metabolismo lipídico III

Question 1

Exogenous lipid pathway ?

Answer 1

Bile emulsifies fats contained in the chyme, then pancreatic lipase cleaves triacylglyceride molecules into two fatty acids and one 2-monoacylglycerol. Enterocytes readily absorb these small molecules from the chymus. Inside of the enterocytes, fatty acids and monoacylglycerides are transformed again into triacylglycerides. Then these lipids (i.e. triacylglycerols, phospholipids, cholesterol, and cholesteryl esters) are assembled into nascent chylomicrons.

*Via apolipoprotein C-II, mature chylomicrons activate lipoprotein lipase (LPL), an enzyme on endothelial cells lining the blood vessels. LPL catalyzes the hydrolysis of triacylglycerol (glycerol covalently joined to three fatty acids) that ultimately releases glycerol and fatty acids from the chylomicrons. Glycerol and fatty acids can then be absorbed in peripheral tissues, especially adipose and muscle, for energy and storage.

**Short-chain fatty acids and medium-chain fatty acids are primarily absorbed through the portal vein(The hepatic portal vein is a blood vessel that conducts blood from the gastrointestinal tract and spleen to the liver.) during lipid digestion, while long-chain fatty acids are packed into chylomicrons and enter lymphatic capillaries, and enter the blood first at the subclavian vein.

Question 2

endogenous lipid pathway

Answer 2

The liver is the central platform for the handling of lipids: it is able to store glycerols and fats in its cells, the hepatocytes. Hepatocytes are also able to create triacylglycerols via de novo synthesis. They also produce the bile from cholesterol.

*In the hepatocytes, triacylglycerols and cholesteryl esters are assembled to form nascent VLDL particles. Nascent VLDL particles are released into the bloodstream where they obtain apolipoprotein C-II and become mature.

**Again, like chylomicrons, VLDL particles circulate and encounter LPL expressed on endothelial cells. Apolipoprotein C-II activates LPL, causing hydrolysis of the VLDL particle and the release of glycerol and fatty acids. These products can be absorbed from the blood by peripheral tissues, principally adipose and muscle.

Question 3

How are fatty acids released from adipose tissue?

Answer 3

Fatty acids are released from adipose by hydrolysis of their stored form, triacylglycerol. Hydrolysis is initiated by activation of the hydrolytic enzyme, hormone sensitive lipase (HSL). HSL is a phospho-dephospho enzyme which is active in the phospho- form. Phosphorylation of HSL is stimulated by the hormones epinephrine, norepinephrine, cortisol and ACTH. These hormones bind to the surface of the adipocyte, where they activate adenyl cyclase, initiating a typical c-AMP-mediated phosphorylation cascade, terminating with phosphorylation of HSL. After release from adipocytes, unesterified fatty acids are transported in the blood bound to serum albumin to tissues such as liver, heart and muscle, where they are taken up and oxidized.

Question 4

Quick recount of effects of insulin and glucagon on lipid metabolism

Answer 4

INSULIN

#significant reduction in release of fatty acids: HSL activity is inhibited\> no lipids degraded in adipose tissue
#more uptake of glucose into adipocytes, providing the glycerol-3-phosphate for TAG synthesis
#expression of lipoprotein lipase is increased in adipose tissue, providing fatty acids for esterification into glycerol
#glucose converted into TAG in liver and secreted into adipose via VLDL.
in short: decreased degradation, expression of more lipoprotein lipase, more TAG synthesis in liver and VLDL secretion, and more glucose uptake in adipose \> glycerol-3-phosphate

GLUCAGON

#FAs released from hydrolysis of TAG are primarily released into blood. bound to albumin, they are transported to a variety of tissues and used as fuel.
#glycerol produced from TAG degradation used as gluconeogenic precursor by liver, which has glycerol kinase.
#FA can also be oxidised to acetyl coA which can enter the TCA cycle producing energy for the adipocyte
FA’s can also be re-esterified to glycerol-3-phosphate generating TAG and reducing plasma FA concentration

Question 5

De novo lipogenesis

Location ?

Regulation ?

Exceptions ?

Answer 5

Fatty acids made in liver are used to synthesize triacylglycerols for very low density lipoprotein (VLDL) assembly; in adipose tissue, fatty acids are converted to triacylglycerols for storage in lipid droplets - both processes are increased upon feeding or during positive energy balance

Liver and adipose are the major sites of fatty acid synthesis

Except: in lactating mammary gland (to supply medium chain FA components of milk); fetal lung (FA to make lung surfactant)

Question 6

source of reducing equivalents for fatty acid biosynthesis ?

Answer 6

NADPH differs from NADH only by the presence of a phosphate group on one of the ribose units

NADPH is produced by

(i) the pentose phosphate pathway in the cytosol;
(ii) during conversion of malate to pyruvate in the cytosol; and
(iii) by a cytoplasmic isoform of isocitrate dehydrogenase

Question 7

cytoplasmic acetyl coA ?

Answer 7

acetyl coA coming from amino acid metabolism in mitochondria or glycose from glycolysis are condensed into citrate via citrate synthase and into the cytoplasm where it splits back into acetyl coA and oxaloacetate at the expense of ATP via citrate lyase .

**first note there are three transporters across inner and outer membranes of mitochondria:

citrate transporter, pyruvate transporter, and malate-alpha ketoglutarate transporter

Question 8

citrate lyase and cancer ?

Answer 8

Many cancers have a high rate of aerobic glycolysis (Warburg effect)

• The excess glycolytic end- product – pyruvate – is diverted to lipid synthesis
• This is controlled by production of cytoplasmic acetyl CoA mediated by ATP citrate lyase (ACL)
• ACL inhibition can suppress tumor growth

Question 9

ACC ?

Answer 9

ACA has 3 functional regions:

THE ACETYL-COA CARBOXYLASE

! Biotin carrier protein (gray)
! Biotin carboxylase, which activates CO2 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction

! Transcarboxylase, which transfers activated CO2 (shaded green) from biotin to acetyl-CoA, producing malonyl-CoA

carboxylation cost is 1 ATP per acetyl coA

Question 10

regulation of fatty acid synthesis ?

Answer 10

REGULATION TARGETS

Synthesis: acetyl-CoA carboxylase (ACC)

Oxidation: carnitine acyl transferase I

*ACC is HORMONALLY activated by insulin and ALLOSTERICALLY activated by citrate while FEED-BACK INHIBITED by palmitoyl coA

**When [acetyl-CoA]mt increases, acetyl-CoA is converted to citrate…citrate exported to cytosol > signals excess energy to be converted into fat

***citrate is an inhibitor of PFK-1

low glucose > glucagon > ACC inactive (phosphorylated; inactive monomers) > no more malonyl coA > CAT I activated

Question 11

beta oxidation Vs. fatty acid synthesis

Synthesis steps ?

Answer 11

oxidation > hydration>oxidation> split

Oxidation (remeber, trying to make beta carbon bond vulnerable):

Synthesis:

Fatty Acid synthase:

A homodimer of a single polypeptide (7 components) with all catalytic activities for the synthetic cycle :

1# Chain transfer/charging
– Malonyl/acetyl-CoA ACP transacylase: acetyl coA is loaded onto KS-SH while malonyl coA is loaded onto ACP-SH

2# attached malonyl + acetyl + beta-ketoacyl ACP synthase –> CONDENSATION 2 carbon unit on -SH and then translocation on ACP

3# beta-ketoacyl — beta-ketoacyl reductase — beta hydroxyacyl > NADPH produced

4# beta-hydroxyacyl— beta-hydroxyacyl dehydratase— enoy

5# enoyl — enoyl reductase — acyl > NADPH produced

6# translocation of elongated products to -SH of enzyme for ACP to be re-charged with malonyl > so on & so forth

7# thioesterase splits off thioester bond between palmitate carboxylate and ACP > palmitate released

Question 12

STOICHIOMETRY OF SYNTHESIS OF PALMITATE (16:0)

Answer 12

carboxylation cost equation

7Acetyl-CoA+7CO2 +7ATP->7Malonyl-CoA+7ADP+7Pi

Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH + 14 H+ ->

Palmitate+7CO2 +8CoA+14NADP+ +6H2O

8 Acetyl-CoA+7ATP+14NADPH+14H+ ->

Palmitate+8CoA+6H2O+7ADP+7Pi +14NADP+

CAVIAT:additional energy cost:transport

So Acetyl-CoA is transported into the cytosol with a cost of 2 ATPs

• Therefore, cost of Fatty Acid synthesis is 3 ATPs per 2-C unit (1 carboxylation 2 transport)

Question 13

long-chain fatty acid synthesis ?

Answer 13

Palmitate is the precursor of stearate#18 and longer-chain saturated fatty acids, as well as the monounsaturated acids palmitoleate and oleate.

Question 14

DESATURATION OF FATTY ACIDS?

how does fatty acyl dehydratase function ?

where does desaturation take place ?

Answer 14

Palmitate(16:0) —> palmitoleate(16: 1; Δ9)

– Catalyzed by fatty acyl-CoA

#desaturase in animals
• Also known as the fatty acid

desaturases

• Requires NADPH; enzyme uses cytochrome b5 and cytochrome b5 reductase (Note that this is a Δ9- desaturase! It reduces the bond between C-9 and C-10).

how desaturases function ?

#both methylene carbons and ferrous cyt b5 are oxidized ; 4 electrons are donated to oxygen which becomes water
#cyt b5 reductase reduces cyt b5 back into the ferrous state and itself gets oxidized
#cyt b5 reductase is re-reduced via NADPH

*both fatty acid and NADPH are oxydized

**e- from fatty acyl CoA & second substrate, NADPH, reduce O2