Fat metabolism
Under fat metabolism (or lipid metabolism ), on the one hand, the breakdown of dietary fats in the digestive tract, i.e. fat digestion and transport via the thoracic duct into the venous blood, on the other hand, the oxidative metabolism in the body for the purpose of energy production and the breakdown and conversion to synthesis precursors understood by vitamins, steroid hormones and bile acids.
Fat digestion
During digestion , fats (lipids) and fat-like substances ( lipoids ) are emulsified by the gastric motor skills and partly already broken down (by gastric lipase ). This continues in the intestine until the smallest fat droplets are finally formed by the bile .
We ingest the following lipids through food:
- Triglycerides (vegetable oils, animal fats, etc.)
- Cholesterol (eggs, meat, etc.)
- Fatty acids of different sizes (chain length of the molecule) and saturation (saturated or unsaturated fatty acids).
For more information on this topic, see the special article fat digestion .
transport
In blood lipids can with the help of lipoproteins are transported. A total of approx. 80% of the triglycerides from digestion are absorbed by muscle and fat tissue. This is done by splitting off free fatty acids from triglycerides by means of the endothelial lipoprotein lipase , which is activated by a certain apolipoprotein (ApoCII) .
Metabolic pathways
Points of distinction
As a point of view for differentiating the fat metabolism processes can serve
- the function of fats and fat-like substances, hereinafter referred to as lipids ;
- the direction of the metabolism , differentiated according to anabolic ( anabolic ) and degrading ( catabolic ) processes, in which the (partially) degraded lipids serve as synthesis precursors for other substrates.
Since one of the most important functions of fats, which is the energy supplier, is at the same time the most important catabolic process and the structure of cell membranes, hormones, etc. is anabolic, the breakdown according to functional aspects is most appropriate.
Functions of lipids
The main functions of lipids are
- Energy storage of the organism for most energy-demanding processes;
- Structural components: as lipid bilayers, they form the basic structure of all cellular membranes;
- Synthesis precursor of a large number of biologically active compounds that serve as hormones or substances with a hormone-like effect.
Energy metabolism
With regard to the supply of energy for physical work and - to a limited extent - for structure-maintaining, energy-consuming processes and ion transport, triglycerides are of considerable importance. In connection with this energy-supplying function, they are of paramount importance as mass storage devices for energy. For this purpose, other substrates, especially carbohydrates , can also be converted into fat.
To release the energy inherent in triglycerides, they must first be broken down further. The fatty ester compounds are broken down by lipases , in the case of triglycerides by pancreatic triacylglycerases . After these have been active, a mixture of fatty acids, glycerine and monoacylglycerine results. Bile acids can now be used to form micelles in the intestinal lumen , which the mucosal cells v. a. of the duodenum.
Only after this dissection and passage of the intestinal endothelium can the triglycerides be resynthesized and "packed" in chylomicrons secreted into the lymph, from where they enter the blood via the thoracic duct .
In addition, triglycerides and cholesterol in the liver can also be obtained from e.g. B. glucose can be synthesized.
Before triglycerides can release energy in the cells, they must in turn be released from lipases
- from their transport "packages", the chylomicrons "released"
- as well as being split again into glycerine and fatty acids.
The most important function is performed by the hormone-sensitive lipase (HSL), which gradually separates fatty acids from glycerol through hydrolysis . For further processing, the fatty acids must be transported into the mitochondria . Mitochondria are the power plants of a cell, because in them the fatty acids are converted into energy. During the energy metabolism, L-carnitine binds long-chain fatty acids, which are formed during fat breakdown, and transports them to the mitochondria. The binding and separation takes place with the help of carnitine acyltransferases on the mitochondrial membrane. The actual membrane transport takes care of the carnitine-acylcarnitine transporter .
In the mitochondrion, the fatty acids first have to be activated before they can finally be broken down into acetyl-CoA by means of β-oxidation and introduced into the citric acid cycle . For this purpose, the fatty acids with the empirical formula CH 3 - (CH 2 ) n -COOH are esterified in two steps to give the thioester CH 3 - (CH 2 ) n -CO-S-CoA.
β-oxidation
The reaction at the β-carbon atom of the fatty acid, i.e. on the 3rd carbon atom in total, is called β-oxidation if you count from the side on which the carboxy group is (the carbon atom of the carboxy group is not counted with this method of counting counted as it is not a stereocenter).
The breakdown of fatty acids takes place gradually. This happens in a repeating sequence of 4 individual reactions.
The reactions take place in the mitochondrial matrix. Long chain fatty acids can not diffuse independently from the cytoplasm through the mitochondrial membranes and are therefore of carnitine bound, transported there in the form of acyl-carnitine.
For more information on this topic, see the special article β-oxidation .
As a result of the β-oxidation, the acetyl-CoA molecules can be introduced into the citric acid cycle and fed to the so-called end oxidation, i.e. H. When oxygen is consumed, the entire energy that can be released is converted into ATP or GTP and is therefore available to the body, e.g. the muscle cell, as energy that can be used for a short time.
Synthesis of lipids and lipoproteins
If the acetyl-CoA molecules produced during glycolysis are not otherwise used, fat reserves can be built up. These can be found everywhere in the body, as lipoproteins in the blood, or as lipids in the respective cells or specialized fat cells . If there is an excess of acetyl-CoA and if there is no food , ketone bodies can also be formed, which can function as energy carriers for the brain after an adjustment phase.
After the fatty acids have been built up (see fatty acid synthesis ), three of them are brought together in triglycerides . This is followed by the incorporation into the lipoproteins (see there).
Structure of cell membranes
See the main article cell membrane .
Steroid and other hormones
See the main article on steroid hormones .
Diseases of fat metabolism
The diseases of lipid metabolism include Gaucher's disease , Niemann-Pick disease , Tay-Sachs syndrome , xanthoma and hypercholesterolemia .
See also
Individual evidence
- ↑ See for example Ludwig Weissbecker: Diseases of the lipoid and fat metabolism (lipoidoses). In: Ludwig Heilmeyer (ed.): Textbook of internal medicine. Springer-Verlag, Berlin / Göttingen / Heidelberg 1955; 2nd edition, ibid. 1961, pp. 1119–1121.
- ↑ Gopinathrao / reactome.org: Import of palmitoyl-CoA into the mitochondrial matrix ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
Web links
- Jassal / Gillespie / Gopinathrao / D'Eustachio / reactome.org: Metabolism of lipids and lipoproteins
- "Fat Burning" in Sports Myth and Truth (Dr. Moosburger)