Title: Expression and Activity of the BioH Esterase of Biotin Synthesis is Independent of Genome Context Cao X, Zhu L, Hu Z, Cronan JE Ref: Sci Rep, 7:2141, 2017 : PubMed
BioH is an alpha/beta-hydrolase required for synthesis of the pimelate moiety of biotin in diverse bacteria. The bioH gene is found in different genomic contexts. In some cases (e.g., Escherichia coli) the gene is not located within a biotin synthetic operon and its transcription is not coregulated with the other biotin synthesis genes. In other genomes such as Pseudomonas aeruginosa the bioH gene is within a biotin synthesis operon and its transcription is coregulated with the other biotin operon genes. The esterases of pimelate moiety synthesis show remarkable genomic plasticity in that in some biotin operons bioH is replaced by other alpha/ss hydrolases of diverse sequence. The "wild card" nature of these enzymes led us to compare the paradigm "freestanding" E. coli BioH with the operon-encoded P. aeruginosa BioH. We hypothesized that the operon-encoded BioH might differ in its expression level and/or activity from the freestanding BioH gene. We report this is not the case. The two BioH proteins show remarkably similar hydrolase activities and substrate specificity. Moreover, Pseudomonas aeruginosa BioH is more highly expressed than E. coli BioH. Despite the enzymatic similarities of the two BioH proteins, bioinformatics analysis places the freestanding and operon-encoded BioH proteins into distinct clades.
Title: A Biotin Biosynthesis Gene Restricted to Helicobacter Bi H, Zhu L, Jia J, Cronan JE Ref: Sci Rep, 6:21162, 2016 : PubMed
In most bacteria the last step in synthesis of the pimelate moiety of biotin is cleavage of the ester bond of pimeloyl-acyl carrier protein (ACP) methyl ester. The paradigm cleavage enzyme is Escherichia coli BioH which together with the BioC methyltransferase allows synthesis of the pimelate moiety by a modified fatty acid biosynthetic pathway. Analyses of the extant bacterial genomes showed that bioH is absent from many bioC-containing bacteria and is replaced by other genes. Helicobacter pylori lacks a gene encoding a homologue of the known pimeloyl-ACP methyl ester cleavage enzymes suggesting that it encodes a novel enzyme that cleaves this intermediate. We isolated the H. pylori gene encoding this enzyme, bioV, by complementation of an E. coli bioH deletion strain. Purified BioV cleaved the physiological substrate, pimeloyl-ACP methyl ester to pimeloyl-ACP by use of a catalytic triad, each member of which was essential for activity. The role of BioV in biotin biosynthesis was demonstrated using a reconstituted in vitro desthiobiotin synthesis system. BioV homologues seem the sole pimeloyl-ACP methyl ester esterase present in the Helicobacter species and their occurrence only in H. pylori and close relatives provide a target for development of drugs to specifically treat Helicobacter infections.
Title: An Atypical alpha/beta-Hydrolase Fold Revealed in the Crystal Structure of Pimeloyl-Acyl Carrier Protein Methyl Esterase BioG from Haemophilus influenzae Shi J, Cao X, Chen Y, Cronan JE, Guo Z Ref: Biochemistry, 55:6705, 2016 : PubMed
Pimeloyl-acyl carrier protein (ACP) methyl esterase is an alpha/beta-hydrolase that catalyzes the last biosynthetic step of pimeloyl-ACP, a key intermediate in biotin biosynthesis. Intriguingly, multiple nonhomologous isofunctional forms of this enzyme that lack significant sequence identity are present in diverse bacteria. One such esterase, Escherichia coli BioH, has been shown to be a typical alpha/beta-hydrolase fold enzyme. To gain further insights into the role of this step in biotin biosynthesis, we have determined the crystal structure of another widely distributed pimeloyl-ACP methyl esterase, Haemophilus influenzae BioG, at 1.26 A. The BioG structure is similar to the BioH structure and is composed of an alpha-helical lid domain and a core domain that contains a central seven-stranded beta-pleated sheet. However, four of the six alpha-helices that flank both sides of the BioH core beta-sheet are replaced with long loops in BioG, thus forming an unusual alpha/beta-hydrolase fold. This structural variation results in a significantly decreased thermal stability of the enzyme. Nevertheless, the lid domain and the residues at the lid-core interface are well conserved between BioH and BioG, in which an analogous hydrophobic pocket for pimelate binding as well as similar ionic interactions with the ACP moiety are retained. Biochemical characterization of site-directed mutants of the residues hypothesized to interact with the ACP moiety supports a similar substrate interaction mode for the two enzymes. Consequently, these enzymes package the identical catalytic function under a considerably different protein surface.
We recently identified a gene (FTN_0818) required for Francisella virulence that seemed likely involved in biotin metabolism. However, the molecular function of this virulence determinant was unclear. Here we show that this protein named BioJ is the enzyme of the biotin biosynthesis pathway that determines the chain length of the biotin valeryl side-chain. Expression of bioJ allows growth of an Escherichia coli bioH strain on biotin-free medium, indicating functional equivalence of BioJ to the paradigm pimeloyl-ACP methyl ester carboxyl-esterase, BioH. BioJ was purified to homogeneity, shown to be monomeric and capable of hydrolysis of its physiological substrate methyl pimeloyl-ACP to pimeloyl-ACP, the precursor required to begin formation of the fused heterocyclic rings of biotin. Phylogenetic analyses confirmed that distinct from BioH, BioJ represents a novel subclade of the alpha/beta-hydrolase family. Structure-guided mapping combined with site-directed mutagenesis revealed that the BioJ catalytic triad consists of Ser151, Asp248 and His278, all of which are essential for activity and virulence. The biotin synthesis pathway was reconstituted reaction in vitro and the physiological role of BioJ directly assayed. To the best of our knowledge, these data represent further evidence linking biotin synthesis to bacterial virulence.
Title: Structure of the enzyme-acyl carrier protein (ACP) substrate gatekeeper complex required for biotin synthesis Agarwal V, Lin S, Lukk T, Nair SK, Cronan JE Ref: Proc Natl Acad Sci U S A, 109:17406, 2012 : PubMed
Although the pimeloyl moiety was long known to be a biotin precursor, the mechanism of assembly of this C7 alpha,omega-dicarboxylic acid was only recently elucidated. In Escherichia coli, pimelate is made by bypassing the strict specificity of the fatty acid synthetic pathway. BioC methylates the free carboxyl of a malonyl thioester, which replaces the usual acetyl thioester primer. This atypical primer is transformed to pimeloyl-acyl carrier protein (ACP) methyl ester by two cycles of fatty acid synthesis. The question is, what stops this product from undergoing further elongation? Although BioH readily cleaves this product in vitro, the enzyme is nonspecific, which made assignment of its physiological substrate problematical, especially because another enzyme, BioF, could also perform this gatekeeping function. We report the 2.05-A resolution cocrystal structure of a complex of BioH with pimeloyl-ACP methyl ester and use the structure to demonstrate that BioH is the gatekeeper and its physiological substrate is pimeloyl-ACP methyl ester.
Title: Evolution of a new function in an esterase: simple amino acid substitutions enable the activity present in the larger paralog, BioH Flores H, Lin S, Contreras-Ferrat G, Cronan JE, Morett E Ref: Protein Engineering Des Sel, 25:387, 2012 : PubMed
Gene duplication and divergence are essential processes for the evolution of new activities. Divergence may be gradual involving simple amino acid residue substitutions or drastic such that larger structural elements are inserted deleted or rearranged. Vast protein sequence comparisons supported by some experimental evidence argue that large structural modifications have been necessary for certain catalytic activities to evolve. However it is not clear whether these activities could not have been attained by gradual changes. Interestingly catalytic promiscuity could play a fundamental evolutionary role a preexistent secondary activity could be increased by simple amino acid residue substitutions that do not affect the enzyme's primary activity. The promiscuous profile of the enzyme may be modified gradually by genetic drift making a pool of potentially useful activities that can be selected before duplication In this work we used random mutagenesis and in vivo selection to evolve the Pseudomonas aeruginosa PAO1 carboxylesterase PA3859 a small protein to attain the function of BioH a much larger paralog involved in biotin biosynthesis. BioH was chosen as a target activity because it provides a highly sensitive selection for evolved enzymatic activities by auxotrophy complementation. After only two cycles of directed evolution mutants with the ability to efficiently complement biotin auxotrophy were selected. The in vivo and in vitro characterization showed that the activity of one of our mutant proteins was similar to that of the wild-type BioH enzyme. Our results demonstrate that it is possible to evolve enzymatic activities present in larger proteins by discrete amino acid substitutions.
Title: Remarkable diversity in the enzymes catalyzing the last step in synthesis of the pimelate moiety of biotin Shapiro MM, Chakravartty V, Cronan JE Ref: PLoS ONE, 7:e49440, 2012 : PubMed
Biotin synthesis in Escherichia coli requires the functions of the bioH and bioC genes to synthesize the precursor pimelate moiety by use of a modified fatty acid biosynthesis pathway. However, it was previously noted that bioH has been replaced with bioG or bioK within the biotin synthetic gene clusters of other bacteria. We report that each of four BioG proteins from diverse bacteria and two cyanobacterial BioK proteins functionally replace E. coli BioH in vivo. Moreover, purified BioG proteins have esterase activity against pimeloyl-ACP methyl ester, the physiological substrate of BioH. Two of the BioG proteins block biotin synthesis when highly expressed and these toxic proteins were shown to have more promiscuous substrate specificities than the non-toxic BioG proteins. A postulated BioG-BioC fusion protein was shown to functionally replace both the BioH and BioC functions of E. coli. Although the BioH, BioG and BioK esterases catalyze a common reaction, the proteins are evolutionarily distinct.
Title: Biotin synthesis begins by hijacking the fatty acid synthetic pathway. Lin S, Hanson RE, Cronan JE Ref: Nat Chemical Biology, 6:682, 2010 : PubMed
Although biotin is an essential enzyme cofactor found in all three domains of life, our knowledge of its biosynthesis remains fragmentary. Most of the carbon atoms of biotin are derived from pimelic acid, a seven-carbon dicarboxylic acid, but the mechanism whereby this intermediate is assembled remains unknown. Genetic analysis in Escherichia coli identified only two genes of unknown function required for pimelate synthesis, bioC and bioH. We report in vivo and in vitro evidence that the pimeloyl moiety is synthesized by a modified fatty acid synthetic pathway in which the omega-carboxyl group of a malonyl-thioester is methylated by BioC, which allows recognition of this atypical substrate by the fatty acid synthetic enzymes. The malonyl-thioester methyl ester enters fatty acid synthesis as the primer and undergoes two reiterations of the fatty acid elongation cycle to give pimeloyl-acyl carrier protein (ACP) methyl ester, which is hydrolyzed to pimeloyl-ACP and methanol by BioH.
