An l-glucitol oxidizing dehydrogenase from Bradyrhizobium japonicum USDA 110 for production of d-sorbose with enzymatic or electrochemical cofactor regeneration

Characterization of the sorbitol dehydrogenase SmoS from Sinorhizobium meliloti 1021

Sinorhizobium meliloti 1021 is a Gram-negative alphaproteobacterium with a robust capacity for carbohydrate metabolism. The enzymes that facilitate these reactions assist in the survival of the bacterium across a range of environmental niches, and they may also be suitable for use in industrial processes. SmoS is a dehydrogenase that catalyzes the oxidation of the commonly occurring sugar alcohols sorbitol and galactitol to fructose and tagatose, respectively, using NAD⁺ as a cofactor. The main objective of this study was to evaluate SmoS using biochemical techniques. The nucleotide sequence was codon-optimized for heterologous expression in Escherichia coli BL21 (DE3) Gold cells and the protein was subsequently overexpressed and purified. Size-exclusion chromatography and X-ray diffraction experiments suggest that SmoS is a tetramer. SmoS was crystallized, and crystals obtained in the absence of substrate diffracted to 2.1 Å resolution and those of a complex with sorbitol diffracted to 2.0 Å resolution. SmoS was characterized kinetically and shown to have a preference for sorbitol despite having a higher affinity for galactitol. Computational ligand-docking experiments suggest that tagatose binds the protein in a more energetically favourable complex than fructose, which is retained in the active site over a longer time frame following oxidation and reduces the rate of the reaction. These results supplement the inventory of biomolecules with potential for industrial applications and enhance the understanding of metabolism in the model organism S. meliloti.

Target highlights from the first post-PSI CASP experiment (CASP12, May-August 2016)

The functional and biological significance of the selected CASP12 targets are described by the authors of the structures. The crystallographers discuss the most interesting structural features of the target proteins and assess whether these features were correctly reproduced in the predictions submitted to the CASP12 experiment.

Structural characterization of the thermostable Bradyrhizobium japonicumD-sorbitol dehydrogenase

Bradyrhizobium japonicum sorbitol dehydrogenase is NADH-dependent and is active at elevated temperatures. The best substrate is D-glucitol (a synonym for D-sorbitol), although L-glucitol is also accepted, giving it particular potential in industrial applications. Crystallization led to a hexagonal crystal form, with crystals diffracting to 2.9 Å resolution. In attempts to phase the data, a molecular-replacement solution based upon PDB entry 4nbu (33% identical in sequence to the target) was found. The solution contained one molecule in the asymmetric unit, but a tetramer similar to that found in other short-chain dehydrogenases, including the search model, could be reconstructed by applying crystallographic symmetry operations. The active site contains electron density consistent with D-glucitol and phosphate, but there was not clear evidence for the binding of NADH. In a search for the features that determine the thermostability of the enzyme, the Tm for the orthologue from Rhodobacter sphaeroides, for which the structure was already known, was also determined, and this enzyme proved to be considerably less thermostable. A continuous β-sheet is formed between two monomers in the tetramer of the B. japonicum enzyme, a feature not generally shared by short-chain dehydrogenases, and which may contribute to thermostability, as may an increased Pro/Gly ratio.

Triacetonide of Glucoheptonic Acid in the Scalable Syntheses of d-Gulose, 6-Deoxy-d-gulose, l-Glucose, 6-Deoxy-l-glucose, and Related Sugars

Ease of separation of petrol-soluble acetonides derived from the triacetonide of methyl glucoheptonate allows scalable syntheses of rare sugars containing the l-gluco or d-gulo structural motif with any oxidation level at the C6 or C1 position of the hexose, usually without chromatography: meso-d-glycero-d-guloheptitol available in two steps is an ideal entry point for the study of the biotechnological production of heptoses.

A thermostable S-adenosylhomocysteine hydrolase from Thermotoga maritima: properties and its application on S-adenosylhomocysteine production with enzymatic cofactor regeneration

S-adenosylhomocysteine (SAH) is an effective sedative, a good sleep modulator, and a new anticonvulsant. SAH can be synthesized from adenosine and homocysteine by using microbial S-adenosylhomocysteine hydrolase (SAHase). The extremely thermostable SAHase and lactate dehydrogenase (LDH) from Thermotoga maritima were successfully overexpressed in Escherichia coli, and purified by heat treatments. The SAHase exhibited the highest activity at 85 °C and pH 8.0 with a specific activity of 6.2 U/mg when NAD concentration was 1mM. However, optimal SAHase reaction conditions shifted to 100 °C and pH 11.2, and its specific activity increased to 36.8 U/mg after NAD concentration was raised to 8mM. Biosynthesis of SAH at 85 °C largely increased the adenosine solubility which was a limiting factor for improving the titer of product. At 85 °C and pH 8.0, 24 μmol of SAH was obtained when 0.5mg of SAHase was applied to a 10 ml reaction mixture. The SAH production was further increased to 153 μmol by adding LDH and pyruvate into the reaction mixture for NAD regeneration. Therefore, extremely thermostable enzymes SAHase and LDH from T. maritima form an efficient NAD consumption and regeneration system for SAH biosynthesis. This method has great potential for industrial-scale enzymatic production of SAH.