About transamination, Vitamin B6 and Pyridoxal Phosphate

Answer to Biochemistry Question AM-03

 

TRANSAMINATION:

 

•         Interchange of the functional groups between an a-keto acid and one amino acid

 

•         ENZYMES THAT CATALYZE THESE REACTIONS:

•         Aminotransferases or transaminases

 

                                                      

 

•         REQUIRED COFACTOR

•         Pyridoxal Phosphate

 

 

Pyridoxal phosphate

 

                                                            

 

Pyridoxal Phosphate is the active form of Vitamin B-6.  This vitamin has three active forms: pyridoxal, pyridoxine (or piridoxol) and  pyridoxamine. (sometimes pyridoxine is used as synonym of Vitamin B6)

 

Besides Transamination, Pyridoxal phosphate participates in several other reactions as cofactor, including:

 

•         Glycogenolysis (cofactor of glycogen phosphorylase but participates with the Phosphate group)

•         Hem synthesis

 

•         METABOLISM OF AMINO ACIDS

                        Decarboxylation

Racemization

                        Non-oxidative deamination

                       

Deficit of Vitamin B6:

 

 

•         Deficit alone is uncommon. It usually appears associated to other nutritional deficits in alcoholics and elderly. Also in TB patients treated with Isoniazid. L-DOPA and Penicillamine also interfere with the metabolism of B6 and can produce deficiency.

•         The deficit is characterized by dermatitis like eruption, neuropathy and frequently anemia.

 

Toxicity of Vitamin B6:

 

Vitamin B6 can also cause neuropathy when taken in excess. It has been established an upper tolerable intake level (UL) of  100 mg/day for adults. 

 

 

Participation of Pyridoxal Phosphate in the Mechanism of Transamination

 

Pyridoxal Phosphate acts as intermediary in the reaction:

 

a)     First, it takes the amino group of the original amino acid (amino acid 1), and gives the oxygen  to the carbon skeleton of the amino acid, yielding an a-ketoacid (a-ketoacid 1). Pyridoxal Phosphate becomes Pyridoxamine Phosphate in the process.

 

b)     In the second part of the reaction, the Pyridoxamine Phosphate gives the amino group to a ketoacid (ketoacid 2), yielding a new amino acid (amino acid 2) while the pirydoxal phosphate is regenerated.

 

             amino acid1 + pyridoxal Phosphate <—-> ketoacid1 + Pyridoxamine Phosphate

 

                 Pyridoxamine Phosphate + ketoacid2 <———-> amino acid 2 + Pyridoxal Phosphate

              ——————————————————————————————————————————–

            amino acid1 + ketoacid2 <———–>  ketoacid1 + amino acid2

 

  

Important couples in Transamination reactions:

 

When the amino acid transaminated is Alanine it yields the ketoacid Pyruvate (and viceversa)

 

When the amino acid transaminated is Aspartate, the reaction yields the ketoacid Oxalacetate (and viceversa)

 

When the amino acid transaminated is Glutamate, the reaction yields the ketoacid a-ketoglutarate

 

 

Importance of Transamination:

 

•         Funneling the a-amino group of amino acids to a-keto glutarate to get glutamate (glutamate plays a central rol in Nitrogen metabolism).

 

•         Synthesis of non essential amino acids

 

•         Interconexion between amino acid metabolism and Krebs Cycle.

 

The following reaction is a very good example of these three former observations:

 

a-amino acid + a-ketoglutarate ó a-ketoacid + glutamate

 

Clinical Importance of Transaminases (Aminotransferases) study:

 

Since amino transferases are intracellular enzymes, abundant in hepatic and cardiac tissues, serum aminotransferases such as serum glutamate-oxaloacetate-aminotransferase (SGOT) (also called aspartate aminotransferase, AST) and serum glutamate-pyruvate aminotransferase (SGPT) (also called alanine transaminase, ALT) classically have been used as clinical markers of these tissue damages, with increasing serum levels indicating an increased extent of damage.  

 

 

More information about these topics can be found in the following links:

 

Dietary Supplement fact Sheet: Vitamin B6

 

Linus Pauling Institute at Oregon State University. Micronutrient Information Center

 

Merck Manual: Vitamin B6

 

Transamination

 

 

Transamination and Deamination

 

Transamination reaction

 

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