lecture 4: lipid digestion & absorption Flashcards
4 major roles of lipids in the body
fuel for energy, structural component of cellular membranes, lubricants, signaling molecules
acylglycerols
major lipid class of the body
concentrated form of energy
tryacylglycerol (TAG) critical in metabolism, primary storage of fat
why is fat the preferred storage form of energy?
more energy per gram
they do not associate with water, less weight (carbs have hydrophilic interactions)
high dietary carbs >
liver synthesizes FA
fatty acid and triacylglycerol metabolism
series of hydrolysis and re-esterification reactions
FA transported across membranes
TAG = major form of lipids in lipoproteins in circulation
resting muscles primarily use what for energy?
glucose
working muscle….
oxidizes fatty acids at higher levels
gastric lipase
secreted in stomach, active in acidic environment
hydrolyzes FA at sn3 position
TAG > DAG + FA
fatty acids stimulate the release of
CCK in the blood > contractions in gallbladder > bile release
bile acids
in bile, interact with surface of lipid droplets and act as emulsifiers
pancreatic lipase
digest most TAG in small intestine
works at the aqueous-lipid interface of emulsified lipid particles
TAG > MAG + 2FA
colipase
activated by trypsin
binds to aqueous-lipid interface, facilitates binding and anchoring of pancreatic lipase (otherwise inhibited by bile acids)
phospholipase A2
digests phospholipids, hydrolyzes sn2 position
carboxyester lipase
excreted by pancreas in active form, products free cholesterol and FA products
acts on substrates in aq-lipid interface of micelles, stimulated by bile acids
micellar solubilization
essential to absorption of hydrophobic products
enables products of digestion to cross barrier presented by water layer surrounding enterocytes
the concentration gradient between apical membrane and intracellular compartment favors…
initial diffusion of products into the cell
digestion products rapidly re-esterified > maintain low intracellular conc > continued lipid uptake
fatty acid translocase CD36
translocates long-chain fatty acids across apical membrane of enterocyte
carrier for uptake of FAs
transport protein NPC1L1
major role in absorption of cholesterol
how are absorbed FAs transported?
most are incorporated into TAGS and transported through lymph in chylomicrons
others are transported to liver through portal vein
portal transport of FAs
most FAs are incorporated in TAG, transported through lymph as chylomicrons
transport to liver by portal vein is the preferential transport by medium chain FAs
double bonds in long chain FAs increase rate of transport
neurotensin
intestinal peptide hormone that enhances lymphatic lipid transport
glucagon-like peptide 2 GLP2
increases lipid absorption and chylomicron secretion
released in response to fat/carb consumption
outlined process of carrier-mediated uptake of FAs
CD36 translocates FA
FABP transports FA to ER
FAs metabolized by FABP and ACSL
ACAT, MGAT, DGAT are ER membrane proteins and facilitate esterification of cholesterol
