Fat synthesis

The mRNA forms a small proportion of the cell s RNA and has only a transient existence. In some microorganisms it may function as a template in synthesis 10-20 times only in mammalian tissues its active life may be much longer and in some cases it may persist for several days. 

The mechanism of protein synthesis discussed above does not involve addition of amino acids to preformed peptides synthesis starts with an amino acid and a polypeptide chain is synthesised by the successive addition of single amino acids. Unless all the amino acids required to synthesise the peptide are present at the right time, synthesis will not take place and the amino acids that are present are removed and may be catabolised. Considerable wastage of amino acids may thus take place if an incomplete mixture is presented for synthesis. 

NB The calculation assumes synchronous provision of the required amino acids efficiency is probably much less under normal conditions. 

The glycerides (triacylglycerols) of the depot fat are derived from preformed glycerides or may be synthesised in the body from fattyacyl-CoAs and L-glycerol-3-phosphate. This can take place in most tissues but is confined mostly to the liver and adipose tissue. 

The introduction of genes that alter inherent biochemical pathways in organisms may be of significant interest in the nutrition of animals, since it could confer to them some ability that they did not previously possess such as producing essential nutrients. For example  

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Respiration of the developing peanut is very rapid during fat synthesis but declines before maturity (72) studies of respiration indicate another decline about 8—10 hours after the harvest of peanuts (73). 

Insulin also plays a role in fat metabolism. In humans, most fatty acid synthesis takes place in the liver. The mechanism of action of insulin involves directing excess nutrient molecules toward metabolic pathways leading to fat synthesis. These fatty acids are then transported to storage sites, predominantly adipose tissue. Finally, insulin stimulates the uptake of amino acids into cells where they are incorporated into proteins. 

Getting ACETYL-CoA OUT OF THE MITOCHONDRIA and into the cytosol for fat synthesis. 

GLYCOGEN INCREASES GLYCOLYSIS FAT SYNTHESIS PROTEIN SYNTHESIS... 

We should note at this point that the TCA cycle is more than just a means of producing NADH for oxidative phosphorylation. The pathway also provides a number of useful intermediates for other, often synthetic, pathways. For example, citrate is the starting substance for fat synthesis (Chapter 9) succinyl-CoA is required for haem production and 2-oxoglutarate and oxaloacetate in particular are involved with amino acid and pyrimidine metabolism. Pathways which have dual catabolic/anabolic functions are referred to as amphibolic . 

Fat synthesis The acetyl-CoA prodnced from amino acid catabolism is also a precursor for fatty acid and triacyl-glycerol synthesis, both in adipose tissne and liver (details of pathways are given in Chapter 11). Unfortnnately, the quantitative significance of this pathway is not known. It is likely to be variable and probably small in hnmans. 

Glycogen synthesis in liver and muscle Glycogen breakdown in liver Fat synthesis in liver Fat breakdown in adipose tissue Protein synthesis in muscle Protein breakdown in muscle... 

Fat synthesis in the liver (right). Fatty acids and fats are mainly synthesized in the liver and in adipose tissue, as well as in the kidneys, lungs, and mammary glands. Fatty acid biosynthesis occurs in the cytoplasm—in contrast to fatty acid degradation. The most important precursor is glucose, but certain amino acids can also be used. 

To meet these changing circumstances, the liver has remarkable metabolic flexibility. For example, when the diet is rich in protein, hepatocytes supply themselves with high levels of enzymes for amino acid catabolism and gluconeogenesis. Within hours after a shift to a high-carbohydrate diet, the levels of these enzymes begin to drop and the hepatocytes increase their synthesis of enzymes essential to carbohydrate metabolism and fat synthesis. Liver enzymes turn over (are synthesized and degraded) at live to ten times the rate of enzyme turnover in other tissues, such as muscle. Extrahepatic... 

When the mass of adipose tissue increases, released leptin inhibits feeding and fat synthesis and stimulates oxidation of fatty acids. When the mass of adipose tissue decreases, a lowered leptin production favors a greater food intake and less fatty acid oxidation. 

Adipose tissue can metabolize glucose by means of the HMP, thereby producing NADPH, which is essential for fat synthesis (see p. 184 and Figure 24.5, ). However in humans, de novo synthesis is not a major source of fatty acids in adipose tissue. 

There is no direct evidence that the consumption of simple sugars is harmful. Contrary to folklore, diets high in sucrose do not lead to diabetes or hypoglycemia. Also contrary to popular belief, carbohydrates are not inherently fattening. They yield 4 kcal/g (the same as protein and less than half that of fat, see Figure 27.5), and result in fat synthesis only when consumed in excess of the body s energy needs. However, there is an association between sucrose consumption and dental caries, particularly in the absence of fluoride treatment. 

As can be seen in Table 4.2, the fatty acids are not randomly distributed among the three positions of the TG in bovine milk. Control of esterification is not understood, but there are several factors known to affect it. The presence of glucose is known to stimulate the synthesis of milk TG (Dimmena and Emery 1981 Rao and Abraham 1975). In the mouse, Rao and Abraham concluded that glucose was supplying factors other than NADPH or acylglycerol precursors that stimulated milk fat synthesis. The fatty acid that is esterified is known to be affected by the concentration of the acyl donors present (Marshall and Knudsen 1980 Bickerstaffe and Annison 1971). However, in studies under various conditions, palmitic acid was consistently esterified at a greater rate than other fatty acids (Bauman and Davis 1974 Moore and Christie 1978 Smith and Abraham 1975). 

Dimick, P. S., McCarthy, R. D. and Patton, S. 1970. Milk fat synthesis. In Physiology of Digestion and Metabolism in Ruminant, Ed. A.T. Phillipson (Editor). Oriel Press, Newcastle on Tyne, p. 534. 

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