Lipid metabolism: Difference between revisions

'''Lipid metabolism''' is the synthesis and degradation of [[lipid]]s in cells, involving the breakdown and storage of fats for energy and the synthesis of structural and functional lipids, such as those involved in the construction of [[cell membrane]]s. In animals, these fats are obtained from food and are synthesized by the [[liver]].<ref name="MM_Lipid_metabolism">{{cite news | url = http://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/lipid-disorders/overview-of-lipid-metabolism|title=Overview of Lipid Metabolism|newspaper=Merck Manuals Professional Edition|access-date=2016-11-01}}</ref> Lipogenesis is the process of synthesizing these fats.<ref name="chemistryexplained.com">{{cite web | url = http://www.chemistryexplained.com/Hy-Kr/Hydrolysis.html | title = Hydrolysis – Chemistry Encyclopedia – structure, reaction, water, proteins, examples, salt, molecule | work = chemistryexplained.com | access-date = 2016-11-01 }}</ref><ref name = "Freifelder_1987">{{cite book | last1 = Freifelder | first1 = David | name-list-style = vanc | title = Molecular biology | date = 1987 | publisher = Jones and Bartlett | location = Boston | isbn = 978-0-86720-069-0 | edition = 2nd | url-access = registration | url = https://archive.org/details/molecularbiology00davi }}</ref> The majority of lipids found in the human body from ingesting food are [[triglycerides]] and [[cholesterol]].<ref>{{cite book | last1 = Baynes | first1 = Dominiczak | name-list-style = vanc | title = Medical Biochemistry | date = 2014|publisher=Saunders, Elsevier Limited|isbn=978-1-4557-4580-7|pages=121–122}}</ref> Other types of lipids found in the body are [[fatty acid]]s and [[membrane lipids]]. Lipid metabolism is often considered the [[digestion]] and absorption process of dietary fat; however, there are two sources of fats that organisms can use to obtain energy: from consumed dietary fats and from stored fat.<ref>{{cite journal | vauthors = Arrese EL, Soulages JL | title = Insect fat body: energy, metabolism, and regulation | journal = Annual Review of Entomology | volume = 55 | pages = 207–25 | date = 2010 | pmid = 19725772 | pmc = 3075550 | doi = 10.1146/annurev-ento-112408-085356 }}</ref> [[Vertebrates]] (including humans) use both sources of fat to produce [[energy]] for organs such as the [[heart]] to function.<ref name="Lehninger_2000">{{cite book | last1 = Lehninger | first1 = Albert L | first2 = David L. | last2 = Nelson | first3 = Michael M. | last3 = Cox | name-list-style = vanc | title = Lehninger Principles of Biochemistry | location = New York | publisher = Worth Publishers | year = 2000 | edition = 3rd | isbn = 978-1-57259-931-4 | url-access = registration | url = https://archive.org/details/lehningerprincip01lehn }}</ref> Since lipids are [[Hydrophobic effect|hydrophobic]] molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with [[hydrolysis]],<ref>{{cite web | last = Ophardt | first = Charles E. | name-list-style = vanc | url = http://chemistry.elmhurst.edu/vchembook/622overview.html | title = Lipid Metabolism Summary | date = 2013 | work = Virtual Chembook | publisher = Elmhurst College }}</ref> which occurs with the help of various enzymes in the digestive system.<ref name="chemistryexplained.com" /> Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals.<ref>{{cite journal | vauthors = Wedding RT | date = May 1972 | title = Reviewed Work: Plant Lipid Biochemistry | jstor = 2430826? | journal = [[The New Phytologist]] | volume = 71 | issue = 3 | pages = 547–548 }}</ref> The second step after the hydrolysis is the absorption of the fatty acids into the [[Epithelium|epithelial cells]] of the [[Gastrointestinal wall|intestinal wall]].<ref name="Lehninger_2000" /> In the epithelial cells, fatty acids are packaged and transported to the rest of the body.<ref name="Jo_2016">{{cite journal | vauthors = Jo Y, Okazaki H, Moon YA, Zhao T | title = Regulation of Lipid Metabolism and Beyond | journal = International Journal of Endocrinology | volume = 2016 | pages = 5415767 | date = 2016 | pmid = 27293434 | pmc = 4880713 | doi = 10.1155/2016/5415767 | doi-access = free }}</ref>

 

 

== Lipid digestion ==

[[Digestion]] is the first step to lipid metabolism, and it is the process of breaking the triglycerides down into smaller [[monoglyceride]] units with the help of [[lipase]] enzymes. Digestion of fats begin in the mouth through chemical digestion by [[lingual lipase]]. Ingested cholesterol is not broken down by the lipases and stays intact until it enters the epithelium cells of the small intestine. Lipids then continue to the stomach where chemical digestion continues by [[gastric lipase]] and mechanical digestion begins ([[peristalsis]]). The majority of lipid digestion and absorption, however, occurs once the fats reach the small intestines. Chemicals from the pancreas ([[pancreatic lipase family]] and [[bile salt-dependent lipase]]) are secreted into the small intestines to help breakdown the triglycerides,<ref>{{cite book | last1 = Pelley | first1 =John W. | name-list-style = vanc | title = Elsevier's Integrated Review Biochemistry | date = 2012 | publisher = Elsevier/Mosby | location = Philadelphia | isbn = 978-0-323-07446-9 | edition = 2nd }}</ref> along with further mechanical digestion, until they are individual [[fatty acid]] units able to be absorbed into the small intestine's [[Epithelium|epithelial cells]].<ref>{{cite book | title = Fundamentals of Biochemistry: Life at the Molecular Level | first1 = Donald | last1 = Voet | last2 = Voet | first2 = Judith G. | last3 = Pratt | first3 = Charlotte W. | name-list-style = vanc | date = 2013 | publisher = Wiley | isbn = 978-0-470-54784-7 | edition = Fourth | location = Hoboken, NJ | oclc = 738349533 }}</ref> It is the pancreatic lipase that is responsible for signalling for the [[hydrolysis]] of the triglycerides into separate free fatty acids and glycerol units.

 

[[File:2431 Lipid Absorption.jpg|thumb|Flowchart showing the lipid absorption process]]

The second step in lipid metabolism is absorption of fats. Short chain fatty acids can be absorbed in the [[stomach]], while most absorption of fats occurs only in the [[small intestine]]s. Once the triglycerides are broken down into individual fatty acids and [[glycerol]]s, along with cholesterol, they will aggregate into structures called [[micelle]]s. Fatty acids and monoglycerides leave the micelles and diffuse across the membrane to enter the intestinal epithelial cells. In the [[cytosol]] of epithelial cells, fatty acids and monoglycerides are recombined back into triglycerides. In the cytosol of epithelial cells, triglycerides and cholesterol are packaged into bigger particles called [[chylomicron]]s which are [[amphipathic]] structures that transport digested lipids.<ref name="Jo_2016" /> Chylomicrons will travel through the bloodstream to enter [[adipose]] and other tissues in the body.<ref name="Lehninger_2000" /><ref name="chemistryexplained.com" /><ref name="Freifelder_1987" />

 

== Lipid storage ==

Lipids are stored in [[white adipose tissue]] as triglycerides. In a lean young adult human, the mass of triglycerides stored represents about 10–20 kilograms. Triglycerides are formed from a backbone of glycerol with three fatty acids. Free fatty acids are activated into acyl-CoA and esterified to finally reach the triglyceride droplet. Lipoprotein lipase has an important role.<ref>Mechanism of Storage and Synthesis of Fatty Acids and Triglycerides in White Adipocytes | Physiology and Physiopathology of Adipose Tissue pp 101–121 | DOI: 10.1007/978-2-8178-0343-2_8</ref>

 

 

== Lipid biosynthesis ==

 

=== Membrane lipid biosynthesis ===

[[Cholesterol]] can be made from [[acetyl-CoA]] through a multiple-step pathway known as [[isoprenoid pathway]]. Cholesterols are essential because they can be modified to form different [[hormone]]s in the body such as [[progesterone (medication)|progesterone]].<ref name="Lehninger_2000" /> 70% of cholesterol biosynthesis occurs in the cytosol of liver cells.{{Citation needed|date=January 2019}}

 

Lipid metabolism is tightly regulated by hormones to ensure a balance between energy storage and utilization.

 

* [[Thyroid hormones|Thyroid Hormone:]] promotes hepatic triglyceride synthesis, enhancing lipolysis, stimulating mitochondrial fatty acid β-oxidation, and regulating cholesterol levels through various mechanisms, including LDL receptor expression and bile acid excretion.<ref name=":0" />

* [[Sex hormone|Sex Hormone:]]

 

== Lipid metabolism disorders ==

 

== Types of lipids ==

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