Hormone-sensitive lipase (HSL) plays a key role in lipid metabolism and overall energy homoeostasis, by controlling the release of fatty acids from stored triglycerides in adipose tissue. Lipases and esterases form a protein superfamily with a common structural fold, called the alpha/beta-hydrolase fold, and a catalytic triad of serine, aspartic or glutamic acid and histidine. Previous alignments between HSL and lipase 2 of Moraxella TA144 have been extended to cover a much larger part of the HSL sequence. From these extended alignments, possible sites for the catalytic triad and alpha/beta-hydrolase fold are suggested. Furthermore, it is proposed that HSL contains a structural domain with catalytic capacity and a regulatory module attached, as well as a structural N-terminal domain unique to this enzyme. In order to test the proposed domain structure, rat HSL was overexpressed and purified to homogeneity using a baculovirus/insect-cell expression system. The purification, resulting in > 99% purity, involved detergent solubilization followed by anion-exchange chromatography and hydrophobic-interaction chromatography. The purified recombinant enzyme was identical to rat adipose-tissue HSL with regard to specific activity, substrate specificity and ability to serve as a substrate for cAMP-dependent protein kinase. The recombinant HSL was subjected to denaturation by guanidine hydrochloride and limited proteolysis. These treatments resulted in more extensive loss of activity against phospholipid-stabilized lipid substrates than against water-soluble substrates, suggesting that the hydrolytic activity can be separated from recognition of lipid substrates. These data support the concept that HSL has at least two major domains.
Hormone-sensitive lipase, the rate-limiting enzyme of intracellular TG hydrolysis, is a major determinant of fatty acid mobilization in adipose tissue as well as other tissues. It plays a pivotal role in lipid metabolism, overall energy homeostasis, and, presumably, cellular events involving fatty acid signaling. Detailed knowledge about its structure and regulation may provide information regarding the pathogenesis of such human diseases as obesity and diabetes and may generate concepts for new treatments of these diseases. The current review summarizes the recent advances with regard to hormone-sensitive lipase structure and molecular mechanisms involved in regulating its activity and lipolysis in general. A summary of the current knowledge regarding regulation of expression, potential involvement in lipid disorders, and role in tissues other than adipose tissue is also provided.
Hormone-sensitive lipase is the key enzyme in the mobilization of fatty acids from adipose tissue, thereby playing a crucial role in the overall energy homeostasis in mammals. Its activity is stimulated by catecholamines through cAMP-dependent phosphorylation of a single serine, a process that is prevented by insulin. This regulatory property is unique to this enzyme among all known lipases and has been acquired during evolution through insertion of a regulatory module into an ancestral lipase. Sequence alignments have failed to detect significant homology between hormone-sensitive lipase and the rest of the mammalian lipases and esterases, to which this enzyme is only very distantly related. In the present work, we report the finding of a remarkable secondary structure homology between hormone-sensitive lipase and the enzymes from a superfamily of esterases and lipases that includes acetylcholinesterase, bile salt-stimulated lipase, and several fungal lipases. This finding, based on the identification of the secondary structure elements in the hormone-sensitive lipase sequence, has allowed us to construct a three-dimensional model for the catalytic domain of hormone-sensitive lipase. The model reveals the topological organization, predicts the components of the catalytic triad, suggests a three-dimensional localization of the regulatory module, and provides a valuable tool for the future study of structural and functional aspects of this metabolically important enzyme.In mammals, free fatty acids derived from adipose tissue triglycerides are the most important fuel and provide more than half of the caloric requirements when dietary substrates are lacking. Hormone-sensitive lipase (HSL) 1 plays a crucial role in the mobilization of free fatty acids from adipose tissue by catalyzing the first and rate-limiting step in the hydrolysis of stored triglycerides. Thus, HSL is a key enzyme not only in lipid metabolism but also in overall energy homeostasis. HSL is under acute hormonal and neural control. Its activity is stimulated by cAMP-dependent phosphorylation of a single serine residue (1), named regulatory site. This phosphorylation is catalyzed by cAMP-dependent protein kinase upon stimulation of -adrenergic receptors by catecholamines. The antilipolytic action of insulin is mediated through a net dephosphorylation of the regulatory site of HSL (1). A second, cAMP-independent, phosphorylation site is present in the HSL protein, two residues C-terminal of the regulatory site (1, 2). Phosphorylation of this site (called the basal site) does not affect the catalytic activity but prevents phosphorylation of the regulatory site, thus having a potential antilipolytic role (3). The regulation of the enzyme activity by reversible phosphorylation is the most remarkable property of HSL, and unique among all known lipases. A second distinctive feature of HSL is its unusual substrate specificity, hydrolyzing cholesteryl esters and triglycerides at similar rates and showing 10 times higher hydrolytic activity toward diglycer...
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