Crystal structure of a ring-cleaving cyclohexane-1,2-dione hydrolase, a novel member of the thiamine diphosphate enzyme family

Building Bridges: Biocatalytic C-C-Bond Formation toward Multifunctional Products

Carbon-carbon bond formation is the key reaction for organic synthesis to construct the carbon framework of organic molecules. The review gives a selection of biocatalytic C-C-bond-forming reactions which have been investigated during the last 5 years and which have already been proven to be applicable for organic synthesis. In most cases, the reactions lead to products functionalized at the site of C-C-bond formation (e.g., α-hydroxy ketones, aminoalcohols, diols, 1,4-diketones, etc.) or allow to decorate aromatic and heteroaromatic molecules. Furthermore, examples for cyclization of (non)natural precursors leading to saturated carbocycles are given as well as the stereoselective cyclopropanation of olefins affording cyclopropanes. Although many tools are already available, recent research also makes it clear that nature provides an even broader set of enzymes to perform specific C-C coupling reactions. The possibilities are without limit; however, a big library of variants for different types of reactions is required to have the specific enzyme for a desired specific (stereoselective) reaction at hand.

Oxidative carbon-carbon bond cleavage is a key step in spiroacetal biosynthesis in the fruit fly Bactrocera cacuminata

The early steps of spiroacetal biosynthesis in the fruit fly Bactrocera cacuminata (Solanum fly) have been investigated using a series of deuterium-labeled, oxygenated fatty acid like compounds. These potential spiroacetal precursors were administered to male flies, and their volatile emissions were analyzed for specific deuterium incorporation by GC/MS. This has allowed the order of early oxidative events in the biosynthetic pathway to be determined. Together with the already well-established later steps, the results of these in vivo investigations have allowed essentially the complete delineation of the spiroacetal biosynthetic pathway, beginning from products of primary metabolism. A fatty acid equivalent undergoes a series of enzyme-mediated oxidations leading to a trioxygenated fatty acid like species that includes a vicinal diol. This moiety then undergoes enzyme-mediated oxidative carbon-carbon bond cleavage as the key step to generate the C9 unit of the final spiroacetal. This is the first time such an oxidative transformation has been reported in insects. A final hydroxylation step is followed by spontaneous spiro-cyclization. This distinct pathway adds further to the complexity and diversity of biosynthetic pathways to spiroacetals.

Observation of a stable carbene at the active site of a thiamin enzyme

Carbenes are highly reactive chemical compounds that are exploited as ligands in organometallic chemistry and are powerful organic catalysts. They were postulated to occur as transient intermediates in enzymes, yet their existence in a biological system could never be demonstrated directly. We present spectroscopic and structural data of a thiamin enzyme in a noncovalent complex with substrate, which implicate accumulation of a stable carbene as a major resonance contributor to deprotonated thiamin.

Role of a highly conserved proline-126 in ThDP binding of Mycobacterium tuberculosis acetohydroxyacid synthase

Acetohydroxyacid synthase (AHAS) of Mycobacterium tuberculosis is a promising target for the development of anti-tuberculosis agents. With the absence of an available bacterial AHAS crystal structure, that of M. tuberculosis, site-directed mutagenesis has been a useful tool for determining its structural and functional features. In this study, a highly conserved proline residue (P126 of M. tuberculosis AHAS) was selected, and the possible role was evaluated by site-directed mutagenesis. P126 was replaced by valine, threonine, alanine, and glutamate to yield P126V, P126T, P126A, and P126E, respectively. All variants were expressed in their soluble forms in Escherichia coli and purified to near homogeneity. The molecular mass (SDS-PAGE) of the purified variants was ∼68 kDa, which is similar to that of wild-type AHAS. The P126V, P126T, and P126A variants exhibited significantly lower activity than wild-type AHAS, whereas P126E was inactive under the tested assay conditions. Furthermore, the P126V and P126T variants showed a significantly decreased preference toward pyruvate and ThDP as substrate and cofactor respectively, whereas the P126A showed similar kinetics to that of wild-type AHAS. Like in AHAS from yeast Saccharomyces cerevisiae (PDB ID: 1N0H), residue P126 is located in the ThDP binding pocket of M. tuberculosis AHAS homology model. Collectively, these results suggest that the conserved P126 plays a significant role in the ThDP binding of M. tuberculosis AHAS.

CC bond formation using ThDP-dependent lyases

The present review summarizes recent achievements in enzymatic thiamine catalysis during the past three years. With well-established enzymes such as BAL, PDC and TK new reactions have been identified and respective variants were prepared, which enable access to stereoisomeric products. Further we highlight recent progress with 'new' ThDP-dependent enzymes like MenD and PigD, which catalyze the Stetter-like 1,4 addition of aldehydes and YerE, which is the first known ThDP-dependent enzyme accepting ketones as acceptors.