Glycolysis - Biochemistry

Dihydroxyacetone phosphate (DHAP) is converted to glyceradehyde-3-phosphate (G3P) by the enzyme triose phosphate isomerase. As the name suggests, this enzyme catalyzes the isomerization of a three-carbon sugar into another three-carbon sugar. Since the molecular formulas of DHAP and G3P are the same, we know that they are isomers of each other.

The balance between DHAP and G3P is extremely important in regulating overall cell metabolism. DHAP is a precursor to triglycerides, and is used in their synthesis, while G3P is an intermediate in glycolysis, an ATP-producing process. In order to favor the conversion of DHAP into G3P, and not the opposite, the cell must keep G3P levels low (Le Chatelier's Principle). Consider the following equilibrium: . This should make sense: if there is lots of ATP around in the cell, there is no need for glycolysis to proceed. Thus the equilibrium will be pushed to the left, increasing the concentration of DHAP in the cell. In humans, DHAP is converted into triglycerides, which get stored as fat. One way to shift this equilibrium to the right is to "create" an ATP need. This can be done by exercising. Exercise utilizes ATP and will thus pull the equilibrium to the right, removing DHAP (which was destined to be converted into fat) and facilitates its conversion into G3P to proceed with cellular respiration.

The tenth and final reaction of glycolysis involves the conversion of phosphoenolpyruvate (PEP) into pyruvate. This step is catalyzed by the enzyme pyruvate kinase. This kinase is going to remove a phosphate group from PEP and put it on ADP to yield ATP. Pyruvate, a three-carbon molecule, is the end product of glycolysis. It can be sent to the pyruvate dehydrogenase complex to be turned into acetyl-CoA, which enters the Krebs cycle. Alternatively, it can be reduced into lactate and/or ethanol (depending on the organism) to regenerate for glycolysis via anaerobic respiration.

Glyceraldehyde phosphate dehydrogenase (GAPDH) is used in the sixth reaction, where G3P is converted to 1,3-bisphosphoglycerate (1,3-BPG). A hydrogen is removed from G3P and added to , yielding NADH. Also, G3P has one phosphate group, while 1,3-BPG has two. The energy released as G3P is oxidized (causing subsequent reduction of ) is highly exergonic. This energy, sometimes referred to as the energy of oxidation, drives the addition of inorganic phosphate onto G3P, yielding the doubly-phosphorylated 1,3-BPG.

Theninth reaction involves the conversion of 2-phosphoglycerate into phosphoenolpyruvate. The enzyme enolase, which produces a double bond by removing the hydroxyl group on 2-phosphoglycerate catalyzes this reaction. Note that the resulting molecule is an enol (double bond -ene, and alcohol - ol).

Isomerases catalyze the isomerization, or rearrangement of atoms within a molecule, of its substrate. Isomerases are seen in glycolysis inn the second step where glucose-6-phosphate is converted into fructose-6-phosphate by phosphoglucose isomerase. Glucose-6-phosphate is rearranged into fructose-6-phosphate such that the molecular formula is unchanged. Another isomerase is triose phosphate isomerase. It catalyzes the isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.

Hexokinase is the first enzyme in the glycolytic pathway and it is responsible for the phosphorylation of glucose to glucose 6-phosphate. The other enzymes catalyze subsequent reactions in glycolysis.

The rate-limiting step of glycolysis is the conversion of glucose-6-phosphate to fructose-6-phosphate. This reaction is catalyzed by the enzyme phosphofructokinase. Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate, pyruvate kinase converts Phosphoenolpyruvate to pyruvate, and lactate dehydrogenase converts pyruvate into lactose.

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 .

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.

To see which of the enzymes in these answer choices may by in glycolysis, let's go through each one and look at their function.

Aldolase - This enzyme is indeed involved in glycolysis. It is responsible for the cleavage of fructose-1,6-bisphosphate into two products, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Thiolase - This enzyme catalyzes the the reversible association of two acetyl-CoA molecules into acetoacetyl-CoA. This is an important part of the mevalonate pathway as well as beta oxidation and ketone body synthesis/degradation.

Fructose-1,6-bisphosphatase - This enzyme is important in gluconeogenesis, a metabolic pathway that runs counter to glycolysis. Although many of the enzymes found in glycolysis are also used in gluconeogenesis, this enzyme is one example of an exception because it bypasses one of the irreversible reactions from glycolysis.

Aconitase - This enzyme is found in the citric acid cycle. Its function is to convert citrate into its isomer, isocitrate.

Hexokinase initiates the first step of glycolysis, which we know is the series of reactions by which glucose is processed to enter the citric acid cycle, and generate energy for the cell. So, in the case of glycolysis, glucose is the substrate of hexokinase. Based upon the name of the enzyme, we can infer that it phosphorylates a six-carbon molecule (which glucose is). By knowing the substrate is glucose, the correct product is glucose-6-phosphate, since its name indicates phosphorylation (addition of a phosphate group) to one of its carbons. Glycogen, fructose, and ATP are not involved in this first step of glycolysis.

Hexokinase, pyruvate kinase, and PFK are regulatory enzymes in glycolysis, but PFK catalyzes the rate-limiting step (the phosphorylation of fructose-6-phosphate). Citrate synthase and isocitrate dehydrogenase are involved in the Krebs cycle, not glycolysis.

Glucokinase specifically phosphorylates the six-carbon sugar glucose. It is involved in glycolysis, but only in hepatocytes; hexokinase is the main enzyme that phosphorylates glucose during the first reaction of glycolysis. Rather, glucokinase's main role is to phosphorylate glucose to glucose-1-phosphate during the process of glycogen synthesis. The other kinases involved in glycolysis is phosphofructokinase. Fructose and mannose are not phosphorylated by glucokinase. Also, note that hexokinase has a higher affinity for glucose than does glucokinase.

Phosphofructokinase catalyzes the third step in glycolysis transforming fructose 6-phosphate to fructose-1,6-bisphosphate. It is an irreversible step, and it is one of the major regulatory points of glycolysis. One way in which it controls the flow of glycolysis is that when there is a high level of ATP, PFK is inhibited. This is because the ultimate goal of glycolysis is to make ATP. Thus, if there is already a high level of ATP, glycolysis should slow down.

Hexokinase catalyzes the first step of glycolysis, and this step requires the input of an ATP molecule. This step is unfavorable, but the steps catalyzed by the rest of the enzymes listed as answer choices are favorable.

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.

Triose phosphate isomerase is responsible for converting dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. It is glyceraldehyde-3-phosphate that continues on through glycolysis to ultimately form a pyruvate molecule. Therefore, if there is no triose phosphate isomerase, the dihydroxyacetone will be unable to continue through glycolysis. The normal net yield of 2 ATP will be halved, the production of NADH will be halved, and only 1 pyruvate molecule will be created. Glycolysis will still be able to function, and the energy investment phase will be unaffected.

Phosphofructokinase-2 converts fructose-6-phosphate to fructose-2,6-bisphosphate. The product, fructose-2,6-bisphosphate activates phosphofructokinase-1, the rate limiting step in glycolysis. Phosphofructokinase-2 is regulated by insulin (activated) and glucagon (inhibited).

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.