Quinonoid, pyridoxal phosphate reaction

Figure 14-5 Some reactions of Schiff bases of pyridoxal phosphate, (a) Formation of the quinonoid intermediate, (b) elimination of a (3 substituent, and (c) transamination. The quinonoid-carbanionic intermediate can react in four ways (1—4) if enzyme specificity and substrate structure allow.

The enzyme tyrosine phenol lyase can convert tyrosine to phenol and phenol back to tyrosine. Working on tyrosine, it also produces pyruvic acid and ammonia. The tyrosine is attached to pyridoxal phosphate during the reaction, and the phenol goes through a quinonoid form. Write the reaction going either from tyrosine or from phenol. This route may be a way by which insects can de-toxify phenol they themselves have produced. 

A second mode of reaction of the quinonoid-carban-ionic intermediate is utilized by plants which synthesize an enzyme that acts on the amino acid S -adenosylmethionine to form a cyclic three-membered ring compound aminocy-clopropane carboxylic acid. This is a major plant hormone. In a third type of reaction a proton is added back to the coenzyme itself (see Fig. 14) to form what is called a ketimine (not illustrated). This is a Schiff base of pyridox-amine phosphate (PMP, Fig. 5) with an a-oxoacid and is an essential intermediate compound in the important process of transamination (Fig. 14). This process is utilized by all living organisms both in the synthesis of amino acids and in the breakdown of excesses of amino acids. The human body forms several amino acids via transamination. As shown in Fig. 15, this is a reversible sequence involving a cyclic interconversion of PLP and PMP in reaction steps of the type illustrated in Fig. 14. 

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