Off-site chime link: Phosphoglycerate Kinase. Reaction 6 - 3-phosphoglycerate Chime in new window Reaction 7: Isomerization. In this reaction the phosphate group moves from the 3 position to the 2 position in an isomerization reaction.
This reaction is catalyzed by phosphoglycerate mutase. Off-site chime link: Phosphoglycerate Mutase. Reaction 7 - 2-phosphoglycerate Chime in new window Reaction 8: Alcohol Dehydration. In this reaction, which is the dehydration of an alcohol, the -OH on C-3 and the -H on C-2 are removed to make a water molecule.
At the same time a double bond forms between C-2 and C This change makes the compound somewhat unstable, but energy for the final step of glycolysis. This reaction is catalyzed by enolase. Off-site chime link: Enolase. This is the final reaction in glycolysis. Again one of the phosphate groups undergoes hydrolysis to form the acid and a phosphate ion, giving off energy. This reaction is catalyzed by pyruvic kinase. Off-site chime link: Pyruvate Kinase. Reaction 9 - pyruvic acid Chime in new window.
Carbohydrate Metabolism Overview. Citric Acid Cycle. Elmhurst College. Glycolysis Summary. Pyruvic Acid - Crossroads. Pyruvate is also converted to oxaloacetate by an anaplerotic reaction, which replenishes Krebs cycle intermediates; also, oxaloacetate is used for gluconeogenesis. These reactions are named after Hans Adolf Krebs, the biochemist awarded the Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes.
The cycle is also known as the citric acid cycle or tri-carboxylic acid cycle, because citric acid is one of the intermediate compounds formed during the reactions. If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms.
Pyruvate from glycolysis is converted by fermentation to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation. Alternatively it is converted to acetaldehyde and then to ethanol in alcoholic fermentation. Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine, and to ethanol.
Therefore, it unites several key metabolic processes. Figure 9. Control of glycolysis is unusual for a metabolic pathway, in that regulation occurs at three enzymatic points:.
Glycolysis is regulated in a reciprocal fashion compared to its corresponding anabolic pathway, gluconeogenesis. Reciprocal regulation occurs when the same molecule or treatment phosphorylation, for example has opposite effects on catabolic and anabolic pathways. Reciprocal regulation is important when anabolic and corresponding catabolic pathways are occurring in the same cellular location. As an example, consider regulation of PFK.
It is activated by several molecules, most importantly fructose-2,6- bisphosphate F2,6BP. This molecule has an inhibitory effect on the corresponding gluconeogenesis enzyme, fructose-1,6-bisphosphatase F1,6BPase.
You might wonder why pyruvate kinase, the last enzyme in the pathway, is regulated. The answer is simple.
Pyruvate kinase catalyzes the most energetically rich reaction of glycolysis. In other words, it takes two enzymes, two reactions, and two triphosphates to go from pyruvate back to PEP in gluconeogenesis. Another interesting control mechanism called feedforward activation involves pyruvate kinase.
Pyruvate kinase is activated allosterically by F1,6BP. This molecule is a product of the PFK reaction and a substrate for the aldolase reaction. Glycolysis is divided into two categories: aerobic chemical reactions that occur with the presence of oxygen and anaerobic chemical reactions that do not require oxygen. An example of anaerobic glycolysis is fermentation. There are three stages in an aerobic glycolysis reaction: 1 decarboxylation of pyruvate 2 Citric Acid Cycle also known as the Krebs Cycle 3 Electron transport chain.
How much of this accrues in glycolysis? The initial phosphorylation reactions steps 1 and 3 in slide 3. One molecule of ATP each is obtained in steps 7 and Since all of the steps from 6 to 11 occur twice per molecule of glucose, the net balance is a gain of two moles of ATP per mole of glucose—a very modest contribution to the final tally.
Still, glycolysis is a viable source of ATP, and it is the major one that operates in our tissues under anaerobic conditions, that is, while oxygen is in short supply.
This concerns mostly skeletal muscle during maximal exercise, such as a meter dash. As noted above, erythrocytes and some other cell types rely on anaerobic glycolysis even under aerobic conditions. The glyceraldehydephosphate dehydrogenase reaction step 6 in slide 3. Under aerobic conditions, the hydrogen is transferred from NADH to one of several carriers that deliver it to the respiratory chain in the mitochondria, and ultimately to oxygen.
These shuttle mechanisms are discussed in detail in section 6. Under anaerobic conditions, this is impossible; therefore, other means for hydrogen disposal are required. In human metabolism, pyruvate serves as a makeshift hydrogen acceptor under anaerobic conditions; it is reduced to lactate by lactate dehydrogenase step 11 in slide 3. The lactate is released into the bloodstream, where it accumulates; it is removed and recycled after restoration of oxygen supply. The muscle pain caused by lactate accumulation forces us to discontinue anaerobic exercise after a short while.
Anaerobic glycolysis also occurs in many microbes, which also face the need to reoxidize NADH. Without the option of reverting to oxidative metabolism within a short time span, they must also deal with the continued accumulation of acid. The yeast Saccharomyces cerevisiae solves this problem through ethanolic fermentation: The acid is converted to a neutral and considerably less toxic compound ethanol via decarboxylation. The CO 2 developed in this reaction makes bread dough rise up, whereas the ethanol does the same to government tax revenue.
As was noted in slide 1. These occur in several sub-types whose properties are tuned to the physiological roles of different organs. A typical carrier for passive substrate transport alternates between two conformations that are open to either side of the membrane. On both sides of the membrane, the substrate reversibly binds to the carrier protein; binding and dissociation are governed by mass action kinetics.
Transport occurs when the protein changes from the outward-facing to the inward-facing conformation, or vice versa, while a substrate molecule is bound to it. The sequence of events in passive transport resembles the pattern of Michaelis-Menten enzyme kinetics—in both cases, a solute binds reversibly to a protein, which then performs some state-altering action on it.
Accordingly, the velocity of transport by facilitated diffusion follows the familiar Michaelis-Menten law:. V max is proportional to the number of carrier molecules.
In many tissues, the number of glucose transporters varies depending on insulin levels. In this way, insulin controls the rate of glucose uptake into cells in these tissues see slide At lower substrate concentrations, the rate of transport becomes dependent on K M , which is inversely related to the affinity of the transporter for the substrate. From equation 3. The lowest K M —or in other words, the highest glucose affinity—occurs with the GLUT3 transporter subtype, which is found in the brain.
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