Glycolysis - Biochemistry

The first and third steps of glycolysis involve energy consumption in the form of ATP. A phosphate group is added to glucose, and fructose-6-phosphate. In the seventh and tenth steps of glycolysis, ADP is phosphorylated at the level of the substrate into ATP. Since this is after glucose had been split into two three-carbon molecules, each molecule of glucose results in four ATP produced. However, since two were consumed early in glycolysis, the net ATP production is 2.

The first step of glycolysis is the addition of a phosphate group to glucose to form glucose-6-phosphate. The third step of glycolysis is the addition of another phosphate group to fructose-6-phosphate to form fructose-1,6-bisphosphate. The conversion of ATP to ADP is needed to supply the phosphate group in both of these reactions. These are the only two reactions in glycolysis where ATP is used to to add phosphate groups.

Glycolysis produces 4 ATP molecules. However, 2 ATP molecules are required to initiate glycolysis. Subtracting these two numbers gives the net ATP yield from glycolysis--2 ATP molecules.

Although glycolysis will ultimately produce 4 ATP, there is an initial requirement of 2 ATP for it to begin. The conversion of glucose to glucose-6-phosphate and the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate both require ATP.

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.

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 .

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.

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.

Glycolysis has a net yield of 2 ATP per glucose molecules. The Krebs cycle produces 2 GTP molecules per glucose. The electron transport chain and ATP synthase are the main producers of ATP in cellular respiration. The pyruvate dehydrogenase complex, however, does not yield any ATP (or any other nucleoside phosphates). It simply produces acetyl-CoA from pyruvate, releasing one molecule of carbon dioxide per pyruvate.

In the liver, glucokinase irreversibly converts glucose in the cell to glucose-6-phosphate. Phosphofructose kinase-1 irreversibly converts fructose-6-phosphate to fructose-1,6-bisphosphate. Pyruvate kinase converts phosphoenolpyruvate to pyruvate. All the other enzymes listed catalyze reversible glycolysis reactions.

Glucose-6-phosphate (G6P) is the first molecule of the pentose phosphate pathway where it is acted upon by glucose-6-phosphate dehydrogenase. G6P is the result of the hexokinase (first) reaction in glycolysis. What is key here is that the tissue in question is muscle. Because muscle cells lack the glucose-6-phosphatase necessary to produce free glucose from G6P, they cannot be said to perform gluconeogenesis. They do, however, perform glycogenesis through conversion of G6P to glucose-1-phosphate followed by conversion to uridine diphosphateglucose for addition to a growing molecule of glycogen.