Catabolic Pathways and Metabolism - Biochemistry

Triose phosphate isomerase catalyzes the isomerization between dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Glucose-6-phosphate dehydrogenase is a regulatory enzyme for the pentose phosphate pathway. Citrate synthase is a regulatory enzyme for the Krebs cycle, catalyzing the synthesis of citrate from acetyl-CoA and oxaloacetate. PFK catalyzes the rate-limiting step in glycolysis.

The conversion of succinate to fumarate is the only reaction that occurs outside of the normal Krebs cycle. Complex II of the electron transport chain has an enzyme known as succinate dehydrogenase. This enzyme is responsible for the conversion of succinate to fumarate. Fumarate is return to the cycle where it is then oxidized to malate continuing the cycle. Each of the other reactions of the Krebs cycle listed all occur in the inner mitochondrial matrix; whereas the conversion of succinate to fumarate occurs at the inner mitochondrial membrane.

Arginase catalyzes the conversion of arginine to ornithine and urea. Therefore, if there is a deficiency in arginase, there will be a buildup of arginine, and a deficiency in ornithine and urea. Without the ability to form urea, the ammonium waste from protein and amino acids degradation will not be able to be converted to urea safely. The buildup of ammonium will cause hyperammonemia, which results in some unfavorable physiological scenarios.

2 molecules of ammonia and 1 molecule of carbon dioxide are converted into 1 molecule of urea in every turn of the urea cycle. In addition, each cycle regenerates 1 molecule of ornithine for use in the next turn.

First, we must realize that the first committed step is the first irreversible reaction of glycolysis that is unique to glycolysis (cannot lead to another process, such as the pentose phosphate pathway). This is the third step, in which fructose-6-phosphate is converted to fructose-1,6-bisphosphate (the correct answer).

Glucose is the beginning reactant of glycolysis, and pyruvate is the final product. Glucose-6-phosphate is the product of the first step of glycolysis overall, but not of the committed step.

Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate is mediated by phosphoglycerate kinase. Conversion of phosphoenolpyruvate to pyruvate is mediated by pyruvate. In both these reactions adenosine diphosphate (ADP) is converted to ATP via substrate level phosphorylation. Conversion of 2-phosphoglycerate to phosphoenolpyruvate, mediated by enolase, does not produce ATP.

As glucose is introduced into the glycolytic pathway, it is first phosphorylated to create glucose-6-phosphate. That will then be converted to fructose-6-phosphate via phosphoglucose isomerase. That product will then be phosphorylated once more via phosphofructokinase-1 to create fructose-1,6-bisphosphate. Glucose-1,6-bisphosphate is never an observed intermediate in glycolysis.

The first and third steps of glycolysis are both energetically unfavorable. This means they will require an input of energy in order to continue forward. Per glucose molecule, 1 ATP is required for each of these steps. Therefore, a total of 2 ATP is needed during the energy investment phase of glycolysis.

Hexokinase and glucokinase are two enzymes that serve similar roles but have different characteristics. Hexokinase is found in all tissues, is inhibited by glucose 6 phosphate, and is not induced by insulin. It has a physiologic role of providing cells with a basal level of glucose 6 phosphate necessary for energy production.

Uncouplers of the electron transport chain decrease the proton gradient and thus decrease ATP synthesis. Most energy from the electron transport chain is released as heat. The most common uncouplers are 2,4-dinitrophenol and aspirin, as well as thermogenin. Carbon monoxide is an inhibitor of the electron transport chain, not an uncoupler. Nitric oxide does not affect directly the electron transport chain.

In the glycolytic pathway, 2 molecules of ATP must be used. The purpose of these molecules is to phosphorylate 2 intermediates in the pathway:

1. Glucose must be phosphorylated to glucose-6-phosphate.

2. Fructose-6-phosphate must be phosphorylated to fructose-1,6-bisphosphate.

The first step in glycolysis is the conversion of glucose to glucose-6-phosphate through the consumption on one ATP molecule. Glucose is reacted upon by the enzyme hexokinase to carry out this step. Kinases are a group of enzymes that add phosphate groups by removing them from an ATP. All of these other enzymes catalyze subsequent reactions in glycolysis.

Glyceraldehyde 3-phosphate dehydrogenase is the only enzyme in glycolysis that carries out a redox reaction. Glyceraldehyde 3-phosphate is oxidized to 1,3-bisphosphoglycerate while is reduced to .

Reactive oxygen species are superoxide, hydrogen peroxide, and hydrogen radicals. They are degraded by catalase, superoxide dismutase, and glutathione peroxidase. Neutrophils use reactive oxygen species to kill bacteria during the phagocytic oxidative burst.

The pyruvate dehydrogenase complex essentially carries out a two part reaction: a decarboxylation and an oxidation. All these choices play important roles in the pyruvate dehydrogenase complex. Thiamine pyrophosphate (TPP) is the only choice, however, that is responsible for the decarboxylation step. Lipoamide acts as transporter, transferring the substrate to a distant active site. FAD then reoxidizes lipoamide for the next substrate. CoA is important in producing the substrate.

In the fourth step of glycolysis, the six-carbon molecule fructose-1,6-bisphosphate is cleaved into two separate three-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This is catalyzed by the enzyme, aldolase.

Glycolysis is the only of the above choices that occurs in the cytoplasm. The remaining occur in different parts of the mitochondria. The Krebs cycle occurs in the mitochondrial matrix. Both oxidative phosphorylation and the electron transport chain occur along the inner mitochondrial membrane.

Oxygen is an electron acceptor. In the absence of oxygen (hypoxia) cells cannot generate ATP in the mitochondria. Instead, they will utilize glycolysis. Oxygen is required to carry out the electron transport chain and produce ATP via oxidative phosphorylation.

Glycolysis, as the name suggests, is the process of lysing glucose into pyruvate. Since glucose is a six-carbon molecule and pyruvate is a three-carbon molecule, two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis. Glycolysis occurs in the cytoplasm of the cell, and does not require oxygen. The net energy production is two ATP per glucose.

After glucose is converted into glucose-6-phosphate by hexokinase, glucose-6-phosphate is converted into fructose-6-phosphate. This reaction is catalyzed by phosphoglucose isomerase.