hepatic metabolism of lipids Flashcards
what are the 3 types of lipids in the body
triglycerides
phospholipids
cholesterol
which lipids are composed of fatty acids
only triglycerides and phospholipids
cholesterol is NOT
what are triglycerides mainly used for
as an energy store for times of increased energy demand
what are phospholipids and cholesterol used for
functions such as the synthesis of the cell membrane and the synthesis of steroid hormones.
where is most of the bodies fat stored
in adipocytes which form tissues called adipose tissues
some fat is stored in hepatocytes
structure of triglycerides
3 fatty acids bound to a glycerol molecule
how much of energy stored in the body is accounted for by triglycerides
78%
how much of dietary lipids are triglycerides
95%
the rest consist of phospholipids, free fatty acids (FFAs), cholesterol and fat
soluble vitamins
what are lipids (chemically)
esters of fatty acids and certain alcohol compounds
functions of lipids (3)
- Energy reserves
- Structural part of cell membrane
- Hormone metabolism
what are lipoproteins
molecules made of lipids and protein
they are used to transport cholesterol in the blood to cells
where is HDL formed
in the liver
where is LDL formed
in the plasma
what do HDL do
remove excess cholesterol from blood and tissue
then deliver this cholesterol to the liver which secretes it into the bile
or converts it into bile salts
what do LDL do
the main cholesterol carriers and they deliver cholesterol to cells throughout the body
bind to plasma membrane receptors specific for a protein component
of the LDLs and are then taken up by the cells via endocytosis
why is LDL “bad:
because high plasma
concentrations can be associated with increased deposition of cholesterol in arterial walls and a higher incidence of heart attacks
why is LDL essential
is essential in supplying cells with the cholesterol they require to synthesise cell membranes and for steroid hormone production in the gonads and adrenal glands + aldosterone + cortisol
why is HDL ‘good’
since it removes cholesterol from plasma
where is VLDL synthesised
in hepatocytes
what do VLDL do
carries triglycerides from glucose in liver to adipocytes
what is fatty acid catabolism/lipolysis
the mechanism by which the body accesses energy stored as triglycerides
how much energy comes from fatty acid catabolism
under resting conditions approximately half the energy used by muscle, liver and the kidneys comes from the catabolism of fatty acids
where is adipose tissue found
in deposits underlying the skin or surrounding internal organs
function of adipocytes
to synthesise and store triglycerides during periods of food uptake and when food is not being absorbed from the small intestine, to release fatty acids and glycerol into the blood for uptake & use by other cells in
order to provide the energy required for ATP formation
where does fatty acid catabolism occur
mitochondria
process of fatty acid catabolism/beta oxidation
- Molecule of coenzyme A link to the carboxyl at the end of a fatty acid
- breakdown of ATP —> AMP + 2Pi
- The coenzyme A derivative of fatty acid then proceeds through beta-
oxidation reactions - A molecule of acetyl coenzyme A is split off from the fatty acid and two pairs of hydrogen atoms are transferred to coenzymes (one pair to FAD and the
other pair to NAD+) - The hydrogen atoms from the coenzymes then enter the oxidative
phosphorylation pathway to form ATP - Another coenzyme A attaches to the fatty acid and the cycle is repeated
- Each passage through this sequence shortens the fatty acid chain by two
carbons atoms until all the carbon atoms have transferred to coenzyme A
molecules - These molecules then lead to the production of CO2 & ATP via the Kreb’s
cycle & oxidative phosphorylation
how are triglycerides broken down
hydrolysed into fatty acids and glycerol by triglyceride lipase, which is stimulated by adrenaline and glucagon.
what stimulates triglyceride lipase
adrenaline and glucagon
what happens to the fatty acids produced from lipolysis
they enter the circulation where they bind immediately to albumin.
when bound to albumin they are soluble in blood and so can be transported to tissues including the liver.
what happens to glycerol when it enters hepatocytes
it is is immediately converted into glycerol-3-phosphate, which then enters the glycolysis pathway.
what is beta oxidation
the process by which fatty acids are oxidised and degraded
what by products are produced in beta oxidation
NADH and FADH2
what happens in beta oxidation
2carbon segments are progressively released from the fatty acid chain until acetyl co-A is generated.
Acetyl-co-A then binds immediately with oxaloacetate to form citrate and then enters the TCA cycle to release energy in the form of ATP.
what happens to excess acetyl coA
it is converted into acetoacetic acid using HMG-coA as an intermediate, and then transported to other tissues.
what is ketogenesis
process through which organisms produce ketone bodies by breaking down fatty acids and ketogenic amino acids.
what happens to acetoacetic acid
some is converted to beta-hydroxybutyric acid, and small amounts to acetone
what is acetoacetic acid
a keto acid, and together with beta-hydroxybutyric acid and acetone the three compounds are known as ketone bodies.
uses of ketone bodies (2)
can travel in the blood to other tissues where they are then used for energy. c
also play a significant role in disease states
what is lipoprotein lipase
an enzyme that hydrolyses triglycerides in lipoproteins (chylomicrons & VLDLs)
into 2 free fatty acids & 1 glycerol molecule
what is hepatic lipase
Converts IDL (intermediate density lipoprotein) into LDL thereby packaging it with more triglycerides to be released in the body
where is hepatic lipase expressed
in the liver and adrenal glands
what is lipogenesis
an essential mechanism that provides an energy store which can be used at times when the body’s energy requirement cannot be met by glucose alone
how much glycogen can the body store
~15g/kg
this can soon become depleted during times of starvation or high activity
compare energy levels of fat with glycogen
much more fat can be stored within adipose tissue, and fat contains 2.5x the energy as the same quantity of glycogen
where are fatty acids synthesized
cytoplasm of hepatocytes
what happens to glucose not converted to glycogen
converted to pyruvate via the glycolysis pathway, and transported into the mitochondria where it is converted to acetyl Co-A
what is the citrate shuttle
if not entering the TCA cycle, acetyl-CoA needs to leave the mitochondria and enter the cytosol.
but the inner mitochondrial membrane is impermeable to acetyl-coA, and therefore it must traverse the membrane in an altered form; as citrate.
- Acetyl-CoA joins with oxaloacetate to form citrate
- Citrate is able to cross the mitochondria membrane
- Citrate is then converted back into acetyl-CoA and oxaloacetate within the cytosol
what happens to acetyl-CoA once it is in the cytosol
it can be converted to malonyl-CoA by acetyl-CoA carboxylase - important in the regulation of lipogenesis as it is allosterically activated by citrate and inhibited by AMP
what does fatty acid synthase do
creates an elongated fatty acid chain from the malonyl-CoA molecules, adding two carbon atoms for each molecule of malonyl-coA. As the malonyl-CoA molecules are added they lose a carbon atom creating CO2
what stimulates lipogenesis
insulin
what inhibits lipogenesis
glucagon and adrenaline
and
negative feedback from the presence of lipoproteins in the blood.
what are the 2 types of adipose tissue
white adipose tissue (WAT)
brown adipose tissue (BAT)
how does WAT store excess energy
as triglycerides
how does BAT store excess energy
dissipates stored energy as heat
what is WAT made of
mainly triglycerides which are stored in a large singular droplet
surrounding the white triglyceride droplet is a thin ring comprising the cytoplasm and nucleus.
what is BAT made of
smaller than white adipocytes and contain multitudes of small (multilocular) lipid droplets.
where is WAT found
under the skin
surrounding inner organs
