Adipose Hormone-sensitive lipase (HSL), also known as

Adipose tissue is a loose connective tissue
composed mostly of adipocytes. Its main function is to store energy in the form
of lipids, but it is also used to cushion and insulate the body. Furthermore, it
has also been found to secrete hormones such as oestrogen and the cutokine TNF?. There are two types of
adipose tissues: white and brown.  White
adipose tissue is made up of large cells in one huge droplet, containing only
few mitochondria and they have poor blood supply. On the other hand, brown
adipose tissue is made up of small polyclonal cells with lipid droplets. In
addition, brown adipose tissue is mainly found in new-borns and hibernating
animals and play a role in non-shivering thermogenesis.

Lipolysis is the breakdown
of lipids by the hydrolysis of triglycerides (TG) into glycerol and free fatty
acids. The fatty acids produced can then be metabolised by cells to yield large
quantities of ATP.

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The rate at which lipids are
broken down is limited by the release of triglyceride lipase which allows for
the rate fatty acids are produced to be regulated. The release of triglyceride
lipase is almost entirely controlled by the hormones adrenalin, nor-adrenalin,
growth hormone and Adrenocorticotropic hormone (ACTH). These work by
phosphorylating the enzyme and thereby activating it. The hormones activate G
protein coupled receptors (GPCR) on the phospholipid bilayer which transmits a
stimulatory signal to adenylate cyclase. This results in generation of cyclic
AMP (cAMP). cAMP binds to protein kinase A (PKA), causing the regulatory
subunits to dissociate from active subunits thereby activating the enzyme. PKA
then subsequently activates triglyceride lipase via phosphorylation.

Hormone-sensitive lipase
(HSL), also known as triglyceride lipase, this hydrolyses ester linkages of
triglycerides, forming diacylglycerol which is then hydrolysed further into
free fatty acids and glycerol. Fatty acids are then released from the cell
where they can be used as an energy source.

Lipolysis can then be
inhibited using the same general principles. The entire reaction relies upon
triglyceride lipase being phosphorylated. When the body no longer needs fatty
acids as an energy source, the hormones essential for triglyceride lipase will
be inhibited and instead insulin will be secreted. this results in
dephosphorylation on triglyceride lipase which prevents the enzyme cascade
therefore, preventing breakdown of triglyceride lipase