Unit 6 - Introduction to Lipids and their Metabolism Flashcards
what is the major form of metabolic energy in humans and where do we get it?
triacylglycerols (AKA triglycerides or TGs)
-obtained from animals and plants
what are the 3 reasons we store metabolic energy as fat?
- carbons in triglycerides have lower oxidation state than carbons in CHO or PRO (9 kcal/g VS 4 kcal/g if dry, and 6x kcal if wet)
- TG are stored in anhydrous state (no water), but CHO have twice their dry weight as bound water
- fats don’t participate in cell’s osmotic balance (no bound water), so they can be stored to a larger fraction of cell volume
what is the “definition” of a fatty acid? what lengths are more abundant? what is the more common structure?
carboxylic acids with alkyl side chains (R-COOH)
- long chain C16 and C18 are more common
- hydrocarbon chains are saturated or unsaturated
- -DB in polyunsaturated are almost always cis (separated by methylene group), and never adjacent or conjugated (or else too easily oxidized)
why are long chain FA rarely “free” for long?
ionized FA form micelles in aqueous solution, and are toxic to cells
- since toxic, non-esterified FA (NEFA) are bound to PRO via albumin (extracellular) or FA-binding PRO (intracellular)
- FA can also be esterified to compounds other than glycerol
what are the number of carbons and double bonds for
- myristic
- palmitic
- palmitoleic
- stearic
- oleic
- linoleic
- linolenic
- arachidonic acids?
myristic: C14, 0 DB
palmitic: C16, 0 DB
palmitoleic: C16, 1 DB
stearic: C18, 0 DB
oleic: C18, 1 DB
linoleic: C18, 2 DB
linolenic: C18, 3 DB
arachidonic: C20, 4 DB
how are fats used as fuel? what organ cannot use it?
FA are derived from TG, and used by most tissues (heart, muscle, kidneys) as source of E
- FA-derived ACoA enters TCA to yield ATP and heat
- brain CANNOT use FFA as fuel b/c of BBB)
what are the 2 ultimate sources of NEFA?
- diet (exogenous pathway)
2. synthesized de novo by liver and adipose (endogenous pathway)
what happens to excess FA?
from either exogenous or endogenous pathway, is stored as TG in adipocytes to be mobilized as NEFA when needed as fuel
what does bile contain?
bile acids/salts
phosphatidyl choline
cholesterol
what fats are more common in diets?
long chain TG (esterified to LCFA) are most common, and rarely MCT (esterified to MCFA)
what does digestion of FA involve?
hydrolysis of ester bonds in TG (catalyzed by TG-specific esterases = lipases)
- 20% in lingual and gastric lipases (have acid pH optimum)
- the rest in SI via bile acids (digestive detergents)
what does pancreatic lipase enzyme activation require?? what does it cleave preferentially? what are its main products?
formation of a complex with colipase and a drop of emulsified lipid to stabilize “open” conformation and allow access to substrate
- partial hydrolysis of LCFA with preference for 1 and 3 positions
- main products are 2-monoacylglycerol (2-MAG) and NEFA
what happens to the products of lipid digestion?
absorption (passage into cells of intestinal mucosa)
- requires bile
- products first incorporated into mixed micelles that cross stationary aqueous boundary layer at intestinal wall
- then FFA, MAG, and glycerol diffuse/transport to intestinal mucosa cells
what are mixed micelles?
formed by bile acids, with nonpolar digestive products (2-MAG and NEFA) allowing translocation across stationary aqueous boundary layer at intestinal wall
-glycerol backbones are oriented toward aqueous phase
what is the major FA transporter in enterocytes? glycerol transport?
FATP5 (fatty acid transport protein 5) for FA
AQP3 (aquaborin family) for glycerol
absorption is predominantly carrier-mediated, but also can be passive via diffusion
