Home » Citric Acid Cycle Pathway: Steps, Products, Significance

Citric Acid Cycle Pathway: Steps, Products, Significance

The citric acid cycle pathway (Krebs cycle or tricarboxylic acid cycle or TCA Cycle) is the most important metabolic pathway for the energy supply to the body. About 65-70% of the ATP is synthesized in Krebs cycle. Citric acid cycle essentially involves the oxidation of acetyl CoA to CO2 and H2O. This cycle utilizes about two thirds of total oxygen consumed by the body. The name TCA cycle is used, since, at the outset of the cycle, tricarboxylic acids (citrate, cis-aconitate and isocitrate) participate.

History of Krebs cycle

The citric acid cycle was proposed by Hans Adolf Krebs in 1937, based on the studies of oxygen consumption in pigeon breast muscle. The cycle is named in his honor (Nobel Prize for Physiology and Medicine in 1953.)

What is Citric Acid Cycle

Krebs cycle or citric acid cycle pathway basically involves the combination of a two carbon acetyl CoA with a four carbon oxaloacetate to produce a six carbon tricarboxylic acid, citrate. In the reactions that follow, the two carbons are oxidized to CO2 and oxaloacetate is regenerated and recycled. Oxaloacetate is considered to play a catalytic role in citric acid cycle.

Krebs cycle or citric acid cycle pathway is a cyclic process. However, it should not be viewed as a closed circle, since many compounds enter the cycle and leave. TCA cycle is comparable to a heavy traffic circle in a national highway with many connecting roads. Each intermediate of the cycle connecting another pathway is a road.

Citric Acid Cycle Location

The enzymes of citric acid cycle pathway are located in mitochondrial matrix, in close proximity to the electron transport chain. This enables the synthesis of ATP by oxidative phosphorylation without any hindrance.

Citric Acid Cycle Reaction

The overall reaction/ equation of the citric acid cycle is:

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

In words, the equation is written as:

Acetyl CoA + Nicotinamide adenine dinucleotide + Flavin adenine dinucleotide + Adenosine diphosphate + Phosphate   →   Pyruvate + Water + Adenosine triphosphate + Nicotinamide adenine dinucleotide + Hydrogen ions

Citric Acid Cycle Enzymes

In eukaryotic cells, the enzymes that speed up the reactions of the citric acid cycle are in the matrix of the mitochondria, except for succinate dehydrogenase and aconitase, which are in the inner mitochondrial membrane. Almost all of the enzymes in the citric acid cycle need Mg2+. This is something they all have in common. 

During the citric acid cycle, the following enzymes speed up the process at different points: Citrate synthase, Aconitase, Isocitrate dehydrogenase, α-ketoglutarate, Succinyl-CoA synthetase, Succinate dehydrogenase, Fumarase and Malate dehydrogenase.

Steps of Citric Acid Cycle

The pyruvate dehydrogenase complex is responsible for the oxidative decarboxylation of pyruvate to acetyl CoA. This step serves as a link between glycolysis and the citric acid cycle pathway.  The citric acid cycle steps are described below (Figure 1).

Citric Acid Cycle Diagram

Citric Acid Cycle Diagram

Figure 1: Citric Acid Cycle Diagram

Citric Acid Cycle Steps

Step 1

Krebs cycle Step 1 Condensation of acetyl CoA with oxaloacetate

Figure 2: Krebs cycle Step 1 Condensation of acetyl CoA with oxaloacetate. Image Source: Lehninger Principles of Biochemistry.

In the first step of the citric acid cycle, the four-carbon complex oxaloacetate (OAA) and the two-carbon compound acetyl CoA are joined. The oxaloacetate combines with the acetyl group of acetyl CoA and water to produce citric acid, CoA, a six-carbon molecule.

The enzyme citrate synthase speeds up the reaction by bringing together the methyl group of acetyl CoA and the carbonyl group of oxaloacetate. This makes citryl-CoA, which is then broken down into free coenzyme A and citrate.

Step 2

Krebs cycle Step 2 Isomerization of citrate into isocitrate

Figure 3: Krebs cycle Step 2 Isomerization of citrate into isocitrate. Image Source: Lehninger Principles of Biochemistry.

In the second step of the Krebs cycle, citrate changes into isocitrate by a process called isomerization. The enzyme aconitase speeds up this process, which changes citrate into isocitrate. This reaction can be turned around, and citrate can be made from isocitrate. This reaction happens in two steps. In the first step, citrate is turned into cis-aconitase by dehydration. In the second step, cis-aconitase is turned back into isocitrate by hydration.

Step 3

Krebs cycle Step 3 Oxidative decarboxylations of isocitrate

Figure 4: Krebs cycle Step 3 Oxidative decarboxylations of isocitrate. Image Source: Lehninger Principles of Biochemistry.

The third phase of the citric acid cycle is the initial of this cycle’s four oxidation-reduction processes.  Isocitrate dehydrogenase catalyses the oxidative decarboxylation of isocitrate to produce the five-carbon molecule -ketoglutarate.

Like the second reaction, this reaction is a two-step reaction. In the first phase, isocitrate is dehydrogenated to oxalosuccinate, and in the second, oxalosuccinate is decarboxylated to -ketoglutarate. Both reactions are conducted by the same enzyme and are irreversible. However, the first process results in the creation of NADH and the second in the release of CO2.

Step 4

citric acid cycle pathway Step 4 Oxidative decarboxylation of α-ketoglutarate

Figure 5: Citric Acid Cycle pathway  Step 4 Oxidative decarboxylation of α-ketoglutarate. Image Source: Lehninger Principles of Biochemistry.

D-ketoglutarate is converted to succinyl CoA via oxidative decarboxylation, which is catalysed by the D-ketoglutarate dehydrogenase complex. This enzyme requires five cofactors: TPP, lipoamide, NAD+, FAD, and CoA. The mechanism of the reaction is similar to that of pyruvate to acetyl CoA conversion.

Step 5

citric acid cycle pathway Step 5 Conversion of succinyl-CoA into succinate

Figure 6: Citric Acid Cycle pathway Step 5 Conversion of succinyl-CoA into succinate. Image Source: Lehninger Principles of Biochemistry.

Succinate thiokinase changes succinyl CoA into succinate. The phosphorylation of GDP to GTP happens at the same time as this reaction. This is a phosphorylation at the level of the substrate. The enzyme nucleoside diphosphate kinase changes GTP into ATP.

Step 6

citric acid cycle pathway Step 6 Dehydration of succinate to fumarate

Figure 6: Citric Acid Cycle pathway  Step 6 Dehydration of succinate to fumarate. Image Source: Lehninger Principles of Biochemistry.

The enzyme succinate dehydrogenase changes succinate into fumarate by oxidising it. FADH2, not NADH, is produced when this reaction happens.

Step 7

citric acid cycle pathway Step 7 Hydration of fumarate to malate

Figure 6: Citric Acid Cycle pathway Step 7 Hydration of fumarate to malate. Image Source: Lehninger Principles of Biochemistry.

Formation of malate: The enzyme fumarase catalyses the conversion of fumarate to malate with the addition of H2O.

Step 8

Citric Acid Cycle pathway Step 8 Dehydrogenation of L-malate to oxaloacetate

Figure 6: Citric Acid Cycle pathway Step 8 Dehydrogenation of L-malate to oxaloacetate. Image Source: Lehninger Principles of Biochemistry.

Conversion of malate to oxaloacetate: Malate is then oxidized to oxaloacetate by malate dehydrogenase. The third and final synthesis of NADH occurs at this stage. The oxaloacetate is regenerated which can combine with another molecule of acetyl CoA, and continue the cycle.

Products of Krebs cycle

The citric acid cycle energetics, since this is a cyclic process, the oxaloacetate produced at the end will condense with acetyl CoA during the subsequent cycle. At each turn of the cycle,

3 NADH,

1 FADH2,

1 GTP (or ATP),

2 CO2

Regulation of Krebs cycle

ATP requirements are critical for regulating the pace of the citric acid cycle. Either enzymes or ADP levels are responsible for this control. The citric acid cycle is regulated by three enzymes, citrate synthase, isocitrate dehydrogenase, and D-ketoglutarate dehydrogenase.

Citrate synthase is inhibited by ATP, NADH, acetyl CoA and succinyl CoA. Isocitrate dehydrogenase is activated by ADP, and inhibited by ATP and NADH. D-Ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH. 

Availability of ADP is very important for the citric acid cycle to proceed. This is due to the fact that unless sufficient levels of ADP are available, oxidation (coupled with phosphorylation of ADP to ATP) of NADH and FADH2 through electron transport chain stops. The accumulation of NADH and FADH2 will lead to inhibition of the enzymes (as stated above) and also limits the supply of NAD+ and FAD which are essential for TCA cycle to proceed.

FAQ

FAQs on Citric Acid Cycle Pathway

The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid cycle, happens in the mitochondria of eukaryotic cells and in the cytoplasm of prokaryotic cells.

The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid cycle, happens in the mitochondria of eukaryotic cells and in the cytoplasm of prokaryotic cells.

The citric acid cycle, also called the Krebs cycle or the tricarboxylic acid cycle, is where acetyl-CoA goes. It is where the cycle begins, and the acetyl group from acetyl-CoA is used to make citrate, which is the first step in the cycle.

Krebs cycle or citric acid cycle pathway basically involves the combination of a two carbon acetyl CoA with a four carbon oxaloacetate to produce a six carbon tricarboxylic acid, citrate.

The citric acid cycle is also known as the Krebs cycle, named after Sir Hans Adolf Krebs who was awarded the Nobel Prize in Medicine for his discovery of the citric acid cycle in 1937.

At each turn of the cycle, 3 NADH, 1 FADH2, 1 GTP (or ATP), 2 CO2.