Obviously, glucose is not the only fuel used by living things. Our foods contain other sugars, such as lactose in milk and fructose in the disaccharide sucrose (table sugar). The success of dieters hinges on the body's ability to use fat as fuel; of course, this is why the body stores fat in the first place. Under starvation conditions, the body obtains its energy for basic cell function by cannibalizing itself, by oxidizing its proteins.
The sugars are converted fairly easily into either glucose or another intermediate in the glycolysis pathway. In the case of sucrose and glycogen, these polysaccharides are split into simple sugars by phosphorolysis (splitting by phosphate) instead of hydrolysis. This results in glucose-6-phosphate, the intermediate in glycolysis that just follows the point where an ATP is reacted with glucose to get things going. Thus, the extra ATP is not needed, and glycolysis yields one more ATP than for glucose itself.
The human body maintains only enough stored glycogen to last about a day. Then it must switch to fats as a fuel. Most fats are stored in the body as triglycerides. Because they are not soluble in water, they must be transported in the blood as lipoproteins. After a fatty meal the concentration in the blood may be sufficient to give it a milky opalescence. Lipase enzymes in the blood or in fatty tissues hydrolyze the lipoproteins to glycerol and to fatty acids bound to blood proteins. The glycerol enters glycolysis after a few steps involving ATP. The fatty acids enter the cell and react with CoA similar to the way pyr-uvate does at the start of the Krebs cycle. The fatty acid-CoA compounds are then transported into the inner compartment of the mitochondria, where a series of reactions split off the last two carbons of the fatty acid, with the CoA, forming acetyl-CoA and a shorter fatty acid-CoA. The process, called beta oxidation, is repeated until the fatty acid has been consumed. The acetyl-CoAs produced then enter the Krebs cycle, where they are further oxidized. In addition, for every acetyl-CoA formed, one NADH2 and one FADH2 are formed, feeding the electron transport system production of ATP. This accounts for the high-energy yield of fats in comparison to carbohydrates.
Proteins are first hydrolyzed into their component amino acids, followed by deamina-tion, removal of the amino group. Finally, each of the 20 amino acids is converted to either pyruvate, acetyl-CoA, or one of the other intermediates in the Krebs cycle, for further oxidation. Free amino acids are not stored in the body. Excess proteins in the diet thus must be eliminated by the mechanism just described. Deamination releases ammonia to the blood, which can be toxic and must be rapidly removed. This can be accomplished by incorporation into new amino acids or by excretion either directly as ammonia (fish), uric acid (birds and reptiles), or urea (mammals).
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