hydrolysis of the peptide bond

The serine proteases, chymotrypsin, trypsin and elastase - 1

computing


Bonds hydrolysed:

trypsin

esters of basic aa

chymotrypsin

esters of aromatic aa

elastase

esters of small neutral aa

The serine proteases, chymotrypsin, trypsin and elastase

The serine proteases, chymotrypsin, trypsin and elastase - 2

computing


substrate sits in a groove

on the enzyme surface

bond to be cleaved

lies over catalytic site


Trypsin

computing


-

Gly

Gly

Asp

-

+

peptide in groove on enzyme surface

trypsin

Bonds hydrolysed:

trypsin

esters of basic amino acids

chymotrypsin

esters of aromatic amino acids

elastase

esters of small neutral amino acids


Chymotrypsin

computing


Gly

Gly

Ser


chymotrypsin

Bonds hydrolysed:

trypsin


chymotrypsin


elastase



Elastase

computing


Val

Thr

Gly


elastase

Bonds hydrolysed:

trypsin


chymotrypsin


elastase



Endopeptidases

computing


Endopeptidases

hydrolyse specific peptide bonds within the protein chain

pepsin gastric juice amides of Phe, Tyr, Trp, Leu, Met

enteropeptidase succus entericus trypsinogen trypsin

trypsin pancreatic juice Arg, Lys esters

chymotrypsin Phe, Tyr, Trp esters

elastase neutral aliphatic esters

pancreatic juice

pancreatic juice

Exopeptidases

computing


Exopeptidases

hydrolyse specific peptide bonds of terminal amino acids

dipeptidases brush border various dipeptides

carboxypeptidase A pancreatic juice C-terminal large neutral aa

carboxypeptidase B pancreatic juice C-terminal Lys, Arg

aminopeptidases various N-terminal aa succus entericus

Peptidases are secreted as inactive

zymogens

computing


Peptidases

secreted as inactive zymogens and activated after secretion

pepsinogen pepsin gastric acid, then pepsin

trypsinogen trypsin enteropeptidase

chymotrypsinogen chymotrypsin trypsin

pro-elastase elastase trypsin

procarboxypeptidases carboxypeptidase trypsin

pro-aminopeptidases aminopeptidases trypsin

substrate-binding cleft

masked by terminal region of protein

in zymogen

End

computing


Introduction to Nutrition and Metabolism, 3rd edition

David A Bender Taylor & Francis Ltd, London 2002

Chapter 4: Digestion and Absorption

End of presentation The peptide sequence simulation on the CD accompanies this Chapter

Documents

computing Chapter 4 - ajums.ac.ir• detects any reducing sugar Glucose oxidase • can be quantitative •specific for glucose • false negative with vitamin C Branched structure