About the Decarboxylation Reaction
A Decarboxylation Reaction is a chemical reaction in which a carboxyl group is removed and carbon dioxide is released (\[CO_{2}\]). Decarboxylation is a process in which carboxylic acids remove a carbon atom from a carbon chain. Carboxylation is the split chemical step in photosynthesis, where \[CO_{2}\] is added to the substance. It is a completely reversible reaction. Decarboxylases, on the other hand, are enzymes that catalyze decarboxylation.
One of the oldest known organic transformations is decarboxylation. It is one of the processes that is thought to occur in conjunction with destructive distillation and pyrolysis (thermal decomposition of substances). Through the intermediacy of metal carboxylate complexes, metal salts, particularly copper compounds, enhance the process. Aryl carboxylates can be decarboxylated to yield the equivalent aryl anion, which can then undergo cross-coupling processes.
The term decarboxylation means the removal of a carboxyl group (-COOH) from any reactant molecule. The chemical reaction where a carboxyl group (-COOH) gets eliminated and carbon dioxide (\[CO_{2}\]) is released at the product end is called Decarboxylation. The liberation of \[CO_{2}\] makes the reaction almost irreversible in many cases. However, the reverse process i.e. Carboxylation is the addition of \[CO_{2}\]. Carboxylation accounts for the very first step of Photosynthesis after the intake of \[CO_{2}\]. Carboxylation results in the formation of Carboxylic acid. Most Decarboxylation Reactions involve carboxylic acids, where a carbon atom is broken off from the carbon chain. This carbon atom is released in the form of \[CO_{2}\].
Decarboxylation – Carboxylic Acid
Carboxylic acids are the organic decarboxylation acids written as RCOOH, where R stands for an alkyl group or Hydrogen. The decarboxylation of a carboxylic acid is one of the oldest reactions discovered in organic chemistry. The reaction process involves the removal of the -COOH group or a carboxylate salt of the given acid. The reaction gives the product RH along with \[CO_{2}\].
\[RCO_{2}H \rightarrow RH + CO_{2}\]
Decarboxylation Reactions are observed with a slightly categorized form in many compounds. The following are some of them.
Krapcho Decarboxylation: This reaction involves activated esters with an electron-withdrawing group and halide anions. The ester is later replaced by a proton or an electrophile.
Hunsdiecker Reaction: It is a reaction where silver salts of carboxylic acid undergo decarboxylation to give an organic halide byproduct. It is also called the Halogenation reaction for the addition of halogen.
Decarboxylation Reaction Mechanism
The decarboxylation mechanism replaces the carboxyl group in a carboxylic acid with hydrogen. The reaction is facilitated by a group of enzymes called decarboxylases or carboxy-lyases.
The regent that helps with the reaction is Soda-lime. It is a mixture of caustic soda and quick lime.
The mechanism of the reaction takes place in three steps. The first one begins with the removal of \[H^{+}\] ion from the carboxylic acid and the addition of Na. Na salt of carboxylic acid and water molecule was released in this step.
In the second step, a negative charge from Oxygen is moved in between the carbon-oxygen bond. So, the bond between the alkyl group(-R) and carbon breaks. \[CO_{2}\] is liberated in this step.
The third step takes place with help of the \[H_{2}O\] generated in the first step. It breaks to give a proton ion that combines with an alkyl group to give an alkane.
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Decarboxylation Enzyme
The enzymes that help in the process of Decarboxylation of several organic acids are called decarboxylases or carboxy-lyases. They function in both adding and removing a carboxyl group from the organic compound. They are usually named according to the substrate they catalyze. Some examples of these enzymes are Ornithine decarboxylase, RuBisCO, Pyruvate decarboxylase, Histidine decarboxylase, etc.
Decarboxylation of Amino Acids
Decarboxylation of amino acid results in the formation of an Amine by the removal of the carboxyl group (-COOH) from the amino acid. Due to the removal of the organic acid group, the byproduct moves up the pH scale for being of alkaline nature. The decarboxylase enzymes facilitate the decarboxylation mechanism i.e. removal of acidic groups. Deaminases, on the other hand, remove the amino groups to give out chemicals acidic in nature.
Decarboxylation Tests
These are the biochemical tests involving the production of enzyme decarboxylase. So, they are also known as decarboxylase tests.
The test is used to differentiate various members of Enterobacteriaceae that produce decarboxylase, from other gram-negative bacteria.
Organisms that can metabolize amino acid by decarboxylation are identified by the formation of decarboxylase enzymes namely, arginine decarboxylase, ornithine decarboxylase, and lysine decarboxylase.
The members are then further differentiated based on their abilities to produce these enzymes.
The basal medium used in the test is Moeller’s formula. Meat peptones and beef extract present in the medium provide nitrogenous nutrition for bacterial growth.
Cresol red and Bromocresol purple are the pH indicators in the media.
Substrates like arginine, ornithine, and lysine are added to the medium to detect decarboxylation.
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FAQs on Decarboxylation Reaction
1. What are Hydro Decarboxylations?
The conversion of a carboxylic acid to the equivalent hydrocarbon is known as hydro decarboxylation. This is similar to the more broad term "decarboxylation" as defined above, except that it requires that the carboxyl group be replaced by hydrogen, as expected. The reaction is particularly common when malonic ester synthesis and Knoevenagel condensations are involved. The reaction comprises a carboxylate ion, a carboxylate base, and an unsaturated electron density receptor, such as a protonated carbonyl group. When a carboxylic acid is heated with strong hydrochloric acid, a direct path is difficult because protonated carbon dioxide is produced. The reaction is most likely to start with the addition of water and a proton in these instances.
2. What is Carboxylation and Decarboxylation in the Metabolic Context?
Gain or loss of a single carbon by an organic molecule in the form of \[CO_{2}\] is the most important carbon-carbon bond forming and breaking in biological chemistry. You must have come across an equation like this in one of your biology classes.
\[6CO_{2} + 6H_{2}O + energy \rightarrow C_{6}H_{12}O_{6} + 6O_{2}\]
It refers to the photosynthetic process, in which plants use sunshine to synthesize glucose from individual carbon dioxide molecules. The carboxylation reaction is the main step in which carbon dioxide gets fixed (condensed with an organic molecule).
You should also be familiar with the chemical equation in reverse:
\[C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + energy \]
The oxidative breakdown of glucose to create water, carbon dioxide, and energy is described by the above chemical reaction in respiration. During the transformation of glucose, every carbon atom is transformed into \[CO_{2}\] molecules. The Decarboxylation Reaction is the main step in which a carbon atom separates from a bigger chemical compound. The citric acid cycle and the pentose phosphate route are the key decarboxylation stages in the conversion of glucose to carbon dioxide.
3. What is the Decarboxylation Reaction mechanism?
In a carboxylic acid, the decarboxylation mechanism exchanges the carboxyl group with hydrogen. A set of enzymes known as decarboxylases or carboxy-lyases aid the procedure. Soda-lime is a regent that aids in the reaction. It's made out of a combination of caustic soda and quick lime. Three steps are involved in the reaction's mechanism.
The first involves removing the \[H^{+}\] ion from the carboxylic acid and replacing it with Na. This procedure resulted in the release of sodium salt of carboxylic acid and a water molecule.
A negative charge from oxygen is transported between the carbon-oxygen bonds in the second stage. As a result, the alkyl group(-R) and carbon bond split. In this stage, \[CO_{2}\] is released. The \[H_{2}O\] created in the previous phase is used in the third step. It splits to release a proton ion, which reacts with an alkyl group to form an alkane.
4. What are Decarboxylation mechanisms in the biological system?
The removal of carbon dioxide from organic acids, known as decarboxylation, is a crucial event in biology. Decarboxylase enzymes play an important role in glucose metabolism and amino acid conversion in both aerobic and anaerobic environments. Our understanding of the processes that enable these critical decarboxylase events has continued to grow and inspire during the last decade. The organic cofactors biotin, flavin, NAD, pyridoxal 5'-phosphate, pyruvoyl, and thiamin pyrophosphate are discussed as catalytic centers in this article. Metal-dependent decarboxylase processes are also given a lot of studies.
5. What is Organocatalytic Amino Acid Decarboxylation as a Pathway to Bio-based Amines and Amides?
Amino acids produced through fermentation or recovered from protein waste hydrolysates are a great renewable resource for bio-based chemical synthesis. We provide a chemocatalytic, metal-free technique for the decarboxylation of amino acids, allowing direct access to primary amines, to recycle both carbon and nitrogen. The amino acid is briefly locked into a Schiff base in the presence of a carbonyl molecule, allowing for easier \[CO_{2}\] removal. Isophorone was recognized as a potent organocatalyst under mild conditions after examining several types of aldehydes and ketones on their activity at modest catalyst loadings (5 mol percent). Many amino acids with a neutral side chain were transformed into 2-propanol at 150 °C in 28–99 percent yield after optimization. The amine is sensitive to N-formylation when the reaction is carried out in DMF.
Amino acids produced through fermentation or recovered from protein waste hydrolysates are a great renewable resource for bio-based chemical synthesis. We provide a chemocatalytic, metal-free technique for the decarboxylation of amino acids, allowing direct access to primary amines, to recycle both carbon and nitrogen. The amino acid is briefly locked into a Schiff base in the presence of a carbonyl molecule, allowing for easier \[CO_{2}\] removal. Isophorone was recognized as a potent organocatalyst under mild conditions after examining several types of aldehydes and ketones on their activity at modest catalyst loadings (5 mol percent). Many amino acids with a neutral side chain were transformed into 2-propanol at 150 °C in 28–99 percent yield after optimization. The amine is sensitive to N-formylation when the reaction is carried out in DMF.
6. Explain the decarboxylation of Histidine and Glutamate.
Histidine and Glutamate are both amino acids decarboxylated to form alkaline amine compounds on the body.
Decarboxylation of Histidine: Histidine gets decarboxylated in presence of the enzyme histidine decarboxylase, catalyzed by a coenzyme \[B_{6}-PO_{4}\]. The reaction produces Histamine and \[CO_{2}\]. The decarboxylation process takes place in the Basophils, gut, gastric mucosa cell, and histaminergic neurons of the CNS. Excessive release of Histamine occurs at the site of a wound, in injured tissues.
Decarboxylation of Glutamate: Glutamate decarboxylation is catalyzed by glutamate decarboxylase enzyme with the help of coenzyme \[B_{6}-PO_{4}\]. The reaction forms the product γ-aminobutyric acid (GABA) along with the liberation of \[CO_{2}\]. The reaction takes place in the gray matter of CNS. GABA acts as an inhibitory transmitter when released from axon terminals of neurons.
7. What are the uses and limitations of the decarboxylation test?
The following are the uses of the Decarboxylation test.
The test is efficient in differentiating members of the Enterobacteriaceae with similar physiological activities.
The decarboxylase test for lysine decarboxylase helps differentiate the bacteria Salmonella (+) and Shigella (-).
Arginine decarboxylase test results in the identification of Enterococcus species based on their Arginine metabolizing activity.
There are some limitations to the test. They are:
The test is time-consuming. Results cannot be interpreted before 18-24 hours of incubation. Microorganisms that do not ferment glucose sometimes show weak decarboxylase activity.
The test fails to measure the intracellular amount of the enzyme. The enzyme presence is only detected by the change in pH.
