Aldehydes pyridoxal

Except for some vitamin B12-dependent reactions, the cleavage or formation of carbon-carbon bonds usually depends upon the participation of carbonyl groups. For this reason, carbonyl groups have a central mechanistic role in biosynthesis. The activation of hydrogen atoms (3 to carbonyl groups permits (3 condensations to occur during biosynthesis. Aldol or Claisen condensations require the participation of two carbonyl compounds. Carbonyl compounds are also essential to thiamin diphosphate-dependent condensations and the aldehyde pyridoxal phosphate is needed for most C-C bond cleavage or formation within amino acids. 

The generic descriptor vitamin Be includes six vitamers the alcohol pyridox-ine, the aldehyde pyridoxal, the amine pyridoxamine, and their 5 -phosphates. There is some confusion in the older literature, because at one timepyridoxine was used as a generic descriptor, with pyridoxol as the specific name for the alcohol. As shown in Figure 9.1, the vitamers are metabolicaUy interconvertible and, as far as is known, they have equal biological activity. 

Vitamin B or pyridoxine participates in over 100 enzymatic reactions as the cofactor, pyridoxal phosphate (PLP). It exists in three forms the alcohol, the amine, or the aldehyde. Pyridoxal phosphate is an essential cofactor for enzymes involved in the synthesis of many neurotransmitters. 

Imines are particularly common as intermediates in many biological pathways, where they arc often called Schiff bases. The amino acid alanine, for instance, is metabolized in the body by reaction with the aldehyde pyridoxal phosphate (PLP), a derivative of vitamin Bg, to yield a Schiff base that is further degraded. 

Interconversion between chair and boat forms of cyclohexane derivatives may have x 10 s at room temperature, while rotation about a C-N bond in an amide linkage may be very slow with x 0.1 s. The nonenzymatic hydration of the aldehyde pyridoxal phosphate via Eq. 13-1 occurs with x = 0.01 - 0.1 s, depending upon the pH. ... 

The major sales form of vitamin B6 is the hydrochlorid salt of the primary alcohol pyridoxine. Another vitamin B6 form introduced in the market is the dihydrochlo-rid salt of pyridoxamine. Both vitamin B6 forms are commercially produced via various straightforward chemical synthesis routes. The biologically active cofactor is the aldehyde pyridoxal-5 -phosphate, which is derived in human or animals from the vitamin B6 forms by oxidation or transamination before or after 5 phosphorylation by pyridoxal kinases. 

At the end of the reaction a keto acid has been converted to an amino acid, and the coenzyme pyridoxamine has been converted to the aldehyde pyridoxal. In enzymatic processes the reaction would then be run in the reverse direction with a different amino acid to regenerate the starting materials, and we found that that was also possible here. Thus this system is truly catalytic, converting a keto acid and a different amino acid into the new amino acid and new keto acid by running the process back and forth. The chiral synthesis of amino acids by this process, so closely related to that in an enzyme, may prove to be of practical interest. 

Vitamin B is the heterocyclic aldehyde pyridoxal (3) and is an essential cofactor to a large number of enzymes which catalyze many diverse reactions... 

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