Deacetoxycephalosporin C synthase isozymes exhibit diverse catalytic activity and substrate specificity

Substrates and Loaded Iron Ions Relative Position Influence the Catalytic Characteristics of the Metalloenzymes Angelica archangelica Flavone Synthase I and Camellia sinensis Flavonol Synthase

Metalloenzymes are a class of enzymes that catalyze through the metal ions they load. Angelica archangelica flavone synthase I (AnFNS I) and Camellia sinensis flavonol synthase (CaFLS), both of which belong to metalloenzymes, have highly similar structures and metal catalytic cores. However, these two enzymes catalyze the same substrate to produce significantly different products. To identify the cause for the differences in the catalytic characteristics of AnFNS I and CaFLS, their protein models were constructed using homology modeling. Structural alignment and molecular docking was also used to elucidate the molecular basis of the differences observed. To analyze and verify the cause for the differences in the catalytic characteristics of AnFNS I and CaFLS, partial fragments of AnFNS I were used to replace the corresponding fragments on CaFLS, and the catalytic characteristics of the mutants were determined by bioconversion assay in E. coli and in vitro catalytic test. The results suggest that the difference in catalytic characteristics between AnFNS I and CaFLS is caused by the depth of the active pockets and the relative position of the substrate. Mutant 10 which present similar dock result with AnFNS I increased the proportion of diosmetin (a flavone) from 2.54 to 16.68% and decreased the proportion of 4′-O-methyl taxifolin (a flavanol) from 47.28 to 2.88%. It was also indicated that the atoms in the substrate molecule that determine the catalytic outcome may be H-2 and H-3, rather than C-2 and C-3. Moreover, it is speculated that the change in the catalytic characteristics at the changes relative spatial position of H-2/H-3 of hesperetin and the loaded carbonyl iron, caused by charged residues at the entrance of the active pocket, is the key factor for the biosynthesis of flavone from flavanone.

Deacetoxycephalosporin C synthase (expandase): Research progress and application potential

Cephalosporins play an indispensable role against bacterial infections. Deacetyloxycephalosporin C synthase (DAOCS), also called expandase, is a key enzyme in cephalosporin biosynthesis that epoxides penicillin to form the hexavalent thiazide ring of cephalosporin. DAOCS in fungus Acremonium chrysogenum was identified as a bifunctional enzyme with both ring expansion and hydroxylation, whereas two separate enzymes in bacteria catalyze these two reactions. In this review, we briefly summarize its source and function, improvement of the conversion rate of penicillin to deacetyloxycephalosporin C through enzyme modification, crystallography features, the prediction of the active site, and application perspective.

Evolution of flavone synthase I from parsley flavanone 3beta-hydroxylase by site-directed mutagenesis

Flavanone 3beta-hydroxylase (FHT) and flavone synthase I (FNS I) are 2-oxoglutarate-dependent dioxygenases with 80% sequence identity, which catalyze distinct reactions in flavonoid biosynthesis. However, FNS I has been reported exclusively from a few Apiaceae species, whereas FHTs are more abundant. Domain-swapping experiments joining the N terminus of parsley (Petroselinum crispum) FHT with the C terminus of parsley FNS I and vice versa revealed that the C-terminal portion is not essential for FNS I activity. Sequence alignments identified 26 amino acid substitutions conserved in FHT versus FNS I genes. Homology modeling, based on the related anthocyanidin synthase structure, assigned seven of these amino acids (FHT/FNS I, M106T, I115T, V116I, I131F, D195E, V200I, L215V, and K216R) to the active site. Accordingly, FHT was modified by site-directed mutagenesis, creating mutants encoding from one to seven substitutions, which were expressed in yeast (Saccharomyces cerevisiae) for FNS I and FHT assays. The exchange I131F in combination with either M106T and D195E or L215V and K216R replacements was sufficient to confer some FNS I side activity. Introduction of all seven FNS I substitutions into the FHT sequence, however, caused a nearly complete change in enzyme activity from FHT to FNS I. Both FHT and FNS I were proposed to initially withdraw the beta-face-configured hydrogen from carbon-3 of the naringenin substrate. Our results suggest that the 7-fold substitution affects the orientation of the substrate in the active-site pocket such that this is followed by syn-elimination of hydrogen from carbon-2 (FNS I reaction) rather than the rebound hydroxylation of carbon-3 (FHT reaction).

A complete library of amino acid alterations at N304 in Streptomyces clavuligerus deacetoxycephalosporin C synthase elucidates the basis for enhanced penicillin analogue conversion

N304 of Streptomyces clavuligerus deacetoxycephalosporin C synthase was mutagenized to alter its catalytic ability. Given that N304A, N304K, N304L, and N304R mutant enzymes exhibited significant improvements in penicillin analogue conversions, we advocate that replacement of N304 with residues with aliphatic or basic side chains is preferable for engineering of a hypercatalytic enzyme.

Controlling the Substrate Selectivity of Deacetoxycephalosporin/deacetylcephalosporin C Synthase

Deacetoxycephalosporin/deacetylcephalosporin C synthase (DAOC/DACS) is an iron(II) and 2-oxoglutarate-dependent oxygenase involved in the biosynthesis of cephalosporin C in Cephalosporium acremonium. It catalyzes two oxidative reactions, oxidative ring-expansion of penicillin N to deacetoxycephalosporin C, and hydroxylation of the latter to give deacetylcephalosporin C. The enzyme is closely related to deacetoxycephalosporin C synthase (DAOCS) and DACS from Streptomyces clavuligerus, which selectively catalyze ring-expansion or hydroxylation reactions, respectively. In this study, structural models based on DAOCS coupled with site-directed mutagenesis were used to identify residues within DAOC/DACS that are responsible for controlling substrate and reaction selectivity. The M306I mutation abolished hydroxylation of deacetylcephalosporin C, whereas the W82A mutant reduced ring-expansion of penicillin G (an "unnatural" substrate). Truncation of the C terminus of DAOC/DACS to residue 310 (Delta310 mutant) enhanced ring-expansion of penicillin G by approximately 2-fold. A double mutant, Delta310/M306I, selectively catalyzed the ring-expansion reaction and had similar kinetic parameters to the wild-type DAOC/DACS. The Delta310/N305L/M306I triple mutant selectively catalyzed ring-expansion of penicillin G and had improved kinetic parameters (K(m) = 2.00 +/- 0.47 compared with 6.02 +/- 0.97 mm for the wild-type enzyme). This work demonstrates that a single amino acid residue side chain within the DAOC/DACS active site can control whether the enzyme catalyzes ring-expansion, hydroxylation, or both reactions. The catalytic efficiency of mutant enzymes can be improved by combining active site mutations with other modifications including C-terminal truncation and modification of Asn-305.

Significance of C-terminal sequence elements forPetuniaflavanone 3β-hydroxylase activity

Flavanone 3beta-hydroxylase (FHT), a 2-oxoglutarate-dependent dioxygenase (2-ODD), catalyzes the hydroxylation of (2S)-flavanones to (2R/3R)-dihydroflavonols in plants as a key step towards the biosynthesis of flavonols, anthocyanins and catechins. Crystallographic studies of 2-ODDs typically revealed a jelly roll in the enzyme core, and the C-terminus of the enzyme polypeptides was proposed to form a lid covering the active site cavity, thereby reducing the chances for oxidative or proteolytic damage and unfolding. Moreover, it has been proposed that in some cases the C-terminus is involved in substrate selectivity of 2-ODDs. In a systematic approach with highly active Petunia FHT, four C-terminally truncated enzyme forms were generated by deletion of five, 11, 24 or 29 amino acids. The recombinant FHTs preserved their substrate selectivity, but the specific activity decreased gradually with the extent of truncation. Then, an enzyme chimera was constructed by domain swapping replacing the C-terminal 52 amino acids of Petunia FHT by the equivalent region of flavonol synthase (FLS) from Citrus unshiu, an enzyme showing ambiguous FLS and FHT activity. The chimeric dioxygenase still revealed exclusively FHT activity, albeit at a moderate level only. The data predict that the selectivity of FHT is not governed by the C-terminal sequence accounting for about 13% of the enzyme polypeptide.