Keto Acid Decarboxylations Not Involving a Metal Ion

The enzymes described above that convert oxaloacetate to pyruvate and CO2 appear to use metal chelation to stabilize the enolate formed by decarboxylation. Many other /3-ketoacid decarboxylases use a similar mechanism. However, there are a few decarboxylations of /3-keto acids or their functional equivalents in which no metal ion is involved. One is the case of acetoacetate decarboxylase, which functions by means of a Schiif base mechanism. A few additional examples are described below. All these cases involve particularly stable enolates. 

A variety of enzymes catalyze the oxidative decarboxylation of jS-hydroxy acids. Isotope effect studies of malic enzyme (29), isocitrate dehydrogenase (63), and 6-phosphogluconate dehydrogenase (64) indicate that all three of these oxidative decarboxylations occur by stepwise mechanisms in which hydride transfer occurs first, forming a j3-keto acid that then undergoes decarboxylation. Hydride transfer and decarboxylation are both partially rate determining. 

From the point of view of catalytic strategy, all three of these facts are probably connected. The product of the reaction is a phenol, rather than an enolate, and metal ion stabilization of the product is apparently not needed. The driving force associated with formation of an aromatic compound is evidently sufficient that decarboxylation can be concerted with hydride transfer. The carbon isotope effect on this reaction is surprisingly small, perhaps because the transition state is quite early (65). The isotope effects also indicate that substrate binding is associated with a conformation change, which may seat the substrate in a reactive conformation in the active site. 

The oxidative decarboxylation of 6-phosphogluconic acid occurs in a stepwise mechanism involving a )8-keto acid intermediate 64). However, it appears that no metal ion is required in the reaction (66). 2-Deoxy-6-phosphogluconate is also a substrate for this enzyme, but the reaction is about two orders of magnitude slower than reaction of the natural substrate. The keto acid produced in the initial oxidation is released into solution and subsequently undergoes slow enzyme-catalyzed decarboxylation (67). 

This enzyme also catalyzes a decarboxylation via a j8-keto acid intermediate, and it appears that no metal is involved (68, 69). 

---