Lysine enamines, enolate equivalent

Many other reactions in nature use enamines, mostly those of lysine. However, a more common enol equivalent is based on thiol esters derived from coenzyme A. 

CoA thiol esters are widely used in nature. Mostly they are acetyl CoA, but other thiol esters are also used to make enols. We will see more of this chemistry in the next chapter. The two enol equivalents that wc have met so far are quite general lysine enamines can be used for any aldehyde or ketone and CoA thiol esters for any ester. Another class of enol equivalent—the enol ester—has just one representative but it is a most important one. 

Nature s enolate equivalents lysine enamines and coenzyme A... 

Many other reactions in nature use enamines, mostly those formed from lysine. However, a more common enol equivalent is based on thiol esters derived from coenzyme A. Coenzyme A is an adenine nucleotide at one end, linked by a S -pyrophosphate to pantothenic acid, a compound that looks rather like a tripeptide, and then to an amino thiol. Here is the structure broken down into its parts. 

The reverse aldol reaction is catalyzed by an enzyme called aldolase. One of the roles of the enzyme is to stabilize the enolate anion intermediate because such ions are too basic to be produced under physiological conditions. In animals, aldolase accomplishes this task by forming an inline bond between the carbonyl group of fructose-1,6-bisphosphate and the amino group of a lysine amino acid of the enzyme. As a result, the product of the reverse aldol step is an enamine derived from DHAP rather than its enolate anion. (Section 20.8 shows that enamines are the synthetic equivalents of enolate anions.) The formation of the strongly basic enolate anion is avoided. This process is outlined here ... 

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