Introduction

The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid cycle, is a fundamental metabolic pathway occurring in the matrix of the mitochondria. This cycle plays a crucial role in aerobic organisms by providing energy through oxidation reactions that generate reduced electron carriers (NADH and FADH2), which are subsequently used in the electron transport chain to produce ATP via oxidative phosphorylation.

Overview of the Krebs Cycle

The Krebs cycle consists of a series of enzyme-catalyzed reactions, which cyclically convert acetyl-CoA into two molecules of CO2, with concomitant energy production in the form of ATP and reduced cofactors (NADH and FADH2). The overall equation for one turn of the cycle is:

Acetyl-CoA + 3 NAD+ + 1 FAD + 3 ADP + 3 Pi + 2 H2O → 2 CO2 + 3 NADH + CoA + ATP + FADH2 + 3 H+

The cycle is initiated by the condensation of acetyl-CoA with oxaloacetate (OAA), yielding citrate, catalyzed by citrate synthase. The subsequent reactions involve repeated additions of two-carbon compounds derived from acetyl-CoA to the four-carbon compound citrate, followed by decarboxylation and cleavage steps that release CO2 and regenerate OAA, completing one cycle.

Details of the Reactions in the Krebs Cycle

Citrate Synthase

The first reaction in the cycle involves the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase (CS). The CoA moiety of acetyl-CoA is transferred to the alpha-carboxyl group of OAA.

Aconitase

The conversion of citrate to isocitrate is catalyzed by aconitase, which hydrates citrate at its beta carbon to produce cis-aconitate, followed by dehydration to yield isocitrate.

Isocitrate Dehydrogenase

Isocitrate is converted into alpha-ketoglutarate and CO2 via oxidative decarboxylation by isocitrate dehydrogenase (IDH). This reaction involves the reduction of NAD+, producing NADH.

α-Ketoglutarate Dehydrogenase Complex

The next step in the cycle is the oxidative decarboxylation of alpha-ketoglutarate to produce succinyl-CoA, CO2, and reducing equivalents (NADH or FADH2). This reaction is catalyzed by the α-ketoglutarate dehydrogenase complex.

Succinyl-CoA Synthetase

The succinyl-CoA formed in the previous step is converted into succinate and CoA, catalyzed by succinyl-CoA synthetase (SCS). This reaction also consumes ATP to produce ADP + Pi.

Succinate Dehydrogenase

The conversion of succinate to fumarate is catalyzed by succinate dehydrogenase (SDH), which involves the simultaneous oxidation of succinate and reduction of FAD. The reduced FAD is subsequently reoxidized by oxygen, generating FADH2 during electron transport in the inner mitochondrial membrane.

Fumarase

The mutual hydration of fumarate to malate and dehydration of malate back to fumarate is catalyzed by fumarase (FA). This reaction provides a mechanism for reversible regulation of the Krebs cycle, as the equilibrium favors malate under low ATP concentrations and high ADP concentrations.

Malate Dehydrogenase

The final conversion of malate back to oxaloacetate is catalyzed by malate dehydrogenase (MDH), which involves the oxidation of malate and reduction of NAD+, producing NADH in the process. This reaction sets up a cycle, as OAA can now react with acetyl-CoA to initiate another turn of the Krebs cycle.

Regulation of the Krebs Cycle

The rate of flux through the Krebs cycle is regulated at multiple levels, including substrate availability, allosteric regulation of enzymes, and feedback inhibition. The most significant regulatory mechanisms include:

• Citrate Synthase: Inhibited by ATP and NADH, facilitating the production of ATP during times of energy demand.
• Pyruvate Dehydrogenase Complex (PDC): Inhibited by acetyl-CoA and NADH, maintaining the proper balance between pyruvate oxidation and lactate production.
• α-Ketoglutarate Dehydrogenase Complex: Allosterically inhibited by acetyl-CoA and succinyl-CoA, preventing excessive ATP production during times of high energy demand.
• Phosphofructokinase (PFK): Inhibited by ATP, ensuring that glucose is metabolized only when needed for ATP production.

Summary

The Krebs cycle is a crucial metabolic pathway in aerobic organisms, providing energy and reduced cofactors necessary for ATP synthesis via oxidative phosphorylation. The cycle consists of a series of enzyme-catalyzed reactions that cyclically convert acetyl-CoA into CO2, with concomitant energy production in the form of ATP and reduced cofactors (NADH and FADH2). Regulation of the Krebs cycle occurs at multiple levels to ensure proper metabolism and energy balance.