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chemistry

Lipolysis

Lipolysis is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lipolysis rather than just read about it. In short: Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes.

Lipolysis — main illustration
Lipolysis — illustration

Key takeaways

  • Lipolysis belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Lipolysis to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Lipolysis from memory before moving on to harder problems.

Reference excerpt

Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes. Fat storage in the body is through adipose tryglicerides and is utilized for heat, energy, and insulation.

Mechanisms

In the body, stores of fat are referred to as adipose tissue. In these areas, intracellular triglycerides are stored in cytoplasmic lipid droplets. When lipase enzymes are phosphorylated, they can access lipid droplets and through multiple steps of hydrolysis, breakdown triglycerides into fatty acids and glycerol. Each step of hydrolysis leads to the removal of one fatty acid. The first step and the rate-limiting step of lipolysis is carried out by adipose triglyceride lipase (ATGL). This enzyme catalyzes the hydrolysis of triacylglycerol to diacylglycerol. Subsequently, hormone-sensitive lipase (HSL) catalyzes the hydrolysis of diacylglycerol to monoacylglycerol and monoacylglycerol lipase (MGL) catalyzes the hydrolysis of monoacylglycerol to glycerol. Perilipins are proteins that act as a physical shield or dynamic scaffold, preventing lipolytic enzymes in the cytosol from accessing the stored triglycerides during periods of energy abundance. Perilipin 1A is a key protein regulator of lipolysis in adipose tissue. This lipid droplet-associated protein, when deactivated, will prevent the interaction of lipases with triglycerides in the lipid droplet and grasp the ATGL co-activator, comparative gene identification 58 (CGI-58) (a.k.a. ABHD5). When perilipin 1A is phosphorylated by PKA, it releases CGI-58 and it expedites the docking of phosphorylated lipases to the lipid droplet. CGI-58 can be further phosphorylated by PKA to assist in its dispersal to the cytoplasm. In the cytoplasm, CGI-58 can co-activate ATGL. ATGL activity is also impacted by the negative regulator of lipolysis, G0/G1 switch gene 2 (G0S2). When expressed, G0S2 acts as a competitive inhibitor in the binding of CGI-58. Fat-specific protein 27 (FSP-27) (a.k.a. CIDEC) is also a negative regulator of lipolysis. FSP-27 expression is negatively correlated with ATGL mRNA levels.

Regulation

Lipolysis can be regulated through cAMP's binding and activation of protein kinase A (PKA). PKA can phosphorylate lipases, perilipin 1A, and CGI-58 to increase the rate of lipolysis. Catecholamines bind to 7TM receptors (G protein-coupled receptors) on the adipocyte cell membrane, which activate adenylate cyclase. This results in increased production of cAMP, which activates PKA and leads to an increased rate of lipolysis. Despite glucagon's lipolytic activity (which stimulates PKA as well) in vitro, the role of glucagon in lipolysis in vivo is disputed. Insulin counter-regulates this increase in lipolysis when it binds to insulin receptors on the adipocyte cell membrane. Insulin receptors activate insulin-like receptor substrates. These substrates activate phosphoinositide 3-kinases (PI-3K) which then phosphorylate protein kinase B (PKB) (a.k.a. Akt). PKB subsequently phosphorylates phosphodiesterase 3B (PD3B), which then converts the cAMP produced by adenylate cyclase into 5'AMP. The resulting insulin induced reduction in cAMP levels decreases the lipolysis rate. Insulin also acts in the brain at the mediobasal hypothalamus. There, it suppresses lipolysis and decreases sympathetic nervous outflow to the fatty part of the brain matter. The regulation of this process involves interactions between insulin receptors and gangliosides present in the neuronal cell membrane. The rate of lipolysis varies between adipose tissue depots due to differences in adrenergic receptor expression. Beta-adrenergic receptors (β1, β2, β3) stimulate lipolysis through Gs-protein-coupled activation of adenylyl cyclase and subsequent cAMP production, while alpha-2 adrenergic receptors (α2A) inhibit lipolysis through Gi-protein-mediated suppression of adenylyl cyclase. Subcutaneous abdominal adipose tissue exhibits a higher α2:β ratio compared to visceral adipose tissue, which is predominantly β3-receptor-expressing and more lipolytically responsive to catecholamine stimulation.

In blood Triglycerides are transported through the blood to appropriate tissues (adipose, muscle, etc.) by lipoproteins such as Very-Low-Density-Lipoproteins (VLDL). Triglycerides present on the VLDL undergo lipolysis by the cellular lipases of target tissues, which yields glycerol and free fatty acids. Free fatty acids released into the blood are then available for cellular uptake. Free fatty acids not immediately taken up by cells may bind to albumin for transport to surrounding tissues that require energy. Serum albumin is the major carrier of free fatty acids in the blood. The glycerol also enters the bloodstream and is absorbed by the liver or kidney where it is converted to glycerol 3-phosphate by the enzyme glycerol kinase. Hepatic glycerol 3-phosphate is converted mostly into dihydroxyacetonephosphate (DHAP) and then glyceraldehyde 3-phosphate (GA3P) to rejoin the glycolysis and gluconeogenesis pathway.

Medical procedures Physical lipolysis involves destruction of fat cells containing the fat droplets and can be used as part of cosmetic body contouring procedures. Currently there are four main non-invasive body contouring techniques in aesthetic medicine for reducing localized subcutaneous adipose tissue in addition to the standard minimally invasive liposuction: low-level laser therapy (LLLT), cryolipolysis, radio frequency (RF) and high-intensity focused ultrasound (HIFU). However, they are less effective with shorter lasting benefits and can remove significantly smaller amounts of fat compared to traditional surgical liposuction or lipectomy. However, future drug developments can be potentially combined with smaller procedures to augment the result.

References

External links Lipolysis at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

Illustrations

Lipolysis: This image illustrates the three separate steps of hydrolysis involved in lipolysis. In the first step, triacylglycerol is hydrolyzed to make diacylglycerol and this is catalyzed by adipose triglyceride lipase (ATGL). In the second step, diacylglycerol is hydrolyzed to make monoacylglycerol and this is catalyzed by hormone-sensitive lipase (HSL). In the last step, monoacylglycerol is hydrolyzed to make glycerol and this is catalyzed by monoacylglycerol lipase (MGL).
This image illustrates the three separate steps of hydrolysis involved in lipolysis. In the first step, triacylglycerol is hydrolyzed to make diacylglycerol and this is catalyzed by adipose triglyceride lipase (ATGL). In the second step, diacylglycerol is hydrolyzed to make monoacylglycerol and this is catalyzed by hormone-sensitive lipase (HSL). In the last step, monoacylglycerol is hydrolyzed to make glycerol and this is catalyzed by monoacylglycerol lipase (MGL).
Lipolysis: Example of a triacylglycerol
Example of a triacylglycerol
Lipolysis: Example of a diacylglycerol
Example of a diacylglycerol
Lipolysis: Example of a monoacylglycerol
Example of a monoacylglycerol
Lipolysis: Illustration of the activation of lipolysis in an adipocyte. Induced by high epinephrine and low insulin levels in the blood, epinephrine binds to beta-adrenergic receptors on the cell membrane of the adipocyte, which causes cAMP to be generated inside the cell. The cAMP activates protein kinases, which phosphorylate and thus activate hormone-sensitive lipases in the adipocyte. These lipases cleave free fatty acids from their attachment to glycerol in the lipid droplet of the adipocyte. The free fatty acids and glycerol are then released into the blood. The activity of hormone sensitive lipase is regulated by the circulating hormones insulin, glucagon,[2] epinephrine, norepinephrine, growth hormone, atrial natriuretic peptide, brain natriuretic peptide, and cortisol.[10]
Illustration of the activation of lipolysis in an adipocyte. Induced by high epinephrine and low insulin levels in the blood, epinephrine binds to beta-adrenergic receptors on the cell membrane of the adipocyte, which causes cAMP to be generated inside the cell. The cAMP activates protein kinases, which phosphorylate and thus activate hormone-sensitive lipases in the adipocyte. These lipases cleave free fatty acids from their attachment to glycerol in the lipid droplet of the adipocyte. The free fatty acids and glycerol are then released into the blood. The activity of hormone sensitive lipase is regulated by the circulating hormones insulin, glucagon,[2] epinephrine, norepinephrine, growth hormone, atrial natriuretic peptide, brain natriuretic peptide, and cortisol.[10]

Worked examples

Example 1 — a first encounter with Lipolysis

Start with the simplest possible case. Write down what Lipolysis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lipolysis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lipolysis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lipolysis

In research
Lipolysis appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Lipolysis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Lipolysis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biochemical reactions, Fatty acids, Hepatology, so understanding it makes those chapters shorter.
In everyday life
Look for Lipolysis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Lipolysis in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Lipolysis means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Lipolysis out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lipolysis in simple terms?

Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in fat adipocytes.

Why does Lipolysis matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lipolysis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lipolysis.

Tags

  • Biochemical reactions
  • Fatty acids
  • Hepatology
  • Lipid metabolism
  • Lipid methods

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