Question

Identify enzyme \(\mathrm{A}\) in the given reaction of \(\mathrm{K}_{\text {rebs' }}\) OAA \(+\) Acetyl- \(\mathrm{CoA}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{A}}{\longrightarrow}\) Citric acid (4 C) (a) Oxaloacetate synthetase (b) Citrate synthase (c) Aconitase (d) Dehydrogenase

Short answer

(b) Citrate synthase

Step-by-step solution

Understand the Process

Identify the process being described. The reaction mentioned is the first step of the Krebs cycle, where oxaloacetate (OAA) combines with Acetyl-CoA and water (H2O) to form Citric acid.

Identify the Enzyme for the Reaction

Correlate the given reactants and product with the enzyme responsible for catalyzing this specific reaction in the Krebs cycle.

Match the Enzyme with its Function

Find the enzyme that facilitates the condensation of oxaloacetate and Acetyl-CoA into Citric acid. This enzyme is known as Citrate synthase.

Key concepts

Citrate Synthase

Citrate synthase plays a pivotal role in the Krebs cycle, acting as the enzyme that catalyzes the initial step of this metabolic pathway. This crucial reaction involves the combination of oxaloacetate (OAA) and Acetyl-CoA along with water to produce citric acid, a six-carbon compound. The significance of citrate synthase lies in its ability to set the stage for the series of reactions that are integral for the production of ATP, which is the primary energy currency of the cell.

As the gatekeeper of the Krebs cycle, citrate synthase is highly regulated. It operates in a manner that is influenced by the energy needs of the cell; when the energy levels are high, the activity of citrate synthase diminishes. Conversely, when the energy needs of the cell escalate, the enzyme becomes more active to drive the Krebs cycle forward, thereby producing more ATP to meet the increased demand.

Krebs Cycle Steps

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of enzymatic reactions that play a key role in cellular respiration. The purpose of this cycle is to extract high-energy electrons from Acetyl-CoA, which are then used in the electron transport chain to generate ATP.

The cycle comprises eight primary steps:

• Combining Acetyl-CoA with Oxaloacetate to form Citrate, catalyzed by Citrate synthase.
• Isomerization of Citrate to Isocitrate, facilitated by Aconitase.
• Oxidative decarboxylation of Isocitrate to \(\alpha\)-Ketoglutarate, powered by Isocitrate dehydrogenase.
• Another oxidative decarboxylation turning \(\alpha\)-Ketoglutarate into Succinyl-CoA, catalyzed by \(\alpha\)-Ketoglutarate dehydrogenase.
• Substrate-level phosphorylation converting Succinyl-CoA to Succinate, catalyzed by Succinyl-CoA synthetase.
• Oxidation of Succinate to Fumarate, executed by Succinate dehydrogenase.
• Hydration of Fumarate to Malate, catalyzed by Fumarase.
• Oxidation of Malate to Oxaloacetate, facilitated by Malate dehydrogenase, which regenerates the molecule required to continue the cycle.

The Krebs cycle is a classic example of a metabolic pathway that is both cyclical and regulated, as the presence of substrates and feedback from the energy status of the cell influence its operation.

Oxaloacetate

Oxaloacetate (OAA) is a four-carbon molecule that holds considerable importance in cellular metabolism. It is both the starting point and the end product of the Krebs cycle, making it a crucial component in this closed loop of reactions.

OAA reacts with Acetyl-CoA to commence the Krebs cycle, forming Citrate through the enzyme citrate synthase, which is the first enzymatic step of the cycle. This reaction not only signifies the start of the energy extraction process but also symbolizes the regeneration capacity of the cycle, as each turn of the cycle replenishes the Oxaloacetate needed for the subsequent cycle.

The abundance of OAA is also critical for gluconeogenesis, a pathway that synthesizes glucose from non-carbohydrate sources, highlighting its versatility in metabolism. Its concentration within the mitochondria can influence the rate at which the Krebs cycle operates, hence acting as a metabolic control point.

Acetyl-CoA

Acetyl-CoA is a two-carbon molecule that serves as a fundamental substrate in the Krebs cycle. It is the product of the breakdown of carbohydrates, fats, and proteins during the processes of glycolysis and \(\beta\)-oxidation. This makes Acetyl-CoA a convergence point for different metabolites in the energy-producing pathways of the cell.

In the Krebs cycle, Acetyl-CoA combines with Oxaloacetate to form Citrate, marking the first step in the sequence of energy-releasing reactions. Notably, the Acetyl-CoA molecule contributes its acetyl group to the cycle while the CoA component is recycled. The acetyl group then undergoes a series of transformations, ultimately leading to the production of multiple molecules of CO2, ATP, NADH, and FADH2, which are essential for the generation of cellular energy.

Due to its central role in metabolism, Acetyl-CoA is tightly regulated to ensure that energy production is matched with the cell's energy demands, thus maintaining metabolic homeostasis.