Reflection 7 | biochemasterz

Glycolysis:

· Image 92

The fates of pyruvate

What is the fate of pyruvate in the cell? Well, that depends on whether the conditions are aerobic or anaerobic…

If oxygen is available, then the pyruvate moves to the mitochondria through active transport. In the mitochondria the pyruvate is changed to Acetyl CoA this is known as the link reaction. During this reaction CO₂ and H ⁺is removed from the pyruvate which is a 3C compound. NAD which stands for nicotinamide adenine dinucleotide is a coenzyme which is needed for the reaction to follow through, NAD is reduced. . The enzyme responsible for this reaction is pyruvate dehydrogenase, the remaining 2C pyruvate combines with the coenzyme A to produce acetyl CoA. Most of the ATP is made through aerobic conditions. The link reaction produces 5 ATP molecules, this is of course theoretically!

Now, what if there was no oxygen present? Well, under anaerobic conditions the pyruvate is turned into lactate. This conversion occur in mature red blood cells which depend on glycolysis for its energy. Why convert pyruvate to lactate? Well, there is limited NAD therefore if the NAD is used up then glycolysis stops… Therefore when the pyruvate is converted to lactate in the presence of lactate dehydrogenase the NAD⁺is regenerated. Thus no ATP is generated! An example of this occurring in the body… during vigorous exercise the decreasing oxygen allows the lactate or lactic acid to accumulate which then causes muscular cramps.

Also, under anaerobic conditions there is another fate for the pyruvate that is; in yeast fermentation ethanol and carbon dioxide are produced. The enzymes; pyruvate decarboxylase and alcohol dehydrogenase are involved. The pyruvate is first changed to acetaldehyde by the enzyme pyruvate decarboxylase and carbon dioxide is removed or decarboxylation occurs. For this to occur it needs TPP(thiamine pyrophosphate), which is a cofactor for the enzyme. In the second step the acetaldehyde is converted to ethanol. Then H⁺ is removed by NAD and it is during this step that the NAD⁺is regenerated.

Image 93

· FEEDER PATHWAYS FOR GLYCOLYSIS:

So far we have spoken extensively about glucose and its metabolism. However, we have failed to recognize that glucose is not the only carbohydrate consumed in the daily human diet. Other carbohydrates such as fructose and galactose also contribute to the energy levels within the human body.

· Metabolism of fructose:

The main source in which fructose is consumed is from sucrose. For fructose to be metabolized by the body it has two steps involved; phosphorylation of fructose and cleavage of fructose-1-phosphate.

In the phosphorylation of fructose, fructokinase is the enzyme that catalyses the reaction. A phosphate group is added to the fructose molecule (i.e.phosphorylation) to produce fructose-1-phospahte. This allows the energy level of the fructose compound to be higher thus allowing it to participated in the second step. The phosphate group is from the ATP molecule. In the next step, another enzyme aldolase B disintegrate the fructose-1-phosphate to the products; dihydroxyacetone phosphate and glyeraldehyde. Note that aldolase B is found in the liver. The dihydroxyacetone phosphate can then enter glycolysis.

· Image 94

Metabolism of galactose:

Image 95

The source of galactose in the diet is from milk products. The metabolism of galactose is similar to that of fructose. For instance the galactose is phosphorylated to galatose-1-phosphate which is catalysed by the enzyme galactokinase. Galactose-1-phosphate is then changed to UDP-galactose. In this reaction the UDP-glucose reacts with the galactose-1-phosphate which produces UDP-galactose and glucose-1-phosphate with the aid of the enzyme galactose-1-phosphate uridyltransferase. Then the UDP-galactose is converted to a C4 epimer which is UDP-glucose by the enzyme UDP-hexose 4-epimerase. The UDP-glucose can then enter the glycolytic pathway.