HSL

=__Hormone Sensitive Lipase __=

Expression and production
Hormone Sensitive Lipase (HSL) is an enzyme that’s activated when lipolysis needs to occur to provide energy to the cell. It’s highly expressed in tissues responsible for lipid storage and steroid production such as adipose tissue and steroidogenic tissue. It’s seen less in cardiac and skeletal muscle, which are more dependent on polysaccharide pathways. [1]

Regulation
Because it’s involved in such a critical process, it is highly regulated. Initial stimulants include catecholamines, and adrenocorticotropic hormones (ACTH), which are hormones involved in the fight-or-flight response, and bodily stress response systems respectively. Whereas, insulin and adenosine are common inhibitors. Insulin is particularly obvious because it’s only expressed when there is a high amount of glucose in the body. High glucose means lots of energy which means that no lipolysis is necessary.[1]

Direct activators include cyclic AMP (cAMP) dependent protein kinase (PKA), which can not only phosphorylate HSL, but also peripilin A which is a protein involved in shielding lipid molecules. Phosphorylation of peripilin causes this barrier to dissociate and the hydrolysis of triglycerides to begin.[1]

HSL then begins to hydrolyze the ester bonds of triglycerides using the catalytic residues Ser-423, Asp 703, His-733 on the C-terminal. It should be noted that the hydrolysis of the first ester bond is slow due to its dependence on hormone activation therefore it is the rate-limiting step in an otherwise fast reaction. 

**Pathway 1**
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