Crystal structure of a thiolase from Escherichia coli at 1.8 Å resolution

Optimum chalcone synthase for flavonoid biosynthesis in microorganisms

Chalcones and the subsequently generated flavonoids, as well as flavonoid derivatives, have been proven to have a variety of physiological activities and are widely used in: the pharmaceutical, food, feed, and cosmetic industries. As the content of chalcones and downstream products in native plants is low, the production of these compounds by microorganisms has gained the attention of many researchers and has a history of more than 20 years. The mining and engineering of chalcone synthase (CHS) could be one of the most important ways to achieve more efficient production of chalcones and downstream products in microorganisms. CHS has a broad spectrum of substrates, and its enzyme activity and expression level can significantly affect the efficiency of the biosynthesis of flavonoids. This review summarizes the recent advances in the: structure, mechanism, evolution, substrate spectrum, transformation, and expression regulation in the flavonoid biosynthesis of this vital enzyme. Future development directions were also suggested. The findings may further promote the research and development of flavonoids and health products, making them vital in the fields of human diet and health.

The 3-ketoacyl-CoA thiolase: an engineered enzyme for carbon chain elongation of chemical compounds

Because of their function of catalyzing the rearrangement of the carbon chains, thiolases have attracted increasing attentions over the past decades. The 3-ketoacyl-CoA thiolase (KAT) is a member of the thiolase, which is capable of catalyzing the Claisen condensation reaction between the two acyl-CoAs, thereby achieving carbon chain elongation. In this way, diverse value-added compounds might be synthesized starting from simple small CoA thioesters. However, most KATs are hampered by low stability and poor substrate specificity, which has hindered the development of large-scale biosynthesis. In this review, the common characteristics in the three-dimensional structure of KATs from different sources are summarized. Moreover, structure-guided rational engineering is discussed as a strategy for enhancing the performance of KATs. Finally, we reviewed the metabolic engineering applications of KATs for producing various energy-storage molecules, such as n-butanol, fatty acids, dicarboxylic acids, and polyhydroxyalkanoates. KEY POINTS: • Summarize the structural characteristics and catalyzation mechanisms of KATs. • Review on the rational engineering to enhance the performance of KATs. • Discuss the applications of KATs for producing energy-storage molecules.

Aspergillus fumigatus Mitochondrial Acetyl Coenzyme A Acetyltransferase as an Antifungal Target

Ergosterol plays an important role in maintaining cell membrane sterol homeostasis in fungi, and as such, it is considered an effective target in antifungal chemotherapy. In yeast, the enzyme acetyl-coenzyme A (CoA) acetyltransferase (ERG10) catalyzes the Claisen condensation of two acetyl-CoA molecules to acetoacetyl-CoA in the ergosterol biosynthesis pathway and is reported as being critical for cell viability. Using yeast ERG10 for alignment, two orthologues, AfERG10A (AFUB_000550) and AfERG10B (AFUB_083570), were discovered in the opportunistic fungal pathogen Aspergillus fumigatus Despite the essentiality of AfERG10B having been previously validated, the biological function of AfERG10A remains unclear. In this study, we have characterized recombinant AfERG10A as a functional acetyl-CoA acetyltransferase catalyzing both synthetic and degradative reactions. Unexpectedly, AfERG10A localizes to the mitochondria in A. fumigatus, as shown by C-terminal green fluorescent protein (GFP) tag fusion. Both knockout and inducible promoter strategies demonstrate that Aferg10A is essential for the survival of A. fumigatus The reduced expression of Aferg10A leads to severe morphological defects and increased susceptibility to oxidative and cell wall stresses. Although the catalytic mechanism of acetyl-CoA acetyltransferase family is highly conserved, the crystal structure of AfERG10A and its complex with CoA are solved, revealing four substitutions within the CoA binding site that are different from human orthologues. Taken together, our combination of genetic and structural studies demonstrates that mitochondrial AfERG10A is essential for A. fumigatus cell viability and could be a potential drug target to feed the antifungal drug development pipeline.IMPORTANCE A growing number of people worldwide are suffering from invasive aspergillosis caused by the human opportunistic fungal pathogen A. fumigatus Current therapeutic options rely on a limited repertoire of antifungals. Ergosterol is an essential component of the fungal cell membrane as well as a target of current antifungals. Approximately 20 enzymes are involved in ergosterol biosynthesis, of which acetyl-CoA acetyltransferase (ACAT) is the first enzyme. Two ACATs in A. fumigatus are AfErg10A and AfErg10B. However, the biological function of AfErg10A is yet to be investigated. In this study, we showed that AfErg10A is localized in the mitochondria and is essential for A. fumigatus survival and morphological development. In combination with structural studies, we validated AfErg10A as a potential drug target that will facilitate the development of novel antifungals and improve the efficiency of existing drugs.

Structural basis for differentiation between two classes of thiolase: Degradative vs biosynthetic thiolase

Thiolases are a well characterized family of enzymes with two distinct categories: degradative, β-ketoadipyl-CoA thiolases and biosynthetic, acetoacetyl-CoA thiolases. Both classes share an identical catalytic triad but catalyze reactions in opposite directions. Moreover, it is established that in contrast to the biosynthetic thiolases the degradative thiolases can accept substrates with broad chain lengths. Hitherto, no residue or structural pattern has been recognized that might help to discern the two thiolases, here we exploit, a tetrameric degradative thiolase from Pseudomonas putida KT2440 annotated as PcaF, as a model system to understand features which distinguishes the two classes using structural studies and bioinformatics analyses. Degradative thiolases have different active site architecture when compared to biosynthetic thiolases, demonstrating the dissimilar chemical nature of the active site architecture. Both thiolases deploy different "anchoring residues" to tether the large Coenzyme A (CoA) or CoA derivatives. Interestingly, the H356 of the catalytic triad in PcaF is directly involved in tethering the CoA/CoA derivatives into the active site and we were able to trap a gridlocked thiolase structure of the H356A mutant, where the CoA was found to be covalently linked to the catalytic cysteine residue, inhibiting the overall reaction. Further, X-ray structures with two long chain CoA derivatives, hexanal-CoA and octanal-CoA helped in delineating the long tunnel of 235 Å² surface area in PcaF and led to identification of a unique covering loop exclusive to degradative thiolases that plays an active role in determining the tunnel length and the nature of the binding substrate.

Crystal structure of mutant carboxypeptidase T from Thermoactinomyces vulgaris with an implanted S1' subsite from pancreatic carboxypeptidase B

A site-directed mutagenesis method has been used to obtain the G215S/A251G/T257A/D260G/T262D mutant of carboxypeptidase T from Thermoactinomyces vulgaris (CPT), in which the amino-acid residues of the S1' subsite are substituted by the corresponding residues from pancreatic carboxypeptidase B (CPB). It was shown that the mutant enzyme retained the broad, mainly hydrophobic selectivity of wild-type CPT. The mutant containing the implanted CPB S1' subsite was crystallized and its three-dimensional structure was determined at 1.29 Å resolution by X-ray crystallography. A comparison of the three-dimensional structures of CPT, the G215S/A251G/T257A/D260G/T262D CPT mutant and CPB showed that the S1' subsite of CPT has not been distorted by the mutagenesis and adequately reproduces the structure of the CPB S1' subsite. The CPB-like mutant differs from CPB in substrate selectivity owing to differences between the two enzymes outside the S1' subsite. Moreover, the difference in substrate specificity between the enzymes was shown to be affected by residues other than those that directly contact the substrate.