Difference between Glycolysis and Krebs Cycle | Metabolic Engineering

Difference between Glycolysis and Krebs Cycle!

Glycolysis:

Glycolysis is the sequence of enzymatic reactions which oxidize the six-carbon sugar glucose into two three-carbon compounds with the production of a small amount of adenosine triphosphate (ATP). Glycolysis has two basic functions if the cell.

First, it metabolizes simple six-carbon sugars to smaller three-carbon compounds that are then either fully metabolized by the mitochondria to produce carbon dioxide and a large amount of ATP or used for the synthesis of fat for storage.

Second, glycolysis functions to produce a small amount of ATP, which is essential for some cells solely dependent on that pathway for the generation of energy.

Glycolytic pathway is catalyzed by soluble enzymes located in the cytosol of cells. The glycolytic pathway operates in both the presence (aerobic) and absence of oxygen (anaerobic). The metabolism of fuel molecules in the cell can be thought of as an oxidation process.

In glycolysis, glucose is the fuel molecule being oxidized. As the glucose is oxidized by the glycolytic enzymes, the coenzyme nicotinamide adenine dinucleotide (NAD+) is converted from its oxidized to reduced form (NAD⁺ to NADH).

When oxygen is available (aerobic conditions) mitochondria in the cell can re-oxidize to NADH to NAD⁺. However, if either oxygen levels are insufficient (anaerobic conditions) or mitochondrial activity is absent, NADH must be re-oxidized by the cell using some other mechanism. In animal cells, the re-oxidation of NADH is accomplished by reducing pyruvate, the end-product of glycolysis, to form lactic acid.

This process is known as anaerobic glycolysis. During vigorous exercise, skeletal muscle relies heavily on it. In yeast, anaerobic conditions result in the production of carbon dioxide and ethanol from pyruvate rather than lactic acid. This process, called as alcoholic fermentation, is the basis of wine production and the reason why bread dough rises.

Although some cells are highly dependent on glycolysis for the generation of ATP, the amount of ATP generated per glucose molecule is actually quite small. Under anaerobic conditions, the metabolism of each glucose molecule yields only two ATPs. In contrast, the complete aerobic metabolism of glucose to carbon dioxide by glycolysis and the Krebs cycle yields up to thirty-eight ATPs.

Therefore, in the majority of cells the most important function of glycolysis is to metabolize glucose to generate three-carbon compounds that can be utilized by other pathways. The final product of aerobic glycolysis is pyruvate. Pyruvate can be metabolized by pyruvate dehydrogenase to form acetyl coenzyme A (acetyl CoA). Under conditions where energy is needed, acetyl CoA is metabolized by the Krebs cycle to generate carbon dioxide and a large amount of ATP. Acetyl CoA can be used to synthesize fats or amino acids, when the cell does not need energy.

Krebs Cycle:

Krebs cycle is a set of enzymatic reactions that catalyzes the aerobic metabolism of fuel molecules to carbon dioxide and water, thereby producing energy for the production of adenosine triphosphate (ATP) molecules. The Krebs cycle is so named because much of its elucidation was the work of the British biochemist Hans Krebs.

Many types of fuel molecules can be drawn into and utilized by the cycle, including acetyl coenzyme A (acetyl CoA), derived from glycolysis or fatty acid oxidation . Some amino acids are metabolized via the enzymatic reactions of the Krebs cycle. In eukaryotic cells, all but one of the enzymes catalyzing the reactions of the Krebs cycle is found in the mitochondrial matrixes.

The sequence of events known as the Krebs cycle is indeed a cycle; oxaloacetate is both the first reactant and the final product of the metabolic pathway (creating a loop). Because the Krebs cycle is responsible for the ultimate oxidation of metabolic intermediates produced during the metabolism of fats, proteins, and carbohydrates, it is the central mechanism for metabolism in the cell.

In the first reaction of the cycle, acetyl CoA condenses with oxaloacetate to form citric acid. Acetyl CoA utilized in this way by the cycle has been produced either via the oxidation of fatty acids, the breakdown of certain amino acids, or the oxidative decarboxylation of pyruvate (a product of glycolysis).

The citric acid produced by the condensation of acetyl CoA and oxaloacetate is a tri Carboxylic acid containing three carboxylate groups. (Hence, the Krebs cycle is also referred to as the citric acid cycle or tri-carboxylic acid cycle.)

After citrate has been formed, the cycle machinery continues through seven distinct enzyme-catalyzed reactions that produce, in order, iso-citrate, a – ketoglutarate, succinyl coenzyme A, succinate, fumarate, malate, and oxaloacetate.

The freshly produced oxaloacetate, in turn, reacts with yet another molecule of acetyl CoA, and the cycle begins again. Each turn of the Krebs cycle produces two molecules of carbon dioxide, one guanosine triphosphate molecule (GTP), and enough electrons to generate three molecules of NADH and one molecule of FADH₂.

The Krebs cycle is present in virtually all eukaryotic cells that contain mitochondria, but functions only as part of aerobic metabolism (when oxygen is available). This oxygen requirement is owing to the close relationship between the mitochondrial electron transport chain and the Krebs cycle. In the Krebs cycle, four oxidation-reduction reactions occur.

A high energy phosphate bond in the form of GTP is also generated. (This high energy phosphate bond is later transferred to adenosine di-phosphate [ADP] to form adenosine triphosphate [ATP].) As the enzymes of the Krebs cycle oxidize fuel molecules to carbon dioxide, the coenzymes NAD⁺, FAD, and coenzyme Q (also known as ubiquinone) are reduced.

In order for the cycle to continue, these reduced coenzymes must become re-oxidized by transferring their electrons to oxygen, thus producing water. Therefore, the final acceptor of the electrons produced by the oxidation of fuel molecules as part of the Krebs cycle is oxygen. In the absence of oxygen, the Krebs cycle is inhibited.

The citric acid cycle is an amphibolic pathway, meaning that it can be used for both, the synthesis and degradation of biomolecules. Besides acetyl CoA (generated from glucose, fatty acids, or ketogenic amino acids), other biomolecules are metabolized by the cycle.

Several amino acids are degraded to become what are intermediates of the cycle. Likewise, odd-chain fatty acids are metabolized to form succinyl coenzyme A, another intermediate of the cycle. Krebs cycle intermediates are also used by many organisms for the synthesis of other important biomolecules.

Some organisms use the Krebs cycle intermediates a -ketoglutarate and oxaloacetate in the synthesis of several amino acids. Succinyl coenzyme A is utilized in the synthesis of porph5Tin rings, used in home manufacture and chlorophyll biosynthesis. Oxaloacetate and maltase are utilized in the synthesis of glucose, in a process called as gluconeogenesis.