Crystal Structure and Molecular Mechanism of Phosphotransbutyrylase from Clostridium acetobutylicum

Acetone-butanol-ethanol (ABE) fermentation by the anaerobic bacterium Clostridium acetobutylicum has been considered a promising process of industrial biofuel production. Phosphotransbutyrylase (phosphate butyryltransferase, PTB) plays a crucial role in butyrate metabolism by catalyzing the reversible conversion of butyryl-CoA into butyryl phosphate. Here, we report the crystal structure of PTB from the Clostridial host for ABE fermentation, C. acetobutylicum, (CaPTB) at a 2.9 Å resolution. The overall structure of the CaPTB monomer is quite similar to those of other acyltransferases, with some regional structural differences. The monomeric structure of CaPTB consists of two distinct domains, the N- and C-terminal domains. The active site cleft was formed at the interface between the two domains. Interestingly, the crystal structure of CaPTB contained eight molecules per asymmetric unit, forming an octamer, and the size-exclusion chromatography experiment also suggested that the enzyme exists as an octamer in solution. The structural analysis of CaPTB identifies the substrate binding mode of the enzyme and comparisons with other acyltransferase structures lead us to speculate that the enzyme undergoes a conformational change upon binding of its substrate.

Photoautotrophic synthesis of butyrate by metabolically engineered cyanobacteria

Direct conversion of carbon dioxide into chemicals using engineered autotrophic microorganisms offers a potential solution for both sustainability and carbon mitigation. Butyrate is an important chemical used in various industries, including fragrance, food, and plastics. A model cyanobacterium Synechococcus elongatus PCC 7942 was engineered for the direct photosynthetic conversion of CO ₂ to butyrate. An engineered Clostridium Coenzyme A (CoA)-dependent pathway leading to the synthesis of butyryl-CoA, the precursor to butyrate, was introduced into S. elongatus PCC 7942. Two CoA removal strategies were then individually coupled to the modified CoA-dependent pathway to yield butyrate production. Similar results were observed between the two CoA removal strategies. The best butyrate producing strain of S. elongatus resulted in an observed butyrate titer of 750 mg/L and a cumulative titer of 1.1 g/L. These results demonstrated the feasibility of photosynthetic butyrate production and expanded the chemical repertoire accessible for production by photoautotrophs.

Identification of a Key Gene Involved in Branched-Chain Short Fatty Acids Formation in Natto by Transcriptional Analysis and Enzymatic Characterization in Bacillus subtilis

Natto as a fermented soybean product has many health benefits for human due to its rich nutritional and functional components. However, the unpleasant odor of natto, caused by the formation of branched-chain short fatty acids (BCFAs), prohibits the wide acceptance of natto products. This work is to identify the key gene of BCFAs formation and develop the guidance to reduce natto odor. Transcriptional analysis of BCFAs synthesis pathway genes was conducted in two Bacillus subtilis strains with obvious different BCFAs synthesis abilities. The transcriptional levels of bcd, bkdAA, and ptb in B. subtilis H-9 were 2.7-fold, 0.7-fold, and 8.9-fold higher than that of B. subtilis H-4, respectively. Therefore, the ptb gene with the highest transcriptional change was considered as the key gene in BCFAs synthesis. The ptb encoded enzyme Ptb was further characterized by inducible expression in Escherichia coli. The recombinant Ptb protein (about 32 kDa) was verified by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis analysis. The catalysis functions of Ptb were confirmed on substrates of isovaleryl-CoA and isobutyryl-CoA, and the higher catalysis efficiency of Ptb on isovaleryl-CoA explained the higher level of isovaleric acid in natto. The optimal activities of Ptb were observed at 50 °C and pH 8.0, and the enzymatic activity was inhibited by Ca2+, Zn2+, Ba2+, Mn2+, Cu2+, SDS, and EDTA. Collectively, this study reports a key gene responsible for BCFAs formation in natto fermentation and provides potential strategies to solve the odor problem.