What is glycolysis?

Glycolysis is a series of reactions that extract energy from glucose by splitting it into two three-carbon molecules called pyruvates. Glycolysis is an ancient metabolic pathway, meaning it evolved a long time ago and is found in the vast majority of living organisms today. In organisms that perform cellular respiration, glycolysis is the first stage of this process. However, glycolysis does not require oxygen, and many anaerobic organisms (organisms that do not use oxygen) also have this pathway.

Importance of glycolysis

Almost all the energy used by living cells comes from the energy of glucose sugar bonds. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters help in the facilitated diffusion of glucose. Glycolysis is the first pathway used in the breakdown of glucose to extract energy.

It takes place in the cytoplasm of prokaryotic and eukaryotic cells. It was probably one of the first metabolic pathways to evolve, as it is used by almost every organism on earth. The process does not use oxygen and is therefore anaerobic. Glycolysis is the first of the main metabolic pathways of cellular respiration to produce energy in the form of ATP. Through two distinct phases, the six-carbon glucose ring is cleaved into two three-carbon pyruvate sugars through a series of enzymatic reactions.

The first phase of glycolysis requires energy, while the second phase completes the conversion to pyruvate and produces ATP and NADH for the cell to use for energy. In general, the process of glycolysis results in a net gain of two pyruvate molecules, two ATP molecules, and two NADH molecules for the cell to use for energy. After the conversion of glucose to pyruvate, the glycolytic pathway is linked to the Krebs cycle, where more ATP will be produced for the energy needs of the cell.

The steps of glycolysis that require energy

In the first half of glycolysis, energy in the form of two ATP molecules is required to transform glucose into two three-carbon molecules.

The first half of glycolysis (steps that require energy)

In the first half of glycolysis, two molecules of adenosine triphosphate (ATP) are used in the phosphorylation of glucose, which then splits into two three-carbon molecules, as described in the following steps.

Step 1. The first step of glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as a phosphate source, producing glucose-6-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins. It can no longer exit the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.

Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate to one of its isomers, fructose-6-phosphate. An enzyme that catalyzes the conversion of a molecule into one of its isomers is an isomerase. (This change from phosphoglucose to phosphoglucose allows for the eventual splitting of the sugar into two three-carbon molecules.)

Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when the ADP concentration is high; it is less active when ADP levels are low and ATP concentration is high. Therefore, if there is “enough” ATP in the system, the pathway slows down. This is a type of end product inhibition since ATP is the end product of glucose catabolism.

Step 4. Newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step of glycolysis uses an enzyme, aldolase, to split 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

Step 5. In the fifth step, an isomerase converts dihydroxyacetone-phosphate to its isomer, glyceraldehyde-3-phosphate. Thus, the route will continue with two molecules of a single isomer. At this point in the pathway, there is a net investment of energy of two ATP molecules in the breakdown of one glucose molecule.

The energy-releasing steps of glycolysis

In the second half of glycolysis, energy is released in the form of 4 ATP molecules and 2 NADH molecules.

The second half of glycolysis (energy release steps)

So far, glycolysis has cost the cell two ATP molecules and produced two small three-carbon sugar molecules. Both molecules will proceed through the second half of the pathway in which enough energy will be extracted to return the two ATP molecules used as the initial investment and, at the same time, produce again for the cell of two additional ATP molecules and two higher energy molecules. NADH molecules.

Step 6. The sixth step in glycolysis oxidizes the sugar (glyceraldehyde-3-phosphate), extracting high-energy electrons, which are taken up by the electron carrier NAD+, producing NADH. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.

Here again, there is a potential limiting factor for this pathway. The continuation of the reaction depends on the availability of the oxidized form of the electron carrier NAD+. Therefore, NADH must continually be oxidized back to NAD+ for this step to continue. If NAD+ is not available, the second half of glycolysis slows or stops. If oxygen is available in the system, NADH will be easily oxidized, albeit indirectly, and the high-energy electrons from the hydrogen released in this process will be used to make ATP. In an oxygen-depleted environment, an alternative pathway (fermentation) can provide oxidation of NADH to NAD+.

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming an ATP molecule. (This is an example of substrate-level phosphorylation.) A carbonyl group in 1,3-bisphosphoglycerate is oxidized to a carboxyl group and 3-phosphoglycerate is formed.

Step 8. In the eighth step, the remaining phosphate group on 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme that catalyzes this step is a mutase (an isomerase).

Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).

Step 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme, in this case, gets its name from the reverse conversion of pyruvate to PEP) and results in the production of a second ATP molecule by phosphorylation at the substrate level and the compound pyruvic acid (or its salt form, pyruvate). Many enzymes in enzymatic pathways are called reverse reactions since the enzyme can catalyze both forward and reverse reactions (these may have been initially described by the reverse reaction taking place in vitro, under non-physiological conditions).

Glycolysis Results

Glycolysis begins with one glucose molecule and ends with two pyruvates (pyruvic acid) molecules, a total of four ATP molecules, and two NADH molecules. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules to use. If the cell can no longer catabolize pyruvate molecules (via the citric acid cycle or the Krebs cycle), it will collect only two ATP molecules from one glucose molecule.

Mature mammalian red blood cells lack mitochondria and are not capable of aerobic respiration, the process in which organisms convert energy in the presence of oxygen. Instead, glycolysis is its only source of ATP. Therefore, if glycolysis is interrupted, red blood cells lose their ability to maintain their sodium and potassium pumps, which require ATP to function, and eventually die. For example, since the second half of glycolysis (which makes the energy molecules) slows or stops in the absence of NAD+, when NAD+ is not available, red blood cells cannot make enough ATP to survive.

Also, the last step of glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient amounts. In this situation, the entire glycolysis pathway will continue, but only two ATP molecules will be produced in the second half (instead of the usual four ATP molecules). Therefore, pyruvate kinase is a rate-limiting enzyme of glycolysis.