Fatty acids are a class of carboxylic acid compounds, consisting of hydrocarbon groups consisting of carbon and hydrogen connected to carboxyl groups. Under sufficient oxygen supply, they can be oxidized and decomposed into CO2 and H2O, releasing large amounts of energy. Therefore, fatty acids are one of the main energy sources of the body.
1. Divided according to the number of carbon atoms on the carbon chain.
① Short chain fatty acids (SCFA, C≤6): The number of carbon atoms on the carbon chain is between 1~6. It is also called Volatile fatty acids (VFA) because of its strong volatile energy. The common short-chain fatty acids are: acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, and capric acid. Short-chain fatty acids are the main metabolites of microorganisms and can inhibit the proliferation of certain bacteria to maintain the stability of microflora in low pH environment. Research on these fatty acids is mainly related to the metabolism of intestinal flora, host microbial co-metabolism and the development of microbial fermentation, food nutrition, drugs or health products.
② Mid chain fatty acids (MCFA, 6 < C ≤ 12): the number of carbon atoms in the carbon chain is between 6 and 12. The common medium chain fatty acids are: octanoic acid, capric acid, undecanoic acid, dodecanoic acid, etc.
③ Long chain fatty acids (LCFA, C>12): the number of carbon atoms on the carbon chain is 12 or more. Common long-chain fatty acids include: myristic acid, myristic acid, palmitoleic acid, stearic acid, oleic acid, etc.
2. Divided according to the number of double bonds between carbon atoms in the carbon chain.
① Saturated fatty acids: no double bond between carbon and carbon in the carbon chain
② Monounsaturated fatty acids: on the carbon chain, there is a double bond between carbon and carbon
③ Polyunsaturated fatty acid: there are two or more double bonds between carbon and carbon in the carbon chain
Fatty acids classification (Johnston et al., 2017)
The energy stored in fats is mainly in fatty acids. The fatty acids produced by lipolysis bind to clear proteins (albumin) to produce very high density lipoproteins, which are transported via the circulation to the tissues that require energy. Tissues take up fatty acids via various transporter proteins (CD36, FATP, FABP, etc.).
Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids. These processes can be mainly divided into (1) catabolic processes that produce energy and (2) anabolic processes.
In catabolism, fatty acids are metabolized to produce energy, mainly in the form of adenosine triphosphate (ATP). Compared to other macronutrient classes (carbohydrates and proteins), fatty acids produce the most ATP on a per-gram-energy basis when they are fully oxidized to CO2 and water via the beta-oxidation and citric acid cycles. Thus, fatty acids (mainly in the form of triglycerides) are the most important form of fuel storage in most animals and to a lesser extent in plants.
Diagram of fatty acid catabolism (Boer et al., 2006).
Most of the fatty acids in the human body come from food and are exogenous fatty acids, which can be used by the body in the body through transformation and processing. The body can also use sugar and protein to transform into fatty acids called endogenous fatty acids, which are used for triglyceride production and energy storage. The main organs that synthesize fatty acids are the liver and the lactating mammary gland. Fatty acids can also be synthesized in adipose tissue, kidney and small intestine. The immediate raw material for fatty acid synthesis is acetyl CoA. ATP and NADPH are consumed to produce the sixteen-carbon soft fatty acids, which are processed to produce various fatty acids in the body. Synthesis takes place in the cytoplasm.
Biosynthesis and degradation of fatty acids and membrane lipids (Janßen et al., 2014)
Lipid metabolism is involved in a variety of cellular processes critical to cellular transformation, tumor development and disease progression. In cancer research, fatty acid metabolism affects cancer cell biology in many ways, particularly the synthesis of membrane lipids, including glycerophospholipids and signal transduction intermediates such as phosphatidylinositol (4,5)-biphosphate, diacylglycerol (DAG) and phospholipids that promote mitogenic and/or oncogenic signaling. Fatty acids are also substrates for mitochondrial ATP and NADH synthesis, arachidonate production and post-translational protein-lipid modifications of signaling proteins. Cancer cells can acquire fatty acids from a range of intracellular and extracellular sources. These alterations in fatty acid metabolism are a feature of tumorigenesis and metastasis.
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