The structure and the mechanism of action of coenzyme B12-dependent diol dehydratases

Recent Progress in the Understanding and Engineering of Coenzyme B12-Dependent Glycerol Dehydratase

Coenzyme B₁₂-dependent glycerol dehydratase (GDHt) catalyzes the dehydration reaction of glycerol in the presence of adenosylcobalamin to yield 3-hydroxypropanal (3-HPA), which can be converted biologically to versatile platform chemicals such as 1,3-propanediol and 3-hydroxypropionic acid. Owing to the increased demand for biofuels, developing biological processes based on glycerol, which is a byproduct of biodiesel production, has attracted considerable attention recently. In this review, we will provide updates on the current understanding of the catalytic mechanism and structure of coenzyme B₁₂-dependent GDHt, and then summarize the results of engineering attempts, with perspectives on future directions in its engineering.

Measurement of crude-cell-extract glycerol dehydratase activity in recombinant Escherichia coli using coupled-enzyme reactions

Glycerol dehydratase (GDHt), which converts glycerol to 3-hydroxypropionaldehyde, is essential to the production of 1,3-propanediol (1,3-PDO) or 3-hydroxypropionic acid (3-HP). A reliable GDHt activity assay in crude-cell extract was developed. In the assay, GDHt converted 1,2-propanediol (1,2-PDO) to propionaldehyde, which was further converted to 1-propionic acid by aldehyde dehydrogenase (KGSADH) or to 1-propanol by yeast-alcohol dehydrogenase (yADH), while the NADH concentration change was monitored spectrophotometrically. Cells should be disintegrated by Bead Beater/French Press, not by chemical methods (BugBuster^®/B-PER™), because the reagents significantly inactivated GDHt and coupling enzymes. Furthermore, in the assay mixture, a much higher activity of KGSADH (>200-fold) or yADH (>400-fold) than that of GDHt should have been maintained. Under optimal conditions, both KGSADH and yADH showed practically the same activity. The coupled-enzyme assay method established here should prove to be applicable to recombinant strains developed for the production of 3-HP and/or 1,3-PDO from glycerol.

A compelling experimental test of the hypothesis that enzymes have evolved to enhance quantum mechanical tunneling in hydrogen transfer reactions: the beta-neopentylcobalamin system combined with prior adocobalamin data

An intriguing but controversial hypothesis has appeared that "The optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of tunneling and thereby accelerate the reaction rate" (Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31, 397). Restated, enzymes may have evolved to enhance quantum mechanical tunneling by coupling to protein low nu modes that squeeze the reacting centers together in, for example, their H(*) atom abstraction reactions. Such a putative "protein squeezing" mechanism would enhance hydrogen quantum mechanical tunneling by reducing the barrier width. An alternative hypothesis is that enzymes do not enhance tunneling, but simply exploit the same amount of tunneling present in their enzyme-free solution reactions, if those reactions occur. A third, conceivable hypothesis is that enzymes might even inadvertently decrease the amount of tunneling as an undesired result of increasing the barrier width while reducing the barrier height. Testing these hypotheses experimentally requires the extremely rare event of being able to measure the amount of tunneling both in the enzyme system and in a very similar if not identical reaction in enzyme-free solution. This has been accomplished experimentally in only one prior case, our recent study of AdoCbl (coenzyme B(12)) and 8-Meo-AdoCbl undergoing enzyme-like H(*) abstraction reactions (Doll, K. M.; Bender, B. R.; Finke, R. G. to J. Am. Chem. Soc. 2003, in press). The data there reveal no change in the level of tunneling within or outside of the enzyme in comparison to the best literature data for an AdoCbl-dependent enzyme, methylmalonyl-CoA mutase. However, that first system suffers from two limitations: the measurement of the KIE (kinetic isotope effect) data in a nonenzymic 80-110 degrees C temperature range; and lower precision data than desired due to the HPLC-MS method required for one of the KIE analyses. These limitations have now been overcome by the synthesis, then thermolysis and KIE study vs temperature of the H(*) abstraction reaction of beta-neopentylcobalamin (beta-NpCbl) in ethylene glycol-d(0) and ethylene glycol-d(4). This is the first experimental test of Klinman's hypothesis using KIE data obtained at enzyme-relevant temperatures. The key data obtained are as follows: deuterium KIEs of 23.1 +/- 3.0 at 40 degrees C to 39.0 +/- 2.3 at 10 degrees C; an activation energy difference E(D) - E(H) of 3.1 +/- 0.3 kcal mol(-)(1); and a pre-exponential factor ratio A(H)/A(D) of 0.14 +/- 0.07. Moreover, our now three sets of data (NpCbl; AdoCbl; 8-MeOAdoCbl) are shown to lie on the same ln KIE vs 1/T linear plot yielding a set of enzyme-temperature-relevant, high-precision KIE, E(D) - E(H), and A(H)/A(D) data over a relatively large, 110 degrees C temperature range. Significantly, the enzyme-free solution KIE, E(D) - E(H), and A(H)/A(D) are identical within experimental error to those for methylmalonyl-CoA mutase. This finding leads to the conclusion that there is no enzymic enhancement of the tunneling in at least this B(12)-dependent enzyme. This B(12) enzyme does, however, exploit the same (unchanged) level of tunneling measured for the nonenzymic, Ado(*) solution H(*) abstraction reaction. A discussion is presented of the still open question of if this first experimental finding, of "no enzymic enhancement of tunneling" in one B(12)-dependent enzymic system, is likely to prove more general or not.

Adenosylcobalamin-dependent isomerases: new insights into structure and mechanism

Adenosylcobalamin-dependent isomerases catalyze a variety of chemically difficult 1,2-rearrangements that proceed through a mechanism involving free radical intermediates. These radicals are initially generated by homolysis of the cobalt-carbon bond of the coenzyme. Recently, the crystal structures of several of these enzymes have been solved, revealing two modes of coenzyme binding and highlighting the role of the protein in controlling the rearrangement of reactive substrate radical intermediates. Complementary data from kinetic, spectroscopic and theoretical studies have produced insights into the mechanism by which substrate radicals are generated at the active site, and the pathways by which they rearrange.