Enzymes as Catalysts
All known enzymes are catalysts. Enzymes have a number of properties that make them catalysts. To begin with, enzymes accelerate the rate of a chemical reaction without being used up them or altered by the reaction (Luciano, 1983).similarly; enzymes accelerate rates of chemical reactions without changing the equilibrium between the products and reactants of the reaction.
Metabolism of Fructose in the Liver
Once fructose is absorbed, it is rapidly taken up by the liver by glucose transporter, GLUT2(Bozzetti,2006).The fructose in the liver is rapidly metabolized by the enzyme fructokinase into fructose -1-phosphate.Aldolase B enzyme acts on the substrate fructose -1-phosphate.The enzyme metabolizes fructose-1-phosphate into dihydroxyacetone phosphate(DHAP) and glyceraldehyde in two different reaction paths.
The dihydroxyacetone phosphate is converted into glyceraldehydes-3-phosphate by the enzyme triose phosphate isomerase. On the other hand, the glyceraldehyde is also phosphorylated into glyceraldehyde-3-phosphate by glyceraldehyde kinase. The glyceraldehyde can also be metabolized into DHAP by the enzyme glycerol phosphate dehydrogenase. The glyceraldehyde-3- phosphate is further broken into pyruvate which enters the citric cycle as acetyl CoA.
Deficiency of Aldolase B
With the deficiency of aldolase B, the amounts of substrates gradually build up as more fructose is absorbed into the body. The quantity of fructose-1-phosphate rises gradually and the formation of more dihydroxyacetone phosphate and glyceraldehyde is hindered.
Primarily, the cause of the symptoms that manifest in hereditary fructose intolerance is due to the trapping of phosphate in the substrate fructose-1-phosphate and consequently the reduction in the pool of adenosine triphosphate through fructokinase action. The action of all cellular processes that depend on phosphorylation and ATP are lowered due to the trapping of inorganic phosphate and the exhaustion of ATP pool.
Considering the role of inorganic phosphate as a substrate in the action of glycogen phosphorylase, the loss of organic phosphate readily hinders glycogen metabolism. This consequently leads to hypoglycaemia in case of fructose ingestion. The exhaustion of the phosphate pool also contributes to the AMP deaminase activation which leads to an increase in nucleotide catabolism. Consequently, this also causes hyperuricemia. High levels of fructose-1-phosphate inhibit the action of fructokinase in a feedback mechanism. This causes a reduction in the uptake of fructose thus fructosemia.
Cori Cycle
If Cori cycle were to take place and remain in a single cell, the amount of ATP in that cell would reduce consequently. Glycolysis part of the cycle produces an overall of 2ATP molecules and 6ATP molecules are produced during the process. This indicates that a net consumption of 4 ATP molecules are required to maintain the entire cycle and therefore the cycle is not able to be sustained indefinitely.
Cori cycle entails a metabolic pathway where lactate produced through anaerobic glycolysis in body muscles is moved to the liver which is then converted to glucose. The glucose is then returned to the muscle and converted back to lactate. The Cori cycle provides an alternative for release of lactic acid and source replenishment of glucose (Hinwood, 1997)
Defect of an Enzyme in Cori Cycle
A defect involving ATP synthase would decrease the overall ATP production. Formation of ATP molecules during kreb’s cycle requires the release of small amounts of energy. Oxaloacetate is formed formed again after carbon (IV) oxide formation through oxidation reactions in electron transport chain. This alludes that any defect a defect in electron transport chain hinders the conversion of ADP to ATP. Adenosine triphosphate is formed when hydrogen ion is moved down to it particular concentration gradient by ATP synthase. Any defect involving enzymes of electron transport are likely to lead to a decrease in ATP production since it inhibits the conversion of ADP to ATP.
Krebs’s cycle produces several products most of which are converted to ATP through oxidative phosphorylation. Both succinate and NADH are from kreb’s cycle and are consequently oxidized to produce energy. The energy released is used by ATP synthase to produce more ATP.
Role of Coenzyme Q10 in Electron Transport Chain
Coenzyme Q10 is an inorganic vitamin like substance found in most cells that serve in the production of body energy (Ley, 1999). Coenzyme Q10 is important in the conversion of energy particularly from fats and carbohydrates into ATP. The process of energy production takes place in the inner mitochondrial membrane
During the process, electrons released from glucose and fatty acid metabolism are accepted by coenzyme Q10.Coenzyme Q10 then converts the electrons into electron acceptors. Coenzyme Q10 initiates formation of a strong proton gradient across the mitochondrial membrane by transferring the proton outside the membrane. Energy is then eventually released when protons flow right back into the interior of the mitochondrion.
Electron Transport Chain and Oxidative Phosphorylation
Electron transport chain involves several compounds involved in transferring electrons from basically electron donors to acceptors through redox reactions. The electron transfer is also coupled entirely with proton transfer particularly across a membrane. This process creates a wide proton gradient that contributes to the driving of the synthesis of ATP and production of chemical energy in the form of ATP. Molecular oxygen eventually becomes the final electron acceptor.
In oxidative phosphorylation, mitochondria make use of their structure, energy and enzymes to produce adenosine triphosphate. During this process, electrons are released from electron donor and transferred to electron acceptors via redox reactions. These redox reactions contribute to the formation of energy which is used to form ATP. These reactions are made possible due to certain protein complexes within the mitochondrial intermembrane space.
Reference
Bozzetti, F., Staun, M., & Gossum, A. . (2006). Home parenteral nutrition. Wallingford, Oxfordshire, UK: CABI Pub.pg.208
Hinwood, B. G., & Hinwood, B. G. (1997). A textbook of science for the health professions. Cheltenham, U.K: Stanley Thorton.pg.373
Ley, B. M. (1999). Coenzyme Q10: All-around nutrient for all-around health! : latest research as a heart strengthener, energy promoter, aging fighter, and much more!. Temecula, CA: BL Publications.pg.58
Luciano, D. S., Vander, A. J., & Sherman, J. H. (1983). Human anatomy and physiology: Structure and function. New York: McGraw-Hill.pg.57
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