The Citric Acid Cycle: Videos & Practice Problems

The Citric Acid Cycle, also known as the Krebs Cycle, is a crucial metabolic pathway consisting of eight biochemical reactions that play a vital role in cellular respiration. The first phase of this cycle involves the formation of citrate, which is initiated by the interaction between oxaloacetate and acetyl CoA. Oxaloacetate, a four-carbon molecule produced in a previous phase, combines with acetyl CoA, derived from carbohydrates, fats, or proteins, to form citrate, a six-carbon compound.

This reaction is significant as it marks the entry point of acetyl CoA into the cycle, facilitating the subsequent transformations that lead to energy production. The formation of citrate is catalyzed by the enzyme citrate synthase, which helps in the condensation of oxaloacetate and acetyl CoA. The overall reaction can be represented as:

$$\text{Oxaloacetate} + \text{Acetyl CoA} \rightarrow \text{Citrate} + \text{CoA}$$

Understanding this initial step is essential, as it sets the stage for the subsequent reactions in the cycle, where citrate undergoes a series of transformations, ultimately leading to the regeneration of oxaloacetate and the production of energy-rich molecules such as NADH and FADH₂.

In the first step of citrate formation, an important biochemical reaction occurs where the acetyl group from Acetyl CoA combines with oxaloacetate to produce citrate. Acetyl CoA, which consists of an acetyl group and a coenzyme A (CoA) portion, plays a crucial role in this process. During this reaction, water is involved, indicating that hydrolysis takes place. Specifically, the acetyl group is transferred to oxaloacetate, resulting in the formation of citrate while regenerating CoA with its thiol group intact.

This reaction is catalyzed by the enzyme citrate synthase, which facilitates the synthesis of new compounds within the cell. The energy required for this reaction is derived from the hydrolysis of Acetyl CoA, particularly from the cleavage of the high-energy carbon-sulfur bond. The energy released during this hydrolysis drives the conversion of oxaloacetate into citrate, highlighting the interconnectedness of metabolic pathways and the importance of enzyme-catalyzed reactions in cellular processes.

The Citric Acid Cycle, also known as the Krebs Cycle, is a crucial metabolic pathway that plays a significant role in cellular respiration. In Phase B of this cycle, which focuses on the formation of succinyl CoA, three key reactions occur: the conversion of citrate to isocitrate, isocitrate to alpha-ketoglutarate, and alpha-ketoglutarate to succinyl CoA.

In the first reaction, citrate is transformed into isocitrate. This is an isomerization process that rearranges the molecular structure without changing the overall formula. The second reaction involves the conversion of isocitrate to alpha-ketoglutarate. This step is significant as it releases one molecule of NADH, an important electron carrier, and one molecule of carbon dioxide (CO₂). The production of NADH is vital as it plays a key role in the electron transport chain, contributing to ATP production.

The third reaction in this phase sees alpha-ketoglutarate being converted into succinyl CoA. Similar to the previous step, this reaction also results in the release of another molecule of NADH and one molecule of CO₂. Overall, Phase B of the Citric Acid Cycle results in the production of two moles of NADH and two moles of CO₂, with one of each produced in both the second and third reactions.

Understanding these transformations is essential for grasping how energy is extracted from organic molecules during cellular respiration. Each step is catalyzed by specific enzymes that facilitate the conversion of substrates, ensuring the cycle continues efficiently and effectively.

In the citric acid cycle, specifically during phase B, reaction 2 involves the isomerization of citrate to isocitrate, a crucial step for subsequent oxidation. Citrate, which is a tertiary alcohol, cannot be oxidized directly due to its structure. This is where the enzyme aconitase plays a vital role. Aconitase facilitates the conversion of citrate into isocitrate by rearranging the hydroxyl (–OH) and hydrogen (–H) groups on the alcohol carbon.

Initially, in citrate, the alcohol carbon is bonded to three other carbons, classifying it as a tertiary alcohol. However, after the action of aconitase, the –OH group moves to a different position, resulting in isocitrate, where the alcohol carbon is now connected to only two other carbons, thus becoming a secondary alcohol. This structural change is significant because secondary alcohols can be oxidized, allowing the cycle to progress to the next reaction.

Overall, reaction 2 is essential for preparing the substrate for oxidation, ensuring that the citric acid cycle continues efficiently. Understanding this transformation highlights the importance of enzyme-mediated reactions in metabolic pathways.

In the metabolic pathway involving isocitrate, we observe a crucial reaction characterized by both oxidation and decarboxylation. Isocitrate, a secondary alcohol, undergoes oxidation facilitated by the enzyme isocitrate dehydrogenase, transforming it into alpha-ketoglutarate. This reaction is significant as it marks the first instance in the cycle where one mole of NADH is produced alongside the release of one mole of carbon dioxide (CO₂).

The process begins with the oxidation of isocitrate, where the secondary alcohol functional group is converted into a carbonyl group (C=O). This transformation is essential as it signifies the change from isocitrate to alpha-ketoglutarate. During this reaction, the coenzyme NAD⁺ is reduced to NADH, indicating that it has gained electrons. The overall reaction can be summarized as follows:

Isocitrate + NAD⁺ → Alpha-ketoglutarate + NADH + CO₂

In summary, the role of isocitrate dehydrogenase is pivotal in this reaction, as it not only facilitates the oxidation of isocitrate but also plays a key role in the decarboxylation process, leading to the release of CO₂. This reaction exemplifies the interconnectedness of oxidation and decarboxylation in metabolic pathways, highlighting the importance of enzymes in catalyzing these transformations.

In the fourth reaction of phase B of the citric acid cycle, a significant process occurs involving oxidation and decarboxylation. This reaction marks the second instance where a molecule of NADH is produced alongside the release of carbon dioxide (CO₂). The substrate involved in this reaction is alpha-ketoglutarate, which is converted into succinyl CoA through the action of the enzyme alpha-ketoglutarate dehydrogenase.

During this transformation, alpha-ketoglutarate undergoes oxidation, resulting in the formation of a highly energetic carbon-sulfur bond in succinyl CoA. This bond is crucial because its hydrolysis can lead to the production of ATP, highlighting the energy-yielding potential of this reaction. The process also involves the reduction of the coenzyme NAD⁺ to NADH, indicating that one mole of NADH is generated. Additionally, the decarboxylation process signifies the loss of one carbon atom as CO₂.

To summarize, the key components of this reaction include the conversion of alpha-ketoglutarate to succinyl CoA, the production of NADH, and the release of CO₂. The formation of succinyl CoA is essential for subsequent reactions in the citric acid cycle, particularly in the context of energy production.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that plays a significant role in cellular respiration. It consists of a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

In the cycle, the first step involves the conversion of oxaloacetate to citrate, which occurs when oxaloacetate interacts with acetyl-CoA. This reaction is classified as reaction 1 in phase A of the citric acid cycle. The formation of citrate is essential as it sets the stage for subsequent transformations.

Following this, citrate undergoes isomerization to form isocitrate, facilitated by the enzyme aconitase. This step, which transforms a tertiary alcohol in citrate to a secondary alcohol in isocitrate, is identified as reaction 2 in phase B. This conversion is crucial as it prepares the molecule for further oxidation.

Next, isocitrate is oxidized to alpha-ketoglutarate through the action of the enzyme isocitrate dehydrogenase. This oxidation reaction is significant as it releases carbon dioxide and produces NADH, a key electron carrier. This step is categorized as reaction 3 in phase B of the cycle.

Finally, the oxidation of alpha-ketoglutarate leads to the production of succinyl-CoA, which is recognized as reaction 4 in phase B. This reaction is vital as it continues the cycle's energy extraction process, ultimately contributing to ATP production through substrate-level phosphorylation.

Overall, these steps illustrate the intricate series of reactions within the citric acid cycle, highlighting the importance of each transformation in the broader context of cellular metabolism.

The Citric Acid Cycle, also known as the Krebs Cycle, is a crucial metabolic pathway that plays a significant role in cellular respiration. In particular, Phase C of this cycle focuses on the regeneration of oxaloacetate, which is essential for the cycle to continue. This phase encompasses reactions 5 to 8, following the earlier steps that involve the conversion of Acetyl CoA into citrate.

In reaction 5, succinyl CoA is converted into succinate, a process that generates GTP (guanosine triphosphate). GTP is important as it can be readily converted into ATP (adenosine triphosphate) through substrate-level phosphorylation, providing energy for cellular activities. The equation for this conversion can be represented as:

ADP + GTP → ATP + GDP

Following this, succinate undergoes oxidation to form fumarate in reaction 6, during which FAD (flavin adenine dinucleotide) is reduced to FADH₂, another important electron carrier that plays a role in the electron transport chain.

In reaction 7, the addition of water converts fumarate into malate. This hydration step is crucial for the subsequent regeneration of oxaloacetate. Finally, in reaction 8, malate is oxidized back to oxaloacetate, releasing NADH (nicotinamide adenine dinucleotide) in the process. The overall reaction can be summarized as:

Malate + NAD⁺ → Oxaloacetate + NADH + H⁺

Through these reactions, the cycle is able to regenerate oxaloacetate, allowing it to combine with Acetyl CoA once again and continue the metabolic process. This regeneration is vital for maintaining the flow of the Citric Acid Cycle and ensuring efficient energy production within the cell.

In the process of hydrolysis, succinyl CoA is converted to succinate through the action of the enzyme succinyl CoA synthase. Hydrolysis involves breaking a high-energy bond, specifically the carbon-sulfur bond in succinyl CoA, which releases energy. This energy is harnessed to produce guanosine triphosphate (GTP). The hydrolysis of GTP subsequently occurs, releasing an inorganic phosphate group that is then used to convert adenosine diphosphate (ADP) into adenosine triphosphate (ATP), a crucial energy carrier in cellular processes.

The overall reaction can be summarized as follows:

Succinyl CoA + H₂O → Succinate + CoA + GTP

Then, the hydrolysis of GTP can be represented as:

GTP + H₂O → GDP + P_i + Energy

Where P_i is the inorganic phosphate. This process not only regenerates coenzyme A but also produces succinate as the final product. The enzyme involved, succinyl CoA synthase, plays a critical role in facilitating this reaction, highlighting the importance of substrate-enzyme interactions in metabolic pathways.

In the process of oxidation, specifically in reaction 6, the enzyme involved is known as dehydrogenase, which plays a crucial role in facilitating these reactions. For instance, succinate dehydrogenase catalyzes the oxidation of succinate to fumarate. During this reaction, one molecule of FAD (flavin adenine dinucleotide) is reduced to FADH₂, highlighting the importance of this coenzyme in the oxidation process.

In this transformation, the succinate molecule undergoes a structural change where two carbon atoms, which initially form single bonds, convert to a double bond in fumarate. This is essential because carbon atoms must form four bonds to achieve stability. The action of succinate dehydrogenase effectively facilitates this conversion by utilizing FAD, demonstrating how dehydrogenases are integral in converting single-bonded carbon structures into double-bonded configurations.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that plays a significant role in cellular respiration. It involves a series of enzymatic reactions that convert acetyl-CoA into carbon dioxide and energy-rich molecules. Understanding the details of each reaction is essential for grasping how energy is produced in cells.

In the cycle, reaction 5 is responsible for converting succinyl-CoA to succinate through hydrolysis, utilizing the enzyme succinyl-CoA synthetase. This reaction is indeed accurate. Following this, reaction 6 involves the oxidation of succinate to fumarate, which is facilitated by the coenzyme FAD, confirming its correctness as well.

It is also important to note that phase C of the citric acid cycle does not result in the loss of any carbon atoms. While there may be transformations where single-bonded carbons become double-bonded, this does not equate to a loss of carbon atoms from the cycle.

However, a common misconception arises regarding the energy released from the hydrolysis of succinyl-CoA. The energy does not come from GTP; rather, it is derived from the hydrolysis of the high-energy carbon-sulfur bond in succinyl-CoA. This released energy is then utilized to synthesize GTP. Subsequently, GTP can be hydrolyzed to provide inorganic phosphate to ADP, forming ATP. Therefore, the statement claiming that the energy released comes from GTP is incorrect.

In summary, while many aspects of the citric acid cycle are accurately described, the misunderstanding regarding the source of energy in the hydrolysis of succinyl-CoA highlights the importance of distinguishing between the energy release and its subsequent utilization in GTP synthesis.

In the enzymatic reaction catalyzed by Fumarate Hydratase, also known as fumarase, fumarate undergoes hydration to form malate. This process involves the addition of a water molecule to the double bond present in fumarate, which consists of two carbon atoms connected by a pi bond. The breaking of this pi bond allows for the addition of water, resulting in the incorporation of a hydroxyl group (–OH) on one carbon and a hydrogen atom (–H) on the other. Consequently, fumarate is transformed into malate through this hydration reaction.

To summarize, the key transformation in this step is the conversion of fumarate to malate via the addition of water, highlighting the significance of hydration in biochemical processes.

In the process of cellular respiration, the conversion of malate to oxaloacetate represents a crucial oxidation reaction catalyzed by the enzyme malate dehydrogenase. During this reaction, malate, which contains a secondary alcohol functional group, undergoes oxidation. This transformation involves the removal of hydrogen atoms, resulting in the formation of a carbonyl group (C=O) from the hydroxyl group (–OH) present in malate.

As malate is oxidized, the coenzyme NAD⁺ is reduced to NADH. This reduction is significant as NADH serves as an important electron carrier in metabolic pathways, facilitating the transfer of electrons in subsequent reactions. The overall reaction can be summarized as follows:

Malate + NAD⁺ → Oxaloacetate + NADH + H⁺

This step is essential in the citric acid cycle, contributing to the regeneration of oxaloacetate, which is necessary for the continuation of the cycle and the production of energy in the form of ATP.

The citric acid cycle, also known as the Krebs cycle, involves a series of enzymatic reactions crucial for cellular respiration. One key reaction is the oxidation of malate to oxaloacetate, catalyzed by the enzyme malate dehydrogenase. This reaction is essential for regenerating oxaloacetate, allowing the cycle to continue efficiently. The overall reaction can be represented as:

$$\text{Malate} + \text{NAD}^+ \rightarrow \text{Oxaloacetate} + \text{NADH} + \text{H}^+$$

In step 6 of the cycle, succinate is oxidized to fumarate, during which it loses two hydrogen atoms. This oxidation process reduces one molecule of FAD to FADH₂, highlighting the role of flavin adenine dinucleotide in energy production:

$$\text{Succinate} + \text{FAD} \rightarrow \text{Fumarate} + \text{FADH}_2$$

Step 5 involves the hydrolysis of succinyl CoA to produce succinate, a reaction that releases energy used to convert GDP to GTP, a high-energy molecule:

$$\text{Succinyl CoA} + \text{H}_2\text{O} \rightarrow \text{Succinate} + \text{CoA} + \text{GTP}$$

Finally, in step 7, fumarate undergoes hydration to form malate, completing the cycle and preparing for the next round of reactions:

$$\text{Fumarate} + \text{H}_2\text{O} \rightarrow \text{Malate}$$

Understanding these steps is crucial for grasping how the citric acid cycle contributes to the overall process of aerobic respiration, facilitating the conversion of biochemical energy from nutrients into ATP.

The citric acid cycle, also known as the Krebs cycle, plays a crucial role in cellular respiration by degrading acetyl groups to produce carbon dioxide and high-energy molecules, specifically NADH, FADH₂, and ATP. Understanding the phases and reactions within this cycle is essential for grasping its function in energy production.

To aid in memorization, consider the mnemonic "Krebs cycle is a big crab," which highlights the three phases of the cycle: A, B, and C. Within this cycle, there are four oxidation reactions that occur, which can be remembered with the phrase "4 owls and 1 C hawk in a circus ring." The four oxidation reactions yield NADH and FADH₂, with two occurring in phase B and two in phase C.

In phase C, a hydrolysis reaction takes place, which is responsible for producing ATP. This cyclic nature of the citric acid cycle is likened to a circus ring, emphasizing its continuous process. Thus, the key takeaways include the four oxidation reactions that yield NADH and FADH₂, and the single hydrolysis reaction in phase C that produces ATP. Understanding these components is vital for comprehending how the citric acid cycle contributes to the overall energy metabolism in cells.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that processes acetyl CoA derived from glucose. Each glucose molecule generates two acetyl CoA molecules, allowing the cycle to occur twice. The cycle begins with the combination of acetyl CoA and oxaloacetate to form citrate.

As the cycle progresses, several key reactions occur. In reactions 3 and 4, each cycle produces one NADH and one carbon dioxide (CO₂). Since the cycle runs twice for each glucose molecule, this results in a total of four NADH and four CO₂ produced. Additionally, one ATP is generated per cycle, leading to a total of two ATP from the two cycles.

FADH₂ is produced in reaction 6, contributing one FADH₂ per cycle, resulting in a total of two FADH₂ after two cycles. Furthermore, reaction 8 generates another NADH, adding to the total count. Therefore, the complete yield from two rounds of the citric acid cycle includes:

• 4 CO₂
• 2 ATP
• 2 FADH₂
• 6 NADH

Ultimately, the cycle regenerates oxaloacetate, allowing the process to continue. This summary encapsulates the essential outputs and transformations that occur during the citric acid cycle, highlighting its significance in cellular respiration and energy production.

The Citric Acid Cycle, also known as the Krebs cycle, is crucial for energy production in cellular respiration, generating high-energy molecules such as NADH, FADH₂, and ATP. A helpful mnemonic to remember where these molecules are produced involves a story about ants and flies: "Under trees in a forest, there lived 5 ants and 6 flies." Here, "under" signifies NADH, as it is produced in reactions 3, 4, and 8 of the cycle. This results in a total of 3 NADH molecules per cycle, and since the cycle occurs twice, the total becomes 6 NADH molecules.

The phrase "5 ants" indicates the production of ATP, which occurs in reaction 5. Lastly, "flies" represents FADH₂, generated in reaction 6. Thus, the high-energy molecules produced in the Citric Acid Cycle can be summarized as follows: NADH from reactions 3, 4, and 8; ATP from reaction 5; and FADH₂ from reaction 6.

Additionally, the cycle also produces carbon dioxide (CO₂). Specifically, one molecule of CO₂ is generated in reactions 3 and 4, leading to a total of 2 CO₂ molecules per cycle. When the cycle is completed twice, this results in a total of 4 CO₂ molecules. Keeping these memory tools in mind will aid in recalling the high-energy molecules and their respective reactions within the Citric Acid Cycle.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that generates high-energy molecules essential for cellular respiration. In this cycle, specific reactions produce key high-energy molecules: NADH, FADH₂, and ATP. To identify these reactions, a mnemonic can be helpful: "Under threes in a forest, so 48, there lived 5 ants and 6 flies."

In this mnemonic, the numbers correspond to the steps in the citric acid cycle where these high-energy molecules are produced. The production of NADH occurs in steps 3, 4, and 8. Specifically, step 3 involves the conversion of isocitrate to α-ketoglutarate, step 4 involves the conversion of α-ketoglutarate to succinyl-CoA, and step 8 involves the conversion of malate to oxaloacetate. Each of these steps results in the formation of NADH, which is vital for the electron transport chain.

Next, the mnemonic indicates that "5 ants" refers to ATP production, which occurs in step 5. In this step, GTP is generated and can be readily converted to ATP through substrate-level phosphorylation, where ADP is phosphorylated to form ATP.

Lastly, "6 flies" represents the production of FADH₂ in step 6, where succinate is converted to fumarate. This reaction is catalyzed by the enzyme succinate dehydrogenase, which facilitates the reduction of FAD to FADH₂.

In total, there are five reactions in the citric acid cycle that produce high-energy molecules: three producing NADH, one producing ATP, and one producing FADH₂. Understanding these steps is essential for grasping how the citric acid cycle contributes to energy production in aerobic respiration.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that plays a key role in cellular respiration. To effectively remember the sequence of reactions and the intermediate metabolites involved, a mnemonic can be utilized: "Oxford, city, ice crates, kegs, and such contain succulent foamy milk." This phrase corresponds to the following metabolites:

Starting with oxaloacetate (Oxford), which is produced in the final step of the cycle, the process begins when acetyl-CoA combines with oxaloacetate to form citrate (city) in reaction 1. In reaction 2, citrate is converted to isocitrate (ice crates). Moving to reaction 3, isocitrate is transformed into alpha-ketoglutarate (kegs). In reaction 4, alpha-ketoglutarate is further converted into succinyl-CoA (such succulent). Reaction 5 sees succinyl-CoA change into succinate (succulent). Following this, in reaction 6, succinate undergoes oxidation to become fumarate (foamy). In reaction 7, fumarate is converted into malate (milk). Finally, in reaction 8, malate is transformed back into oxaloacetate, completing the cycle.

This mnemonic not only aids in memorizing the order of the compounds but also reinforces the understanding of the transformations that occur throughout the citric acid cycle, highlighting its importance in energy production within the cell.

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that plays a significant role in cellular respiration. Understanding the enzymes involved in this cycle can be simplified by associating them with their respective substrates and the types of reactions they catalyze.

In the first reaction, the formation of citrate is catalyzed by an enzyme known as citrate synthase. This enzyme is classified as a synthase because it facilitates the synthesis of a new compound. Following this, the second reaction involves the isomerization of citrate, which is catalyzed by aconitase. This step is essential for rearranging the molecular structure of citrate.

Reactions three, four, six, and eight are characterized as oxidation reactions. In these cases, the enzymes responsible for catalyzing these reactions are known as dehydrogenases. These enzymes play a vital role in the removal of hydrogen atoms, which is a key aspect of oxidation processes.

In reaction five, a hydrolysis reaction occurs, which harnesses energy to drive the reaction forward. Interestingly, this reaction is catalyzed by a synthase rather than a hydrolase, making it an exception in the cycle. Lastly, reaction seven involves hydration, where water is added to the substrate, further contributing to the cycle's progression.

These insights into the types of enzymes and their corresponding reactions provide a helpful framework for understanding the citric acid cycle and its significance in energy production within the cell.