The conversion of an imine back to an aldehyde or ketone is a hydrolysis, and mechanistically is simply the reverse of imine formation:. This is closely analogous to hemiacetal and hemiketal formation.
Draw a mechanism for the Schiff base transfer reaction shown above. The DHAP substrate binds to the enzyme first, and forms a Schiff base with a specific active site lysine residue: The conversion of an imine back to an aldehyde or ketone is a hydrolysis, and mechanistically is simply the reverse of imine formation: Notice that THF in this reaction serves as an acceptor of a single carbon group - in this case, formaldehyde.
Exercise 11. In many PLP-dependent enzymatic reactions, one of the first things that happens is that PLP forms a Schiff base link with a lysine residue on the enzyme. More than one mechanism has been proposed for this reaction, but one likely pathway involves free formaldehyde as an intermediate.
The electrophilic carbon atoms of aldehydes and ketones can be targets of nucleophilic attack by amines. Notice that PLP has an aldehyde group. The mechanism for hydrazone formation is analogous to that of imine formation.
In the serine to glycine direction, the formaldehyde intermediate is incorporated into tetrahydrofolate through the formation of what could be termed a 'cyclic nitrogen acetal' in the resulting 5,10-methylene-tetrahydrofolate. Most enzymes that interact with PLP catalyze reactions involved in the metabolism of amino acids.
Imines - the general picture The electrophilic carbon atoms of aldehydes and ketones can be targets of nucleophilic attack by amines. This type of compound is known as an imine , or Schiff base. First, the amine nitrogen acts as a nucleophile, attacking the carbonyl carbon.
Hydrazones are close relatives to imines, but are not abundantly in biological molecules. For example, when plants convert carbon in the form of CO 2 into carbohydrate, one of the early reactions that takes place is the condensation of the four-carbon sugar erythrose-4-phosphate E4P with dihydroxyacetone phosphate DHAP to form the seven-carbon sugar sedoheptulose-1,7-bisphosphate:.
Mechanistically, the formation of an imine involves two steps. We will study the reactivity of PLP in more depth in section 14. Guanafuracin, a known antibiotic compound, is a hydrazone, and can be prepared easily by combining equimolar amounts of the appropriate aldehyde and hydrazine: