Step 4: Formation of triacylglycerol/triglyceride
6.5.2. MOBILIZATION OF STORED TRIGLYCERIDES
Hydrolysis of stored triglycerides and release of FFA into the blood is referred to as
“mobilization”.
TRIGLYCERIDE GLYCEROL
+
3 H 2O HC
CH2OH
CH2 OH
H O R
_ _ C H
C
_O
O C_ O H2 _
CH2_ O_ C O
_
C O
_
R R
lipases
+
FREE FATTY ACIDS O H C O R _
3
Fig.6.5.10. Generalized reaction for the hydrolysis of triglycerides by lipases.
Mobilization of TG from adipose tissue is a function of a critical enzyme Hormone-Sensitive Lipase (HSL). As the name implies, the activity of this lipase is regulated by hormones.
Epinephrine/glucagon act through a cAMP-mediated pathway to activate protein kinase A (PKA). Subsequently PKA carries out two phosphorylations that promote hydrolysis of TGs:
phosphorylation of HSL: causing a 2-3 fold increase in its catalytic activity
phosphorylation of perilipin A: enables HSL to move to the surface of the lipid droplet, stimulating its lipolytic activity by about 50 times
Fig.6.5.11. Schematic diagram of the mobilization of free fatty acids (FFA) from adipose tissue to myocytes:
Hormones like glucagon trigger a cAMP-dependent enzyme pathway (1-4) causing phosophorylation of hormone-sensitive lipase and surface perilipin molecules on the lipid droplet. Hydrolysis of triacyl- glycerol
follows (5) and FFA is released into the blood. Bound FFAs (6) in blood enter myocytes with help of a transporter (7) and undergo β-oxidation. (Source: Nelson and Cox 2005, p 634 fig 17-3)
The products of TG hydrolysis are fatty acids and glycerol. Adipose tissue releases a part of the FFA produced into the blood while the rest is converted to acyl-CoA for re-use in TG synthesis. Blood FFA is bound by serum albumin, and transported to the muscle, liver, renal cortex and other tissues. At the level of the cells, the fatty acid dissociates from albumin and is transported across the plasma membrane into the cytosol by a specific fatty acid transporter.
Glycerol produced in lipolysis cannot be metabolized by adipocytes but is released into the blood for use by cells of other tissues. Utilization of glycerol is possible only when it is converted to glycerol 3-phosphate. This requires the action of glycerol kinase, an enzyme which is absent in adipose tissue and muscle but present in other tissues like the liver and kidney. For the same reason, glycerol that is released from its bound form in lipoprotein complexes by lipoprotein lipase, cannot be used by adipose tissue.
Fig.6.5.12. Conversion of glycerol to glycerol 3-phosphate; subsequently the product can be utilized in glycolysis, gluconeogenesis or in the synthesis of glycerides.
Regulation of HSL
The lipolytic activity of HSL is regulated by its reversible phosphorylation/de- phosphorylation. Phosphorylated HSL is catalytically active while dephosphorylated HSL is inactive. Activation of HSL is mainly by a cAMP-mediated cascade mechanism (as for glycogenolysis, refer chapter 5 section 5.A), triggered by hormones like epinephrine. Other lipolytic hormones include glucagon, the thyroid hormones, growth hormone and vasopressin. Glucocoticoids promote lipolysis through a different pathway which involves synthesis of a new lipase and activation of suitable genes, but does not involve cAMP.
C H
C
C Glycerol
glycerol 3-P dehydrogenase NA D+ NA DH.H+
P C CH2OH
CH2O O A PT A DP
glycerol kinase H2OH
H2 OH
H
O P
O H C H
CH2OH
CH2O Glycerol 3-phosphate
DHA P
GLUCO - NEOGENESIS
TRIGLYCERIDE SYNTHESIS
GLYCOLYSIS
_
FFA
+ GLUCOCORTICOIDS
FFA
_
DG
+
FFA TG HSL2-MG
+
FFA+
FFA GLYCEROL 2-MG Lipase
GH TSH ACTH
MSH
A T P cA M P +
Adenylyl cyclase
INSULIN _ THYROID HORMONES
+
+ EPINEPHRINE
NOREPINEPHRINE +
G LUCAGON
P K A +
A T P Mg2+
D A P
active H S L P inactive
H S L
O H2 P i
Lipase phosphatase
+ INSULIN +
INSULIN
5 AMP'
Phospho - diesterase
Fig.6.5.12. Regulation of hormone-sensitive lipase (HSL): Continuous arrows show significant steps in activation and catalytic action of HSL. Broken arrows indicate sites of action of major hormones viz epinephrine, norepinephrine, glucagon and insulin. Thyroid and pituitary hormones have a facilitatory role.
Glucocorticoids activate HSL by a cAMP-independent route. Stimulation and inhibition are shown by symbols. (PKA = protein kinase A; TG/DG/MG = Tri-/di-/mono-acylglycerols respectively)
Insulin is a major inhibitor of lipolysis in adipose tissue. It inhibits HSL indirectly in three ways:
Inhibition of cAMP formation by adenylyl cyclase
Stimulation of the breakdown of cAMP to 5’AMP by phosphodiesterase
Stimulation of lipase phosphatase, which in turn, dephosphorylates and inactivates
Since insulin secretion is stimulated by high blood levels of glucose, hence the nutritional state of the body has a major role in controlling the rate of lipolysis vs the rate of lipogenesis.
Insulin promotes glucose uptake and its increased conversion to TGs in adipose tissue, concommittantly decreasing the release of FFA from TG stores.
It is believed that a hormone, leptin, which is secreted by the adipose tissue and signals energy sufficiency, may also be important in regulation.
Re-cycling of TGs
Fig.6.5.13. Schematic representation of short- and long-range re-cycling of triacylglycerol between adipose tissue and liver. (Source: Nelson and Cox, 2005, p 806 fig 21-20)
It is remarkable that the adipose tissue, generally regarded as rather “low-profile”, carries out a continuous cycle of lipolysis and re-esterification in its cells and still maintains fairly constant levels of stored lipids over long periods of time. 70% of all fatty acids that are released by lipolysis are re-esterified to form triglycerides, either by short-re-cycling within the adipose tissue, or by a systemic long-range re-cycling between the liver and adipose tissue. Net release of FFA from the adipose tissue into the blood occurs only when hormones stimulate high activity of HSL to the extent that [FFA] exceeds the ability of the adipocyte to re-synthesize triacylglycerols. The liver re-synthesizes TGs and releases them into the blood within VLDL. Breakdown of VLDL by lipoprotein lipase in the capillary walls again makes FFA available to the adipose tissue.
TO SUM UP SECTIONS 2, 3, 4 AND 5:
We end this discourse on the metabolism of the simple lipids with a composite diagram to clarify your concepts:
Fig.6.5.14. Summary of the metabolism of triacylglycerols and fatty acids in adipose tissue and liver. Red and green circles are points of inhibition and stimulation respectively.
(Source: Voet and Voet, 1995, p 691 fig 23-35)