Lectures 12-18 2014 Human Biochemistry

Lecture 12: Fat and Cholesterol Metabolism !!� !• Lipogenesis is not very active in people on a Western diet!• lipogenic enzyme expression is down regulated by fat consumption!• most of our fat comes from the diet ~100g/day!• most fat in WAT will have come from dietary fat and not de novo lipogenesis!• fat is hydrophobic; causing problems for digestion and transport !• digestive enzymes need fat to be in an emulsion!• fat needs to be carried around the bloodstream within lipoproteins (lipid droplets)!

� !• emulsions are small droplets of fat surrounded by a casing of a molecule that is ampiphilic !• an ampiphile is a term describing a chemical compound possessing both hydrophilic and

lipophilic properties, such a compound is called ampiphilic or ampipathic!• i.e. they are molecules that have hydrophilic and hydrophobic characteristics!• phospholipids are the most common emulsions, they are found in cell membranes!• amphiphilic molecules act as detergents that emulsify fat into tiny particles e.g. micelles!• so when you put fat in water, little micelles (spheres) in which the middle of the sphere is very

oily and the outside is very hydrophilic, happily interacting with the outside environment!• to clarify, micelles are lipid molecules that arrange themselves in a spherical form in aqueous

solution!

� !• micelles and phospholipid bilayers are both examples of emulsions!• consider the diagram below; in both structures, polar heads are facing the aqueous

environment while the hydrophobic tails are buried in the core!• micelles can also be formed using bile salts (bile salts are ampiphilic)!

� !

� !• Fat is contained in the core of micelles that form with bile salts (i.e. fat on the inside and bile

salts on the outside, proving a great big surface area for digestion)!

Dietary Fat

Formation of Emulsions

Bilayers & Micelles

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WARNINGThis material has been reproduced and communicated to you by or on behalf of the

University of Sydney pursuant to Part VB of the Copyright Act 1968 (the Act).The material in this communication may be subject to copyright under the Act. Anyfurther reproduction or communication of this material by you may be the subject of

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Fat and CholesterolMetabolism

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Dietary Fat

• Lipogenesis is not very active in people on aWestern diet

– Lipogenic enzyme expression is down-regulatedby fat consumption

– Most of our fat comes from the diet ~100g/day

– Most fat in white adipose tissue will have come from dietaryfat and not de novo lipogenesis

• Fat is hydrophobic

– Problems for digestion and transport

– Digestive enzymes need fat to be in an emulsion

– Fat needs to be carried around the bloodstreamwithin lipoproteins

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Formation of Emulsions

• Molecules have hydrophilic andhydrophobic characteristics– amphiphilic

– amphipathic

• Phospholipids in cell membranes– As is the phosphatidic acid and

lysolecithin in salad dressing

• Amphiphilic molecules act asdetergents– emulsify fat into tiny ‘particles’

– micelles

Bilayers and Micelles

• In both structures, polar heads are facing theaqueous environment while the hydrophobic tailsare buried in the core

• Micelles can also be formed using bile salts

Fat Digestion

• Fat is contained in the core of micelles that formedwith bile salts

• Churning of dietary fat with bile salts in the intestine– Chyme

– Emulsion

– Easy for lipase to interact with

• Pancreatic Lipase– Hydrolyses fat into FA and glycerol

– Plus mixture of mono- and di-acyl glycerols

Bile Salts

• Produced in the liver

• Made from cholesterol

– Cholesterol itself it not amphiphilic enough to be adetergent - needs modification by addition of polar groups

• Stored in the gall bladder

• After digestion of fat

– Reabsorbed and taken back to the liver

– Via hepatic portal vein

Bile Salts

• Polar groups are added to cholesterol to make it moreamphiphilic

• The only way to get rid of cholesterol is to make them intobile salts (chol cannot be oxidised)

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Undigested Fat

• If gall bladder is blocked by gall stones– no bile salts secreted � less fat digestion � lower

calorie intake

• Inhibitors of fat digestion as weight-loss drugs– Orlistat (brand name Xenical)

– Listed side effects include• oily spotting; orange colored oil in your stool;

• gas with oily discharge;

• an urgent need to go to the bathroom; an inability to control bowelmovements, an increased number of bowel movements.

Olestra

• Olestra (Olean) is a fat substitute– FA attached to sucrose

– Not attacked by lipases• Olestra will passes through gut undigested

– Carries with it fat soluble vitamins

• D, E, K need to be added as supplements

– See anti-Olestra sites

– eg. www.cspinet.org/olestraolestra/

Amusing Aside…

• From Wikipedia entry….

• Because olestra is made by chemically combining sugar and vegetable oil, it releasesno toxic fumes and could potentially become a safe and environmentally friendlyreplacement for petrochemicals

• It is currently used as a base for deck stains and a lubricant for small power tools, andthere are plans to use it on larger machinery

Lipoproteins• Mixture of phospholipid and Apoproteins

– Apoproteins role - Enzymes, Structural, Docking

• Different types of lipoproteins characterised by size and by types ofapoproteins

• First lipoproteins made by intestinal cells– Chylomicrons

– Enter lymphatic system

• Contains fat and cholesterol esters

Bile salts are amphiphilic

Fat Digestion

Page ! of !1 46

Lectures 12-18 2014 Human Biochemistry

� !• if gall bladder is blocked by gall stones (blockage of bile duct) and no bile salts are discrete, then

there is less fat digestion, equating to a lower calorie “intake” (less calories from fat get assimilated)!

• i.e. fat will go straight to the large intestine, producing lots of gas and oily diarrhoea as a result!• poses the effect of inhibitors of fat digestion as “weight loss drugs” e.g. orlistat (brand name:

Xenical)!• probably the best thing about these products is they teach you to avoid fatty foods in the first

place because the nasty side effects are so bad!!!

� !• olestra (olean) is a fat substitute which tastes and behaves in cooking like ordinary fat, but

doesn’t get attacked by pancreatic lipase in digestion!• olestra is FA attached to sucrose!• if it is not attacked by lipases, it will “pass straight through you” undigested!• however, it carries fat soluble vitamins with it (A, D, E, K), so these would need to be taken as

supplements!• however you would still get all the nasty side effects as mentioned above!!

� !• once fat is digested, it gets packaged up!• lipoproteins are mixtures of phospholipids and

apoproteins!• different types of lipoproteins exist and are

characterised by size and by types of apoproteins they contain!

• the first lipoproteins made by intestinal cells after fat is digested: chylomicrons (enter lymphatic system)!

• chylomicrons contain fat and cholesterol esters!!(See diagram below) - cholesterol needs to be esterified and packaged in a chylomicron (i.e. “wack” a FA onto a

cholesterol)!- this lipoprotein structure allows us to move all our fatty hydrophobic molecules around the

blood stream!- After you have eaten a fatty meal, it takes a couple of hours for chylomicrons to appear at a

max concentration in the blood - they are very big and make plasma look very opaque !!

Undigested Fat

Fat Substitute - Olestra

Lipoproteins

Page ! of !3 46

Copyright

COMMONWEALTH OF AUSTRALIACopyright Regulation

WARNINGThis material has been reproduced and communicated to you by or on behalf of the

University of Sydney pursuant to Part VB of the Copyright Act 1968 (the Act).The material in this communication may be subject to copyright under the Act. Anyfurther reproduction or communication of this material by you may be the subject of

copyright protection under the Act.

Do not remove this notice

Fat and CholesterolMetabolism

Loading...

www.taste.com.au

Dietary Fat

• Lipogenesis is not very active in people on aWestern diet

– Lipogenic enzyme expression is down-regulatedby fat consumption

– Most of our fat comes from the diet ~100g/day

– Most fat in white adipose tissue will have come from dietaryfat and not de novo lipogenesis

• Fat is hydrophobic

– Problems for digestion and transport

– Digestive enzymes need fat to be in an emulsion

– Fat needs to be carried around the bloodstreamwithin lipoproteins

Loading...

Formation of Emulsions

• Molecules have hydrophilic andhydrophobic characteristics– amphiphilic

– amphipathic

• Phospholipids in cell membranes– As is the phosphatidic acid and

lysolecithin in salad dressing

• Amphiphilic molecules act asdetergents– emulsify fat into tiny ‘particles’

– micelles

Bilayers and Micelles

• In both structures, polar heads are facing theaqueous environment while the hydrophobic tailsare buried in the core

• Micelles can also be formed using bile salts

Fat Digestion

• Fat is contained in the core of micelles that formedwith bile salts

• Churning of dietary fat with bile salts in the intestine– Chyme

– Emulsion

– Easy for lipase to interact with

• Pancreatic Lipase– Hydrolyses fat into FA and glycerol

– Plus mixture of mono- and di-acyl glycerols

Bile Salts

• Produced in the liver

• Made from cholesterol

– Cholesterol itself it not amphiphilic enough to be adetergent - needs modification by addition of polar groups

• Stored in the gall bladder

• After digestion of fat

– Reabsorbed and taken back to the liver

– Via hepatic portal vein

Bile Salts

• Polar groups are added to cholesterol to make it moreamphiphilic

• The only way to get rid of cholesterol is to make them intobile salts (chol cannot be oxidised)

Loading...

Undigested Fat

• If gall bladder is blocked by gall stones– no bile salts secreted � less fat digestion � lower

calorie intake

• Inhibitors of fat digestion as weight-loss drugs– Orlistat (brand name Xenical)

– Listed side effects include• oily spotting; orange colored oil in your stool;

• gas with oily discharge;

• an urgent need to go to the bathroom; an inability to control bowelmovements, an increased number of bowel movements.

Olestra

• Olestra (Olean) is a fat substitute– FA attached to sucrose

– Not attacked by lipases• Olestra will passes through gut undigested

– Carries with it fat soluble vitamins

• D, E, K need to be added as supplements

– See anti-Olestra sites

– eg. www.cspinet.org/olestraolestra/

Amusing Aside…

• From Wikipedia entry….

• Because olestra is made by chemically combining sugar and vegetable oil, it releasesno toxic fumes and could potentially become a safe and environmentally friendlyreplacement for petrochemicals

• It is currently used as a base for deck stains and a lubricant for small power tools, andthere are plans to use it on larger machinery

Lipoproteins• Mixture of phospholipid and Apoproteins

– Apoproteins role - Enzymes, Structural, Docking

• Different types of lipoproteins characterised by size and by types ofapoproteins

• First lipoproteins made by intestinal cells– Chylomicrons

– Enter lymphatic system

• Contains fat and cholesterol esters

Once fat is digested it

gets packaged up

Chylomicron a are a type of

lipoprotein

Inside is incredibly

hydrophobic (where

the fat is stored)

Lectures 12-18 2014 Human Biochemistry

� !• There is a belief that reducing cholesterol intake through the diet will reduce our body

cholesterol levels!• but remember, we are VERY GOOD cholesterol making machines, and the total amount we

have in our bodies at any one time far exceeds the amount anyone could eat in one day!• Even if you happened to decrease your dietary cholesterol, your body has an amazing capacity

to make up for any shortfall e.g. vegetarians who hardly eat any cholesterol still will have similar if not same body cholesterol levels as a meat eater!

• similarly, people who eat tonnes of cholesterol may only have a little bit more than normal in their bodies, but nothing significant (it is possible that in the long term it could be harmful, but the problem really doesn't lie in the amount of cholesterol we get through our diet)!

• Total body cholesterol ~ 140 grams!• only ~1 g of cholesterol enters the body each day from the diet, and only 0.5 g of this is

actually absorbed!• ~18 g bile salts are secreted into gut each day, and 17.5 g reabsorbed per day!• net loss of bile salt is very little (0.5 g/day)!• therefore, amount absorbed = amount lost as bile salts (hence, a reduction in dietary

cholesterol intake will most likely be met by an increase in endogenous cholesterol synthesis)!• compare the store size to the intake!

- 140 g vs 0.5 g!- vs CHO for which the store size and take are similar magnitude!- vs fat (intake 100 g < store 15 000 g)!!!

� !• Cholesterol is important for: steroid hormone synthesis and regulating membrane fluidity!• membrane fluidity is important for: structural integrity and receptor/enzyme activity!• nb: membranes form compartments which not only need to be strong, they need to be flexible

too (fluid membranes) - think of all the things membranes allow!!• membrane imagery: ’proteins are the icebergs floating around, and the sea they are floating in

is the phospholipids’!• consider membranes made with saturated FA vs membranes made with unsaturated FA and

understand why unsaturated fats are the “good fats”, and then see how cholesterol fixes the problems of saturated FA!!!!

Cholesterol Flux

Importance of Cholesterol

Page ! of !11 46

Lectures 12-18 2014 Human Biochemistry

Lecture 13: Introduction to Nitrogen Metabolism !!

� !20 AA in proteins!more involved in metabolism!loads of different side chains - so pathways of formation and disposal are varied and complex!synthesis and degradation of each protein is under different control (i.e. each protein responds to different signals)!proteins of our bodies are like buildings of a city viewed over hundreds of years (periods of growth, demolition, industrialisation, etc)!the liver is very important - it is the first place that AAs go to from the intestine (this is where amino acids get processed)!Degradation of enzymes have very high Km (so a high concentration of an AA automatically means that it will be degraded)!“not controlled” (i.e. if it’s in excess, it gets degraded - that’s the only control)!!

� !Turnover varies massively between tissues: muscle (2%), intestine (15%)!however, muscle is the greatest bulk of protein!diet intake: 100g protein (16g nitrogen)!turnover: 300g protein/day (~10 kg pool)!loss from body: 100g (skin 0.5g nitrogen, urea 14g nitrogen, faeces 2g nitrogen = 16.5g nitrogen)!

� !

� !In the liver: conversion of amino acids (especially glucose) and processing of amine groups (urea synthesis)!nb: liver can strip off the amine group and turn it into urea, it can also take the carbon backbones leftover from AA and turn them into something useful e.g. CHO!bits to process:!

- Amino group (fixed nitrogen is quite precious, so it is recycled if possible - however ammonia is toxic, so need to convert to a non-toxic product i.e. urea)!

- carbon skeleton (normally a 2-oxo acid which is oxidised or converted to CHO or fat)!

Amino Acids

Daily Flux

Daily Flux

Intake 100 g

16 g nitrogen

Loss 100 g

skin 0.5 g nitrogen urea 14 g nitrogen faeces 2 g nitrogen

10 kg pool

300 g turnover 3% per day

Turnover varies massively between tissues: muscle 2%, intestine 15% But muscle is greatest bulk of protein.

Processing of Amino Acids

Page ! of !13 46