3/4/23 - Lipids
Lipids
Generally insoluble in water
Important source of energy (dietary and storage)
Cell membrane (lipid bilayer)
Microbial phospholipids
Supply essential fatty acids
Synthesis of hormones
Absorption of fat soluble vitamins
↑ palatability of feed
Too much lipid ↓ palatability
Condition feed (↓ dust, ↓ particle separation (fines))
Fatty acids
Straight chain hydrocarbons
Terminate in a carboxyl group
(CH₃) - (CH₂)ₙ - COOH
Make up complex lipids
Saturated fatty acids (SFA)
Contain no C=C double bonds
Contain all of the hydrogens they can contain
Unsaturated fatty acids (UFA)
Contain one or more cis C=C double bonds
Each double bond is missing 2 hydrogens
Monounsaturated fatty acids
MUFA
Contain 1 C=C double bond
Polyunsaturated fatty acids
PUFA
Contain >1 C=C double bonds
Double bonds
Important for membrane fluidity
More double bonds = more fluidity
More double bonds = less energy contained
↑ number of double bonds → ↓ melting point
Oils = liquid at room temp - more double bonds
Fats = solid at room temp - fewer double bonds
Triacylglycerol (TAG) - triglycerides
How lipids are stored
95% of lipid intake (humans)
Combine a glycerol backbone with 3 fatty acids - releases 3H₂O
Breakdown of TAG
A lipase inserts 3H₂O to break the ester bond [circled on diagram (O-C=O)]
Sn positions are important because enzymes act at a specific location
Phospholipids
Non Ruminant Lipids
TAG → [release FFA/free fatty acid, (hydrolyzed by lipase)] → DAG (diacylglycerol) → [release FFA (hydrolyzed by lipase)] → MAG (monoacylglycerol)
Stomach and small intestine
Absorb the FFAs and MAG in the small intestine
Mouth
Lingual lipase (LL)
Produced by salivary glands
Stomach
Gastric lipase (GL)
Produced by chief cells in the stomach
Shared characteristics of LL and GL
Produced continuously
Production ↑ with ↑ fat or lipid intake
Stable and active at the low pH of the stomach
Both act on fatty acids at the SN3 position and prefer short chain fatty acids\
Important in newborns
Emulsification
Body’s attempt to ↑ the surface area of lipid droplets by making and maintaining many small droplets
Two types
Physical emulsification
Mixing that occurs in the stomach and as food enters the small intestine
Chewing
Chemical emulsification
Bile
Mixed with digesta in the duodenum
Stabilize the fat droplet
Bile is stored in the gallbladder except in the horse
Bile synthesized from cholesterol in the liver
Small intestine
Primary site of lipid digestion and absorption
Stabilized fat droplets (bc of bile)
Pancreatic lipase
Hydrolyzes fatty acids from the Sn1 and SN3 positions
Does SN3 first and SN1 second
Requires colipase for optimal activity
Procolipase → [tripsin (proteolytic enzyme)] → colipase
Procolipase - produced by the pancreas
Not an enzyme - it is a protein
Contains a hydrophobic region
Allows it to attach to the fat droplet →
→ Creates a binding site for pancreatic lipase
Summary
Mouth (TAG)
Minimal digestion
Lingual lipase (SN3)
Insert water into TAG using LL, producing DAG and a FFA
Stomach
More digestion (TAG and small amount of DAG and FFA)
Gastric lipase (SN3)
Insert water into TAG using GL and LL, producing DAG and a FFA
Small intestine
Most digestion and absorption (TAG and a medium amount of DAG and FFA)
Phase 1: emulsification
Physical (chewing and mixing) and chemical (bile)
Phase 2: enzymatic digestion
Pancreatic lipase and colipase (protein)
Act on SN1 and SN3
Procolipase → (tripsin) → colipase
Insert water into TAG using pancreatic lipase, producing DAG and FFA, insert water into DAG using pancreatic lipase, producing MAG and FFA
Phase 3: absorption
MAG and FFA diffuse into enterocyte
Bile is absorbed in the ileum
Large intestine/cecum
Undigested lipids are acted on by microbes and MPL is synthesized
3 enzymes break ester bonds
LL, GL, pancreatic lipase
4 enzymes required for lipid digestion
LL, GL, pancreatic lipase, tripsin
5 proteins required for lipid digestion
LL, GL, pancreatic lipase, tripsin, colipase
9 proteins and what they are doing
Lingual lipase (E)
Hydrolyzes TAG Sn3 bonds, producing DAG and FFA in the mouth and stomach
Gastric lipase (E)
Hydrolyzes TAG Sn3 bonds, producing DAG and FFA in the stomach
Tripsin (E)
Activates procolipase into colipase in the small intestine
Colipase
Attaches on fat droplets to create a binding site for pancreatic lipase in the small intestine
Pancreatic lipase (E)
Hydrolyzes TAG at Sn1 and Sn3 to produce DAG and FFA, then hydrolyzes DAG to produce MAG and FFA in small intestine
AcylCoA synthetase (E)
Forms FA ≥12C into TAG in the enterocyte by forming ester bonds at Sn1 and Sn3
B-48
Binds to the fat droplet in the ER within the enterocyte, stabilizes lipids, and confers specificity (like a name tag)
Lipoprotein lipase (E)
Hydrolyzes TAG Sn3 into DAG and FFA in the muscle and adipose
Albumin
Binds to FA ≤ 10C in the blood to transport it
Microbial lipid utilization
TAG is difficult for ruminants to digest
Generally less than 5% of total diet is TAG
Glycolipids
Found in grass and growing plants
Contains a sugar at the Sn1 position
Primary lipid source consumed in all grazing animals
Phospholipids
Primary lipid source digested in the small intestine
Comes from microbial phospholipid production
Microbial digestion of lipids
1) hydrolysis of the glycerol and FAs
TAG → (hydrolyzed by lipase ) → glycerol + 3 FFA
Glycolipid → (hydrolyzed by galactolipase) → glycerol + 2 FFA + sugar
Phospholipid → (hydrolyzed by phospholipase) → glycerol + 2 FFA + polar head group
Glycerol + sugar are fermented by the microbes into VFAs
VFAs are the only product absorbed by the fermentation chamber
Hydrolysis is rapid and complete
2) hydrogenation (biohydrogenation)
Slow and incomplete
UFA → (hydrogen added by hydrogenase) → SFA
18:3 FA (least energy) → (H2 added) → 18:2 → (H2 added) → 18:1 → (H2 added) → 18:0 (most energy)
18:3 FA - 18 carbons and 3 double bonds
Added 3 H2 or 6 H
Prevented 1.5 methanes (CH4)
a) UFAs act as a hydrogen sink - accept H2 to decrease CH4
Increase in supply of energy to the animal
b) UFAs are toxic to cellulolytic ruminal microbes
Decrease SCHO digestion → decrease acetate → decrease CH4
3) MPL synthesis
Increase microbial growth → increased MPL synthesis
Molecules that contain an ester bond or can form an ester bond
Acyl CoA synthetase
TAG
DAG
MAG
Proteins secreted by a mammal digesting lipids (into the small intestine)
Lingual lipase
Gastric lipase
Pancreatic lipase
Procolipase
Molecules that hydrolyze an ester bond at the Sn3 position
Lingual lipase
Gastric lipase
Pancreatic lipase
Lipase (microbial)
Galactolipase
Molecules that hydrolyze an ester bond at the Sn1 position
Pancreatic lipase
Lipase (microbial)
Phospholipase
Molecules that commonly hydrolyze an ester bond at the Sn2 position
Lipase (microbial)
Galactolipase
Phospholipase
Ruminal utilization of lipids results in a decrease in the amount of _____ absorbed by the small intestine
MAG (absorbed by the microbes)
PUFA (bc of hydrogenase, turning them into SFAs)
MUFA (because of hydrogenase, turning them into SFAs)
Not TAG bc it’s not absorbed in the small intestine
Who forms the most microbial phospholipid in the large intestine/cecum?
Horse
Rank the following fatty acids by weight given that they contain the same number of carbons?
PUFA > MUFA > SFA
PUFA has more double bonds, as you add hydrogen the weight increases
To increase microbial phospholipid synthesis in calves, what two polysaccharides would be best to feed?
Amylose
Amylopectin
Why?
More digestible, therefore more fermentable
Calves are already eating cellulose
Molecules expected to be part of a chylomicron
B-48
TAG
Who absorbs the most MPL in the large intestine
Cow = horse = pig