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Western Governors University

Hereditary Fructose Intolerance
Metabolism is the process of cellular respiration. It involves three steps, 1) glycolysis, 2) the citric acid or Krebs cycle and 3) electron transport system. The glycolytic pathway or glycolysis is a metabolic process that leads to the formation of the energy source adenosine triphosphate (ATP) in the body. ATP is essential to the cell and the cellular processes used by the cell. The first step, glycolysis is the process by which each molecule of glucose is converted to two pyruvate molecules, creating two ATPs and two nicotinamide adenine dinucleotide (NADH). NADH is also converted to four ATPs during the electron transport chain. An enzyme is a protein or other substance that will bind to a substrate, or substance that needs to be changed, to create a new, different, product. This product then becomes the next substrate for another enzyme and so on. An enzyme lowers the activation energy of a substance and acts as a catalyst (Hudon-Miller, 2012).

The process of metabolism of sugar (sucrose) includes two monosaccharaides, glucose and fructose. Glucose and fructose both go through glycolysis, however, fructose if first undergoes a conversion to glyceraldehyde and dihydroxyacetone phosphate (DHAP). Fructose is almost completely metabolized in the liver and is used towards replenishing glycogen in the liver and in triglyceride synthesis. There are three steps to fructose breakdown by the liver. First, the fructose has a phosphate added by the enzyme fructokinase to form fructose-1-phosphate. Next, the 6-carbon fructose is split into the 3-carbon molecules glyceraldehyde and DHAP by the enzyme aldolase B. Lastly, glyceraldehyde is then phosphorylated again by another enzyme so that it can enter the glycolytic pathway (New World Encyclopedia, n.d.). In Hereditary Fructose Intolerance (HFI) the body lacks the enzyme needed to break down fructose in the liver. This enzyme, aldolase B, is mutated and therefore cannot easily form

into tetramers altering the ability of the enzyme to work efficiently. Aldolase B catalyzes the breakdown of fructose-1-phosphate. Without it, fructose-1-phosphate is built up in the liver cells. This can cause toxicity resulting in cell death (, n.d.). It also does not allow fructose-1-phosphate to break down and create phosphate needed for Adenosine Triphosphate needed for cell energy.

Figure 1. depicts how enzymes work to help in processes like the breakdown of fructose. The substrate molecules attach to the active site on the enzyme creating at catalyst reaction that alters the molecules. The new molecule is then released and becomes active again. Figure 2. Depicts the effect of enzymes on enzyme activity. The enzyme catalyzes the reaction, meaning that it increases the efficiency of the system. Without enzyme the reaction takes much longer to occur. For example in HFI, the absence of Aldolase B enzyme greatly diminishes the body’s ability to breakdown fructose-1-phosphate to glyceraldehyde and dihydroxyacetone. Aldolase B specifically acts on the substrate fructose-1-phosphate decreasing its ability to catalyze the breakdown to glyceraldehyde and dihydroxyacetone. Reduced Aldolase B creates fructokinease inhibition and accumulation of fructose-1-phosphate in the cells. Too much accumulation in the cells can cause cell death. The accumulation of fructose-1-phosphate in the blood also changes the (ATP) to adenosine monophosphate (AMP) ratio in the cells. This results in an increase of uric acid and symptoms of hyperuricemia in HFI (Genetic People, 2011).


Figure 1 Depicts the lock and key mechanism of how
enzymes involved in the breakdown of molecules.


Figure 2 Depicts the effect of the enzyme on the amount of activation energy.

The Cori Cycle
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