[Show abstract][Hide abstract]ABSTRACT: The formation of an activated cis-3-cyclohexylpropenoic acid by Plm1, the first extension module of the phoslactomycin polyketide synthase, is proposed to occur through an L-3-hydroxyacyl-intermediate as a result of ketoreduction by an A-type ketoreductase (KR). Here, we demonstrate that the KR domain of Plm1 (PlmKR1) catalyzes the formation of an L-3-hydroxyacyl product. The crystal structure of PlmKR1 revealed a well-ordered active site with a nearby Trp residue characteristic of A-type KRs. Structural comparison of PlmKR1 with B-type KRs that produce D-3-hydroxyacyl intermediates revealed significant differences. The active site of cofactor-bound A-type KRs is in a catalysis-ready state, whereas cofactor-bound B-type KRs are in a precatalytic state. Furthermore, the closed lid loop in substrate-bound A-type KRs restricts active site access from all but one direction, which is proposed to control the stereochemistry of ketoreduction.
Full-text Article · Jun 2013 · Chemistry & biology
[Show abstract][Hide abstract]ABSTRACT: The DEBS1-TE fusion protein is comprised of the loading module, the first two extension modules, and the terminal TE domain of the Saccharolyspora erythraea 6-deoxyerythronolide B synthase. DEBS1-TE produces triketide lactones that differ based on the starter unit selected by the loading module. Typical fermentations with plasmid-based expression of DEBS1-TE produce a 6:1 ratio of propionate to isobutyrate-derived triketide lactones. Functional dissection of the loading module from the remainder of DEBS1-TE results in 50% lower titers of triketide lactone and a dramatic shift in the production to a 1:4 ratio of propionate to isobutyrate derived products. A series of radiolabeling studies of the loading module has shown that transfer from the AT to the ACP occurs much faster for propionate than for isobutyrate. However, the equilibrium occupancy of the AT favors isobutyrate such that propionate is outcompeted for ACP occupancy. Thus propionyl-ACP is the kinetic product while isobutyryl-ACP is the thermodynamic product. A slowed transfer from the loading domain ACP to first extension module KS due to functional dissection of DEBS1-TE allows this isobutyryl-ACP favored equilibrium to be realized and likely accounts for the observed shift in triketide lactone products.
[Show abstract][Hide abstract]ABSTRACT: The biosynthesis of GTP derived metabolites such as tetrahydrofolate (THF), biopterin (BH(4)), and the modified tRNA nucleosides queuosine (Q) and archaeosine (G(+)) relies on several enzymes of the Tunnel-fold superfamily. A subset of these proteins includes the 6-pyruvoyltetrahydropterin (PTPS-II), PTPS-III, and PTPS-I homologues, all members of the COG0720 family that have been previously shown to transform 7,8-dihydroneopterin triphosphate (H(2)NTP) into different products. PTPS-II catalyzes the formation of 6-pyruvoyltetrahydropterin in the BH(4) pathway, PTPS-III catalyzes the formation of 6-hydroxylmethyl-7,8-dihydropterin in the THF pathway, and PTPS-I catalyzes the formation of 6-carboxy-5,6,7,8-tetrahydropterin in the Q pathway. Genes of these three enzyme families are often misannotated as they are difficult to differentiate by sequence similarity alone. Using a combination of physical clustering, signature motif, phylogenetic codistribution analyses, in vivo complementation studies, and in vitro enzymatic assays, a complete reannotation of the COG0720 family was performed in prokaryotes. Notably, this work identified and experimentally validated dual function PTPS-I/III enzymes involved in both THF and Q biosynthesis. Both in vivo and in vitro analyses showed that the PTPS-I family could tolerate a translation of the active site cysteine and was inherently promiscuous, catalyzing different reactions on the same substrate or the same reaction on different substrates. Finally, the analysis and experimental validation of several archaeal COG0720 members confirmed the role of PTPS-I in archaeosine biosynthesis and resulted in the identification of PTPS-III enzymes with variant signature sequences in Sulfolobus species. This study reveals an expanded versatility of the COG0720 family members and illustrates that for certain protein families extensive comparative genomic analysis beyond homology is required to correctly predict function.
Full-text Article · Jan 2012 · ACS Chemical Biology
[Show abstract][Hide abstract]ABSTRACT: OleD is shown to play a key reductive role in the generation of alkenes (olefins) from acyl thioesters in Stenotrophomonas maltophilia. The gene coding for OleD clusters with three other genes, oleABC, and all appear to be transcribed in the same direction as an operon in various olefin producing bacteria. In this study, a series of substrates varying in chain length and stereochemistry were synthesized and used to elucidate the functional role and substrate specificity of OleD. We demonstrated that OleD, which is an NADP(H) dependent reductase, is a homodimer which catalyzes the reversible stereospecific reduction of 2-alkyl-3-ketoalkanoic acids. Maximal catalytic efficiency was observed with syn-2-decyl-3-hydroxytetradecanoic acid, with a k(cat)/K(m) 5- and 8-fold higher than for syn-2-octyl-3-hydroxydodecanoic acid and syn-2-hexyl-3-hydroxydecanoic acid, respectively. OleD activity was not observed with syn-2-butyl-3-hydroxyoctanoic acid and compounds lacking a 2-alkyl group such as 3-ketodecanoic and 3-hydroxydecanoic acids, suggesting the necessity of the 2-alkyl chain for enzyme recognition and catalysis. Using diastereomeric pairs of substrates and 4 enantiopure isomers of 2-hexyl-3-hydroxydecanoic acid of known stereochemistry, OleD was shown to have a marked stereochemical preference for the (2R,3S)-isomer. Finally, experiments involving OleA and OleD demonstrate the first 3 steps and stereochemical course in olefin formation from acyl thioesters; condensation to form a 2-alkyl-3-ketoacyl thioester, subsequent thioester hydrolysis, and ketone reduction.
[Show abstract][Hide abstract]ABSTRACT: Polyketide natural products generated by type I modular polyketide synthases (PKSs) are vital components in our drug repertoire. To reprogram these biosynthetic assembly lines, we must first understand the steps that occur within the modular "black boxes." Herein, key steps of acyl-CoA extender unit selection are explored by in vitro biochemical analysis of the PikAIV PKS model system. Two complementary approaches are employed: a fluorescent-probe assay for steady-state kinetic analysis, and Fourier Transform Ion Cyclotron Resonance-mass spectrometry (FTICR-MS) to monitor active-site occupancy. Findings from five enzyme variants and four model substrates have enabled a model to be proposed involving catalysis based upon acyl-CoA substrate loading followed by differential rates of hydrolysis. These efforts suggest a strategy for future pathway engineering efforts using unnatural extender units with slow rates of hydrolytic off-loading from the acyltransferase domain.
[Show abstract][Hide abstract]ABSTRACT: GTP cyclohydrolase I (GCYH-I) is an essential Zn(2+)-dependent enzyme that catalyzes the first step of the de novo folate biosynthetic pathway in bacteria and plants, the 7-deazapurine biosynthetic pathway in Bacteria and Archaea, and the biopterin pathway in mammals. We recently reported the discovery of a new prokaryotic-specific GCYH-I (GCYH-IB) that displays no sequence identity to the canonical enzyme and is present in approximately 25% of bacteria, the majority of which lack the canonical GCYH-I (renamed GCYH-IA). Genomic and genetic analyses indicate that in those organisms possessing both enzymes, e.g., Bacillus subtilis, GCYH-IA and -IB are functionally redundant, but differentially expressed. Whereas GCYH-IA is constitutively expressed, GCYH-IB is expressed only under Zn(2+)-limiting conditions. These observations are consistent with the hypothesis that GCYH-IB functions to allow folate biosynthesis during Zn(2+) starvation. Here, we present biochemical and structural data showing that bacterial GCYH-IB, like GCYH-IA, belongs to the tunneling-fold (T-fold) superfamily. However, the GCYH-IA and -IB enzymes exhibit significant differences in global structure and active-site architecture. While GCYH-IA is a unimodular, homodecameric, Zn(2+)-dependent enzyme, GCYH-IB is a bimodular, homotetrameric enzyme activated by a variety of divalent cations. The structure of GCYH-IB and the broad metal dependence exhibited by this enzyme further underscore the mechanistic plasticity that is emerging for the T-fold superfamily. Notably, while humans possess the canonical GCYH-IA enzyme, many clinically important human pathogens possess only the GCYH-IB enzyme, suggesting that this enzyme is a potential new molecular target for antibacterial development.
Full-text Article · Sep 2009 · Journal of bacteriology
[Show abstract][Hide abstract]ABSTRACT: GTP cyclohydrolase I (GCYH-I) is the first enzyme of the de novo tetrahydrofolate biosynthetic pathway present in bacteria, fungi, and plants, and encoded in Escherichia coli by the folE gene. It is also the first enzyme of the biopterin (BH4) pathway in Homo sapiens, where it is encoded by a homologous folE gene. A homology-based search of GCYH-I orthologs in all sequenced bacteria revealed a group of microbes, including several clinically important pathogens, that encoded all of the enzymes of the tetrahydrofolate biosynthesis pathway but GCYH-I, suggesting that an alternate family was present in these organisms. A prediction based on phylogenetic occurrence and physical clustering identified the COG1469 family as a potential candidate for this missing enzyme family. The GCYH-I activity of COG1469 family proteins from a variety of sources (Thermotoga maritima, Bacillus subtilis, Acinetobacter baylyi, and Neisseria gonorrhoeae) was experimentally verified in vivo and/or in vitro. Although there is no detectable sequence homology with the canonical GCYH-I, protein fold recognition based on sequence profiles, secondary structure, and solvation potential information suggests that, like GCYH-I proteins, COG1469 proteins are members of the tunnel-fold (T-fold) structural superfamily. This new GCYH-I family is found in approximately 20% of sequenced bacteria and is prevalent in Archaea, but the family is to this date absent in Eukarya.
Article · Jan 2007 · Journal of Biological Chemistry