Lipid metabolism

Source: Wikipedia, the free encyclopedia.

Lipid metabolism is the synthesis and degradation of

Vertebrates (including humans) use both sources of fat to produce energy for organs such as the heart to function.[6] Since lipids are hydrophobic molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with hydrolysis,[7] which occurs with the help of various enzymes in the digestive system.[2] Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals.[8] The second step after the hydrolysis is the absorption of the fatty acids into the epithelial cells of the intestinal wall.[6] In the epithelial cells, fatty acids are packaged and transported to the rest of the body.[9]

Metabolic processes include lipid digestion, lipid absorption, lipid transportation, lipid storage, lipid catabolism, and lipid biosynthesis. Lipid catabolism is accomplished by a process known as

mitochondria and peroxisome cell organelles
.

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,[10] along with further mechanical digestion, until they are individual fatty acid units able to be absorbed into the small intestine's epithelial cells.[11] It is the pancreatic lipase that is responsible for signalling for the hydrolysis of the triglycerides into separate free fatty acids and glycerol units.

Lipid absorption

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

adipose and other tissues in the body.[6][2][3]

Lipid transportation

Due to the hydrophobic nature of

VLDL) carry the triglycerides synthesized by our body and low-density lipoproteins (LDL) transport cholesterol to our peripheral tissues.[6][1] A number of these lipoproteins are synthesized in the liver, but not all of them originate from this organ.[1]

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.[13]

Lipid catabolism

Once the chylomicrons (or other lipoproteins) travel through the tissues, these particles will be broken down by

capillaries to release triglycerides.[14] Triglycerides will get broken down into fatty acids and glycerol before entering cells and remaining cholesterol will again travel through the blood to the liver.[15]

[15] Breakdown of fatty acids by beta oxidation
[15] Breakdown of fatty acids by beta oxidation

In the cytosol of the cell (for example a muscle cell), the

3-ketoacyl-CoA thiolase.[15] The diagram to the left shows how fatty acids are converted into acetyl-CoA. The overall net reaction, using palmitoyl-CoA
(16:0) as a model substrate is:

7 FAD + 7 NAD+ + 7 CoASH + 7 H2O + H(CH2CH2)7CH2CO-SCoA → 8 CH3CO-SCoA + 7 FADH2 + 7 NADH + 7 H+

Lipid biosynthesis

In addition to dietary fats, storage lipids stored in the adipose tissues are one of the main sources of energy for living organisms.[17] Triacylglycerols, lipid membrane and cholesterol can be synthesized by the organisms through various pathways.

Membrane lipid biosynthesis

There are two major classes of membrane lipids: glycerophospholipids and sphingolipids. Although many different membrane lipids are synthesized in our body, pathways share the same pattern. The first step is synthesizing the backbone (sphingosine or glycerol), the second step is the addition of fatty acids to the backbone to make phosphatidic acid. Phosphatidic acid is further modified with the attachment of different hydrophilic head groups to the backbone. Membrane lipid biosynthesis occurs in the endoplasmic reticulum membrane.[18]

Triglyceride biosynthesis

The phosphatidic acid is also a precursor for triglyceride biosynthesis. Phosphatidic acid phosphotase catalyzes the conversion of phosphatidic acid to diacylglyceride, which will be converted to triglycerides by acyltransferase. Triglyceride biosynthesis occurs in the cytosol.[19]

Fatty acid biosynthesis

The precursor for fatty acids is

palmitate
(16:0) as a model substrate is:

8 Acetyl-coA + 7 ATP + 14 NADPH + 6H+ → palmitate + 14 NADP+ + 6H2O + 7ADP + 7P¡

Cholesterol biosynthesis

isoprenoid pathway. Cholesterols are essential because they can be modified to form different hormones in the body such as progesterone.[6] 70% of cholesterol biosynthesis occurs in the cytosol of liver cells.[citation needed
]

Lipid metabolism disorders

Lipid metabolism disorders (including

cerebrotendinous xanthomatosis.[22]

Types of lipids

The types of lipids involved in lipid metabolism include:

References

  1. ^ a b c d e "Overview of Lipid Metabolism". Merck Manuals Professional Edition. Retrieved 2016-11-01.
  2. ^ a b c "Hydrolysis – Chemistry Encyclopedia – structure, reaction, water, proteins, examples, salt, molecule". chemistryexplained.com. Retrieved 2016-11-01.
  3. ^
    ISBN 978-0-86720-069-0
    .
  4. ISBN 978-1-4557-4580-7
    .
  5. PMID 19725772
    .
  6. ^
    ISBN 978-1-57259-931-4
    .
  7. ^ Ophardt CE (2013). "Lipid Metabolism Summary". Virtual Chembook. Elmhurst College.
  8. JSTOR 2430826?
    .
  9. ^
    PMID 27293434
    .
  10. ISBN 978-0-323-07446-9
    .
  11. OCLC 738349533
    .
  12. ISBN 978-90-481-8621-1
    .
  13. ^ 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
  14. PMID 26247089
    .
  15. ^ a b c "Fatty Acid beta-Oxidation – AOCS Lipid Library". lipidlibrary.aocs.org. Archived from the original on 2019-01-21. Retrieved 2017-11-28.
  16. ^
    ISBN 978-0-470-04073-7
    .
  17. PMID 27148161
    .
  18. ^
    PMID 20919643
    .
  19. ^
    PMID 1269065
    .
  20. ^ a b "Lipid Metabolism Disorders". MedlinePlus. Retrieved 2016-11-20.
  21. ISBN 978-0-443-02297-5
    .
  22. ^ a b "Disorders of Lipid Metabolism". Merck Manuals Consumer Version. Retrieved 2016-11-20.
  23. ^
    ISBN 978-0-8153-3218-3
    .
  24. ^
    PMID 10712926
    .
  25. PMID 12543708
    .
  26. S2CID 34803631
    .

Lipid+metabolism at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

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