Transcript Lecture 6: Lipids and Fatty Acids

Lecture 5: Lipids and
Carbohydrates
Craig Kasper
Fish Nutrition
Part 1: Lipid Characteristics
Lipid = a compound that is insoluble in
water, but soluble in an organic solvent
(e.g., ether, benzene, acetone,
chloroform)
 “lipid” is synonymous with “fat”, but
also includes phospholipids, sterols, etc.
 chemical structure: glycerol + fatty
acids
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Lipid Molecule
Nutritional Uses of Lipids
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We already know that lipids are concentrated sources
of energy (9.45 kcal/g)
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other functions:
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1) provide means whereby fat-soluble nutrients (e.g.,
sterols, vitamins) can be absorbed by the body
2) structural element of cell, subcellular components
3) components of hormones and precursors for
prostaglandin synthesis
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Lipid Classes
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simple: FA’s esterified with glycerol
compound: same as simple, but with other
compounds also attached
phospholipids: fats containing phosphoric acid and
nitrogen (lecithin)
glycolipids: FA’s compounded with CHO, but no N
derived lipids: substances from the above derived
by hydrolysis
sterols: large molecular wt. alcohols found in nature
and combined w/FA’s (e.g., cholesterol)
Saturated vs. Unsaturated Fatty Acids
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saturated: the SFA’s of a lipid have no double
bonds between carbons in chain
polyunsaturated: more than one double bond
in the chain
most common polyunsaturated fats contain the
polyunsaturated fatty acids (PUFAs) oleic, linoleic
and linolenic acid
unsaturated fats have lower melting points
stearic (SFA) melts at 70oC, oleic (PUFA) at 26oC
Fatty Acids Commonly Found in Lipids
Sat. Fatty Acids
Butyric
Palmitic
Stearic
Unsat. Fatty Acids
Oleic
Linoleic
Linolenic
Formula
C4H8O2
C16H22O2
C18H36O2
Formula
C18H34O2
C18H32O2
C18H30O2
Melting Point (oC)
Liquid
63
70
Melting Point (oC)
Liquid
Liquid
Liquid
Saturated vs. Unsaturated Fats
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saturated fats tightly packed, clog arteries as
atherosclerosis
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because of double bonds, polyunsaturated fats do
not pack well -- like building a wall with bricks (sat.)
vs. irregular-shaped objects (unsat.)
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plant fats are much higher in PUFA’s than animal fats
Saturated vs. Unsaturated FA’s Plant
vs. Animal Fat
corn
Sat. FA’s
Myristic
Palmitic
Stearic
Unsat. FA’s
Oleic
Linoleic
Linolenic
Arachid.
soy
tallow
lard
7.0
2.4
8.5
3.5
3
27
21
32.2
7.8
45.6
45.0
17
54.4
7.1
40
2
0.5
48
11
0.6
Lipid Digestion/Absorption
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Fats serve a structural function in cells, as sources of
energy, and insulation
the poor water solubility of lipids presents a problem
for digestion: substrates are not easily accessible to
digestive enzymes
even if hydrolyzed, the products tend to aggregate to
larger complexes that make poor contact with the cell
surface and aren’t easily absorbed
to overcome these problems, changes in the physical
state of lipids are connected to chemical changes
during digestion and absorption
Lipid Digestion/Absorption
Five different phases:
hydrolysis of triglycerides (TG) to free fatty acids
(FFA) and monoacylglycerols
 solubilization of FFA and monoacylglycerols by
detergents (bile acids) and transportation from the
intestinal lumen toward the cell surface
 uptake of FFA and monoacylglycerols into the cell and
resynthesis to triglyceride
 packaging of TG’s into chylomicrons
 exocytosis of chylomicrons into lymph

Enzymes Involved in Digestion
of Lipids
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lingual lipase: provides a stable interface with
aqueous environment of stomach
pancreatic lipase: major enzyme affecting
triglyceride hydrolysis
colipase: protein anchoring lipase to the lipid
lipid esterase: secreted by pancreas, acts on
cholestrol esters, activated by bile
phospholipases: cleave phospholipids, activated
by trypsin
What about Bile???
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These are biological detergents synthesized by the liver
and secreted into the intestine
they form the spherical structures (micelles) assisting in
absorption
hydrophobic portion (tails of FA) are located to the inside
of the micelle, with heads (hydrophillic portion) to the
outside
they move lipids from the intestinal lumen to the cell
surface
absorption is by diffusion (complete for FA and
monoglycerides, less for others)
Factors Affecting Absorption of Lipids
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amount of fat consumed (fat=digestion=absorption)
age of subject ( age =  digestion)
emulsifying agents ( digestion =  absorption)
chain length of FA’s (> 18C =  digestibility)
degree of saturation of FA ( sat =  digestibility)
overheating and autooxidation (rancidification (rot) at
double bond)
optimal dietary calcium = optimal FA absorption (high Ca =
 absorption)
Lipid Metabolism/Absorption
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short chain FA’s are absorbed and enter the portal
vein to the liver
those FA’s with more than 10 carbons are
resynthesized by the liver to triglycerides
they are then converted into chylomicrons and
pass to the lymphatic system
some FA’s entering the liver are oxidized for
energy, others stored
blood lipids: 45% P-lipids, 35% triglycerides, 15%
cholestrol esters, 5% free FA’s
Lipid
Digestion/
Absorption
Lipid
Digestion/
Absorption
Characteristics of Fat Storage
Most of the body’s energy stores are
triglycerides
 storage is in adipose, source is dietary
or anabolism (synthesis) from COH or
AA carbon skeletons
 remember obesity?
 adipose can remove FA’s from the blood
and enzymes can put them back
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Fatty Acid Nomenclature
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Nomenclature reflects location of double
bonds
also used are common names (e.g., oleic,
stearic, palmitic)
linoleic is also known as 18:2 n-6
this means the FA is 18 carbons in length,
has 2 double bonds, the first of which is on
the 6th carbon
arachidonic = 20:4 n-6
What’s in a Name??
Fatty Acid Nomenclature
Essential Fatty Acids
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Only recently determined as essential (1930)
body can synthesize cholesterol,
phospholipids
research: same as AA’s but via addition
(EFA’s added improved growth, NEFA’s didn’t)
requirement determined by depleting fat
reserves of subject animal: difficult
Essential Fatty Acids (fish)
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Most NEAA found in marine food webs
Essential fatty acids (to date):
– linoleic (18:2 n-6; terrestrials; fish - not really)
– linolenic (18:3 n-3; terrestrials; fish)
– arachidonic (20:4 n-6; marine maybe)
– eicosopentaenoic acid (20:5 n-3, marine)
– docosohexaenoic (22:6 n-3, marine)
Why? Because elongation beyond 18 carbons is very
difficult in marine fish (lack pathways)
actual EFA requirement is a matter of whether the fish is
FW/SW or WW/CW
Essential Fatty Acids (most
animals)
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salmonids need n-3 FA’s for membrane
flexibility in cold water (why does this work?)
trout can elongate and desaturate n-3 FA’s
Linoleic acid (18:2 n-6) is the most essential
addition of arachidonic is also helpful in
deficient diets, but can be synthesized from
linoleic (maybe sparing effect)
EFA’s, like EAA’s, must be dietary
Essential Fatty Acids
LINOLEIC
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
18:2 n-6
LINOLENIC
CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
18:3 n-3
EICOSOPENTAENOIC ACID
CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH
20:5 n-3
DOCOSOHEXAENOIC ACID - YOU CAN DO THIS ONE!
Lipids as Crustacean Energy
Sources
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Largely, n-6 FA’s (linoleic) used for energy
as temperature drops, requirement for
monounsaturated and PUFA’s increases
change in temperature = change in diet
cold water species = increased dietary HUFA’s
maturation animals: increased requirement for
20:4 n-6, 20:5 n-3 and 22:6 n-3 for proper
spawning
Part 2: Carbohydrate
Characteristics
From: Lovell; D’Abramo et al.
General Comments
Carbohydrates often written as “COH”
 much of what we need to know about
them, besides their structure, was
covered in “Bioenergetics, Parts 1&2”
 here, we cover structure
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Carbohydrate Structure
Basic chemical structure consists of
sugar units
 found as aldehydes or ketones derived
from polyhydric alcohols
 contain: C, H, O
 often shown as aliphatic or linear
structures, but exist in nature as ringed
structures
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Glucose Structure
O
CH2OH
C-H
H- C-OH
O
H
OH
HO-C-H
H
OH
HO
H-C-OH
H
OH
H-C-OH
Haworth perspective
CH2OH
H
Carbohydrate Classification
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Usually by the number of sugar units in the
molecule:
– monosaccharides (glucose)
– disaccharides (2 units)
 maltose (2 glucose units)
 sucrose (glucose + fructose)
– polysaccharides (long chain polymers of
monosaccharides
– most important polysaccharides to animals are
starch and cellulose
Starch and Cellulose
CH2OH
O
H
O
CH2OH
OH
H
H
OH
H
O
H
O
OH
H
H
OH
CH2OH
O
starch
CH2OH
H
O
H
H
O
O
O
OH
H
OH
H
H
OH
O
H
H
H
OH
cellulose
Starch and Cellulose
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Starch contains -D-glucose linkage
Cellulose has a -D-glucose linkage
we store starch in muscle tissues as
glycogen, peeled off by enzymes when
needed
cellulose is primary component of plant
tissue, largely indigestible to monogastrics
must have enzyme, “cellulase”