Sheet: Digestion of Lipids, Proteins,& Carbohydrates

Done by: Obadah Abubaker & Yousef Qandeel

In this lecture, we are going to talk about the biochemical aspects of digestion

of carbohydrates, lipids and proteins.

يال طالعة ...المحاضرة واللً بعدها سهالت وان شاء هللا ما بوخذوا منكم وقت كبٌر فتفاءلوا و هاي

So we will understand the process of digestion of the major dietry intake

conserning the lipids, proteins and carbohydrates.

Once you understand these processes and recognize the organs in which they

are taking place as well as the enzymes and end products of these processes

you will be able to practice any defect in the digestive process ,you will be able

to understand any disorder and what are the consequences of the rest order

in that process .

Lipid digestion

What is the major lipid component in our diet? Triacylglycerol (TAG)

Mostly we are taking TAG and it represents 90% of our dietary lipids intake,

and the 10% are composed of some fatty acids, cholesterol, cholesterol esters

and phospholipids.

*example of phospholipids:

-Phosphatidylcholine: it is a phosphatidic acid attached to it a base

choline, serine, ethanolamine, inositol … all these are bases that are attached

to phosphatidyl acid that will produce different types of phospholipids such as

phosphatidylserine, phosphatidylinositol, etc.

what is the phosphatidic acid?

الدكتور طلب نقرأ عنه... ونصٌحة للشٌت بشكل عام انه تركزوا على زبدة المحاضرة واالشٌاء اللً بتكون

موجودة بالسالٌدات والشرح مع بعض .

-Cholesterol ester: it is a cholesterol attached to it a fatty acid.

The main end products of the digestion of these lipids are:

- 2-monoacylglycerol - free fatty acids

- cholesterol - glycerylphosphoryl base

2-monoacylglycerol: it is a glycerol backbone and one fatty acid is esterified on

carbon number 2.

(glycerol has 3 carbon atoms )

glycerylphosphoryl base: we get this after we break the ester bond that binds

the phosphate in glycerophospholipid

this table summarizes the process:

*You are supposed to memorize the enzymes in each organ.

In mouth, lipid digestion will not take place, it will take place in stomach and

small intestine.

-Only 30% of the lipids are digested in stomach. Why?

Because lipases are not active at low pH.

**In the intestine

The enzymes which are involved in small intestine are pancreatic enzymes.

Co-lipase is not an enzyme, it is a protein that will be attached to lipase during

Emulsification and digestion, and without this co-lipase, lipase will be inactive.

-The substrate of cholesterol esterase is cholesterol ester that (cholesterol

esterase) will hydrolyze the ester bond and release the fatty acid and

cholesterol.

-The substrate of phospholipase A2 is phospholipid and usually it hydrolysis the

fatty acid which is esterified at carbon number 2.

-The substrate of lysophospholipase is *lysophospholipid that is going to

hydrolyze the ester bond that links the fatty acid at carbon number 1 and

produces phosphatidic acid.

*lyzophospholipid is a type of phospholipid that is hydrolyzed at carbon

number 2 and has a fatty acid that is esterified at carbon number 1

مالحظة : هذه الصور ليست ضمن ساليدات

الدكتور هي فقط للتوضيح ال أكثر , اذا عجقتك

اتركها .

Concerning the lingual and gastric lipases , only they are important to hydrolyze

fatty acids of short or medium carbon atoms, which are found usually in milk

,so they are important in neonates and infants because of the milk lipid

digestion, but they are not working in the adult stage.

so many lipids consist of short or medium carbon atom fatty acids, while the

adult Triglyceride intake is composed of long chain carbon atom fatty acids

(such as stearic acid, linoleic acid, palmitic acid, etc.)

In order for lipases to work there must be emulsification process

Emulsification process: bile salts will increase the surface area of interaction

between the lipase , co-lipase and the lipids (lipids are amphipathic)

And by that letting the lipase, co-lipase hydrolyzing the lipids (like TAG)

Note: Lingual & Gastric Lipases act only on short and medium chain fatty acids therefore, their effect on TAG has little significance in adults. But important for digestion of milk fat in neonates and infants because

Milk fat has short and medium chain fatty acids.

After emulsification we’ll have what is called “mixed micelle” which has

increase surface area.

فٌها زٌت و ماء اذا حركناها رح ٌختلط الزٌت بالمً على شكل قطرات صغٌرة و هاي هاي شبه كاسة

و مساحة السطح الها بتزٌد بانه بدل ما تكون قطعة وحدة mixed micelleالقطرات الصغٌرة هً ال

كبٌرة بتصٌر قطع صغٌرة.

*Bile salt = bile acid + glycine or taurine

*Cholesterol is the source of bile acid

so we have to sites :-

1- stomach (lingual and gastric lipases important for infants because it’s

specific to hydrolyze short and medium chain fatty acids)

2- small intestine (four enzymes from pancreas that digest different types of

lipids)

substrates

products

for long chain fatty acids to enter systemic circulation it must be absorbed to

the cells in small intestine, re-synthesis of TAG, assembly & secretion of

chylomicrons

so first thing is the absorption of fatty acids by the epithelial cells after the TAG

is hydrolyzed into 2-monoacylglycerol and fatty acid (as you see in the previous

table) then in the intestinal cells what happens is re-synthesizing of TAG by first

forming of fatty acyl CoA (activation of FA) then they will be linked to

2-monoacylglycerol.

Now TAG, cholesterol, and phospholipids will be assembled into chylomicrons

that will enter the lymphatic system and then to bloodstream.

for better understanding watch this video:

https://www.youtube.com/watch?v=i0kHf_5s3J8

Proteins are hydrolyzed into single amino acids by protease (not denatured)

*renin found in infants and it’s for the digestion of casein in milk.

and in adults we have pepsin to hydrolyze proteins into oligopeptides

*pancreas synthesis proteases as zymogens (inactive protease) that needs

activation and these zymogens are chymptrypsinogen, proelastase, and

procaboxypeptidase A&B.

Protease are either endoprotease or exoprotease

Endoprotease(enteropeptidase) :- hydrolyze internal peptide bonds in

oligopeptides

Exoprotease(exopeptidase) :- hydrolyze peptide bonds from the ends

(N-terminal & C-terminal)

In order for these pancreatic enzymes to function they need to be activated

first by covalent modification (cleavage) and they are

1-trypsinogen trypsin

2- chymotrypsinogen chymotrypsin

3- proelastase elastase

4- procaboxypeptidase A&B caboxypeptidase A&B

Each one of these enzymes hydrolyze proteins at specific site for example

trypsin breaks the bond after basic AA and chymotrypsin after aromatic AA.

and they all have an active site where substrates bind and the catalysis

happen.

After digestion of proteins the products are amino acids and Di & Tri-peptides

and there are some mechanisms to transport those amino acids using

gamma-glutamyl cycle.