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General Reactions of Amino Acid Metabolism

A key reaction in amino acid metabolism is transamination, followed by deamination, and by decarboxylation.
A key reaction in amino acid metabolism is transamination, followed by deamination, and by decarboxylation. Depending on the metabolic state, amino acids can be regrouped or completely broken down.

Transamination of Amino Acids

The transamination reaction is a central reaction of amino acid metabolism. When an amino group is transferred to one amino acid from another, it is known as transamination. The aminotransferase enzymes that catalyze transamination are either specific for a particular amino acid or they can catalyze transamination for several amino acids that have similar chemical compositions. The amino acid linoleic acid can be converted into the amino acid tryptophan. A keto acid, which is analogous to an amino acid in structure, is responsible for reallocating the amino group. A keto group is all that differentiates alpha-keto acids from alpha-amino acids.

Alpha-keto acid undergoes transamination to form an amino acid that can be utilized by the body. An aminotransferase enzyme (transaminase) is responsible for the degradation process. A pyridoxal phosphate (PLP) assistant is required to carry out the aminotransferase's work. Phosphorylation of vitamin B6 (pyridoxine) produces this coenzyme.

The transamination of PLP with an amino acid involves a reaction with its aldehyde group (C=O-H) (with a slight reduction of H2O). This is the process of forming the Schiff base (R-NH2). When the hydrogen atom migrates from the amino acid, a double bond is shifted and thus the former aldimine (H-C=O) is converted into ketamine (R-C=O). Then, the alpha-keto acid is formed by adding water across this double bond. Pyridoxamine phosphate is formed when PLP is reduced. Likewise, the reverse reaction occurs when the pyridoxamine phosphate is reacted with an alpha-keto acid to produce another amino acid.

Pyridoxal Phosphate

The coenzyme pyridoxal phosphate, abbreviated PLP, plays an essential role in amino acid metabolism. A pyridoxal is an aldehyde form of Vitamin B6 and PLP is the biologically active form. Vitamin B6 also occurs as alcohol (pyridoxine) and amine (pyridoxamine). The B6 derivatives can be combined into one another. Pyridoxal and pyridoxine are ingested and are found in animals and vegetables.

Vitamin B6 is relatively abundant in wheat germs. Transamination, decarboxylation (which forms biogenous amines), and deamination of amino acids involve the coenzyme PLP. The alanine transaminase (ALT) attaches to a lysine residue in a PLP reaction, creating a Schiff base. PLP contains nitrogen in the pyrimidine ring, which has an electrophilic effect and results in the shifting of bonds. As a result of this reaction, toxic ammonia is formed; its concentration must be kept to a minimum at all times, and nitrogen should not be disposed of. The urea cycle is thus initiated and leads to amino acids being deaminated.

Deamination of Amino Acids

Amino acids contain excess nitrogen that needs to be excreted by the body. A process known as deamination is responsible for this breakdown of amino acids. Nevertheless, this process produces ammonia that must be quickly converted to urea by metabolism. Urea synthesis, which requires a great deal of energy, is carried out by the liver. The excess nitrogen must be transported from the peripheral tissues to the liver. Aspartate, glutamine, and alanine (synthesized from pyruvate) are the three amino acids involved in this transport system.

Deamination can be classified into three types:
  • Hydrolytic deamination
  • Oxidative deamination
  • Eliminative deamination

Oxidative Deamination

By oxidizing an amino group to form an iminium group (C=N), Schiff bases are produced similar to transamination with a dehydrogenase. In this reaction, electron acceptors are the coenzymes NADH/H or NADPH/H which are reduced to NADH/H or NADP+ respectively. When ammoniac (NH3) is added to an Imina group, it transforms into an alpha-keto group.

Hydrolytic Deamination

The amino group reacts with water in hydrolytic deamination. This results in an irreversible attachment between the hydroxyl (OH) group and the amino group. The glutaminase enzyme is responsible for converting glutamine into glutamate. It is convenient that this enzyme is also known as asparaginase, since there are no differences between these two amino acids.

Eliminative Deamination

Additionally, a small amino acid, such as serine or cysteine, can be liberated by the deamination of its nitrogen (in the form of ammonia) and by removal of water, or hydrogen sulfide in the case of sulfurous amino acids. Once again, PLP has to be involved in this reaction. A double bond is formed upon hydration, followed by hydrolysis to make another alpha-keto acid.

Decarboxylation of Amino Acids

One amine and CO2 are formed as side products when an amino acid carboxyl group is cleaved. A cofactor known as PLP is used by enzyme decarboxylase to catalyze the reaction. Moreover, these biogenic amines play important roles in the human body, which is why they are known as biogenous amines. Histamine is an example of a constituent that is formed by decarboxylation of the basic amino acid histidine. Accordingly, the enzyme responsible for that process is histidine decarboxylase. The histamine hormone plays a vital role in hypersensitivity reactions, e.g., the reaction that occurs immediately after exposure to an allergen. Aside from GABA (gamma-aminobutyric acid, derived from glutamine acid), other biogenous amines are of relevance for metabolism, such as dopamine (derived from 3,4-dihydroxyphenylalanine).
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