Glycolysis - Biochemistry

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.

Sucrose is the linking of glucose and fructose. Recall from the glycolytic pathway that fructose is further downstream than glucose, and therefore would allow for faster production of ATP.

In this question, we're shown a portion of glycolysis. We're asked to identify an intermediate in glycolysis based on the intermediate that comes before it and the one that comes after it.

To answer this, we'll need to know the pathway of glycolysis. The first intermediate shown here, glyceraldehyde-3-phosphate, is acted on by the enzyme glyceraldehyde-3-phosphate dehydrogenase. The product of this reaction is 1,3-bisphosphoglycerate, which is thus the correct answer. This intermediate is then acted on by the enzyme phosphoglycerate kinase to produce 3-phosphoglycerate.

Cellular respiration is a long process, and so it is easiest to break it into the following steps:

Step 1: Glycolysis

Step 2: Pyruvate decarboxylation

Step 3: Krebs cycle

Step 4: Oxidative phosphorylation

In the above steps, ATP is only produced by substrate-level phosphorylation in glycolysis and during the Krebs cycle.

In glycolysis, two molecules of pyruvate are produced for every molecule of glucose oxidized. During this process, two ATP molecules are consumed, but four are produced via substrate-level phosphorylation.

In the Krebs cycle, each pass of pyruvate through the cycle generates one molecule of GTP, which is subsequently used to generate a molecule of ATP via substrate-level phosphorylation. Thus, one molecule of ATP is produced via substrate-level phosphorylation per molecule of pyruvate oxidized. But remember that glycolysis produces two molecules of pyruvate for each molecule of glucose oxidized. Hence, the Krebs cycle will contribute a total of two molecules of ATP per glucose molecule oxidized.

Since we have a total of four moles ATP from glycolysis and two moles of ATP from the Krebs cycle (one per pyruvate), we have a cumulative production of six moles of ATP generated by substrate-level phosphorylation per mole of glucose oxidized.

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.