Substrate Induced Movement of the Metal Cofactor between Active and Resting State

Regulation of enzyme activity is vital for living organisms. In metalloenzymes, far-reaching rearrangements of the protein scaffold are generally required to tune the metal cofactor's properties by allosteric regulation. Here structural analysis of hydroxyketoacid aldolase from Sphingomonas wittichii RW1 (SwHKA) revealed a dynamic movement of the metal cofactor between two coordination spheres without protein scaffold rearrangements. In its resting state configuration (M²⁺ _R ), the metal constitutes an integral part of the dimer interface within the overall hexameric assembly, but sterical constraints do not allow for substrate binding. Conversely, a second coordination sphere constitutes the catalytically active state (M²⁺ _A ) at 2.4 Å distance. Bidentate coordination of a ketoacid substrate to M²⁺ _A affords the overall lowest energy complex, which drives the transition from M²⁺ _R to M²⁺ _A . While not described earlier, this type of regulation may be widespread and largely overlooked due to low occupancy of some of its states in protein crystal structures.

Assessing the Thiamine Diphosphate Dependent Pyruvate Dehydrogenase E1 Subunit for Carboligation Reactions with Aliphatic Ketoacids

The synthetic properties of the Thiamine diphosphate (ThDP)-dependent pyruvate dehydrogenase E1 subunit from Escherichia coli (EcPDH E1) was assessed for carboligation reactions with aliphatic ketoacids. Due to its role in metabolism, EcPDH E1 was previously characterised with respect to its biochemical properties, but it was never applied for synthetic purposes. Here, we show that EcPDH E1 is a promising biocatalyst for the production of chiral α-hydroxyketones. WT EcPDH E1 shows a 180-250-fold higher catalytic efficiency towards 2-oxobutyrate or pyruvate, respectively, in comparison to engineered transketolase variants from Geobacillus stearothermophilus (TK_GST). Its broad active site cleft allows for the efficient conversion of both (R)- and (S)-configured α-hydroxyaldehydes, next to linear and branched aliphatic aldehydes as acceptor substrates under kinetically controlled conditions. The alternate, thermodynamically controlled self-reaction of aliphatic aldehydes was shown to be limited to low levels of conversion, which we propose to be due to their large hydration constants. Additionally, the thermodynamically controlled approach was demonstrated to suffer from a loss of stereoselectivity, which makes it unfeasible for aliphatic substrates.

Anomeric Selectivity of Trehalose Transferase with Rare l-Sugars

Retaining LeLoir glycosyltransferases catalyze the formation of glycosidic bonds between nucleotide sugar donors and carbohydrate acceptors. The anomeric selectivity of trehalose transferase from Thermoproteus uzoniensis was investigated for both d- and l-glycopyranose acceptors. The enzyme couples a wide range of carbohydrates, yielding trehalose analogues with conversion and enantioselectivity of >98%. The anomeric selectivity inverts from α,α-(1 → 1)-glycosidic bonds for d-glycopyranose acceptors to α,β-(1 → 1)-glycosidic bonds for l-glycopyranose acceptors, while (S)-selectivity was retained for both types of sugar acceptors. Comparison of protein crystal structures of trehalose transferase in complex with α,α-trehalose and an unnatural α,β-trehalose analogue highlighted the mechanistic rationale for the observed inversion of anomeric selectivity.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction

Transketolase catalyzes asymmetric C−C bond formation of two highly polar compounds. Over the last 30 years, the reaction has unanimously been described in literature as irreversible because of the concomitant release of CO₂ if using lithium hydroxypyruvate (LiHPA) as a substrate. Following the reaction over a longer period of time however, we have now found it to be initially kinetically controlled. Contrary to previous suggestions, for the non‐natural conversion of synthetically more interesting apolar substrates, the complete change of active‐site polarity is therefore not necessary. From docking studies it was revealed that water and hydrogen‐bond networks are essential for substrate binding, thus allowing aliphatic aldehydes to be converted in the charged active site of transketolase.