Fatty acids transport within cells

Fatty acids Despite the fact that fatty acids are lipid soluble, so that they will diffuse across membranes without a transporter, one is present in the plasma membrane to speed up entry into the cells, so that it is sufficient to meet the demand for fatty acid oxidation. Triacylglycerol transport into cells also depends on the fatty acid transporter. Since it is too large to be transported per se, it is hydrolysed within the lumen of the capillaries in these tissues and the resultant fatty acids are taken up by the local cells via the fatty acid transporter (Chapter 7). Hence the fatty acid transporter molecule is essential for the uptake of triacylglycerol. 

Figure 7.8 Comparison of oxygen transport from lung to a cell and then into a mitochondrion with fatty acid transport from an adipocyte to a cell and then into the mitochondria in various tissues/ organs. Fatty add is transported in blood bound to albumin, oxygen is transported in blood bound to haemoglobin. Fatly add is transported within the cell attached to the fatty acid-binding protein (BP), oxygen is transported within a cell attached to myoglobin (Mb). Alb represents albumin, Hb haemoglobin.

Fatty acids and lipids usually are not transported between the cells of higher plants, but are transported within cells from chloroplasts to sites on the endoplasmic reticulum and back into the chloroplasts (Somerville and Browse, 1991). 

Fatty acid utilized by muscle may arise from storage triglycerides from either adipose tissue depot or from lipid stores within the muscle itself. Lipolysis of adipose triglyceride in response to hormonal stimulation liberates free fatty acids (see Section 9.6.2) which are transported through the bloodstream to the muscle bound to albumin. Because the enzymes of fatty acid oxidation are located within subcellular organelles (peroxisomes and mitochondria), there is also need for transport of the fatty acid within the muscle cell this is achieved by fatty acid binding proteins (FABPs). Finally, the fatty acid molecules must be translocated across the mitochondrial membranes into the matrix where their catabolism occurs. To achieve this transfer, the fatty acids must first be activated by formation of a coenzyme A derivative, fatty acyl CoA, in a reaction catalysed by acyl CoA synthetase. 

The FFA released by the adipocytes is collected by albumin and is transported to the various tissues in the blood. The albumin-FFA complex is able to cross the endothelial barrier in the capillaries and enter the interstitial space and so deliver this important fuel to the plasma membrane of the cell. To facilitate the transport of free fatty acids across the plasma membrane and within the cell, other transport proteins are present these are known as fatty acid binding proteins (FABP). 

After release from the adipocyte, the fatty acids are transported in the blood as a complex with albumin, as described above. They are then taken up by cells for oxidation This involves transport through the plasma membrane, the cytosol and finally the inner mitochondrial membrane of the cell for oxidation of the fatty acids within the mitochondria. 

The very low solubility of fatty acids poses a problem for their transport within the cell, as it does in the blood. The problem is solved by the presence of the fatty acid binding protein (FABP). It binds fatty acids at the inner surface of... 

Medium-chain acyl-CoA synthetase, which is present within the mitochondrial matrix of the liver, activates fatty acids containing from four to ten carbon atoms. Medium-chain length fatty acids are obtained mainly from triacylglycerols in dairy products. However, unlike long-chain fatty acids, they are not esterified in the epithelial cells of the intestine but enter the hepatic portal vein as fatty acids to be transported to the liver. Within the liver, they enter the mitochondria directly, where they are converted to acyl-CoA, which can be fully oxidised and/or converted into ketone bodies. The latter are released and can be taken up and oxidised by tissues. 

Fats (triacylglycerol, 1) are esters of glycerol with three fatty acids (see p.48). Within the cell, they mainly occur as fat droplets. In the blood, they are transported in the hydrophobic interior of lipoproteins (see p. 278). 

Most steroid hormones exist in part as sulfate esters and may also become esterified with fatty acids.256 The fatty acid esters may have relatively long lives within tissues.256 A special sex hormone-binding globulin transports sex hormones in the blood and regulates their access to target cells.256a b... 

Vitamin E, like neutral lipids, requires apoB lipoproteins at every stage of its transport (Fig. 27-2). Dietary vitamin E becomes emulsified in micelles produced during the digestive phase of lipid absorption and permeates the intestinal epithelium, similar to fatty acids and cholesterol. Uptake of vitamin E by enterocytes appears to be concentration dependent. Within intestinal cells, vitamin E is packaged into chylomicrons and secreted into lymph. During blood circulation of chylomicrons, some vitamin E may be released to the tissues as a consequence of partial lipolysis of these particles by endothelial cell-anchored lipoprotein lipase. The rest remains associated with chylomicron remnants. Remnant particles are mainly endocy-tosed by the liver and degraded, resulting in the release of fat-soluble vitamins. 

Vitamin E plays an important role in cell metabolism as an antioxidant for the elimination of reactive oxygen intermediates. Subsequent to intestinal resorption, vitamin E is transported in chylomicrons into the liver, from where it reaches other organs together with VLDL. Vitamin E deficiency is observed in chronic liver diseases caused by alcohol, Wilson s disease, haemochromatosis and abetalipoproteinaemia. In vitamin E deficiency, neurologic disturbances (areflexia, dysbasia, ocular palsy, reduced perception of vibration) occur haemolysis can likewise be induced or become more pronounced due to epoxide formation of unsaturated fatty acids within the erythrocyte membranes. 

These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains. Site for Kreb s Citric Acid Cycle Electron transport system and Oxidative Phosphorylation Fatty acid oxidation Amino acid catabolism Interconversion of carbon skeletons. 

The stratum corneum is the outermost layer of the epidermis and has a thickness of 10-15 pm. It is the principal barrier for the transport of most solutes (except for very lipophilic compounds) across the skin. The stratum corneum is a continuous heterogeneous structure that consists of approximately 10-25 layers of closely packed dead keratinized cells (corneocytes) cemented together by intercellular lipids. The intercellular lipids in the stratum corneum are in the form of multiple lamellar bilayers composed mainly of ceramides, cholesterol, and fatty acids. Proteins in the stratum corneum are largely concentrated within the corneocytes as keratin fibrils. The transport of lipophilic compounds across the stratum corneum is related to the intercellular lipids (lipoidal or intercellular pathways). On the other hand, it is believed that the transport of polar and ionic compounds is related to pathways with aqueous properties (the polar or pore pathways) when the stratum corneum is under a hydrated state. ... 

The following brief overview describes the modes of transport of FFAs in the body and serves as a preview to the description of the mitochondrial transport system. FFAs are generated by lipolysis of TGs stored in lipocytes of adipose tissue (adipocytes) and in lipocytes of other organs. Fatty acids are also released by the action of lipoprotein lipase in the bloodstream. Special membrane-bound proteins mediate the transfer of FFAs across the membranes of various cells, such as enterocytes and hepatocytes. Within the cell, FFAs are carried on special proteins called fatty acid-binding proteins. These proteins have low molecular weights, about 15,000, and can account for 5% of cytosolic protein (Bemlohr et ah, 1997). 

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