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Orotate phosphoribosyltransferase (OPRT) catalyzes the reaction that adds the pyrimidine base to the ribose in the penultimate step of the de novo biosynthesis of pyrimidine nucleotides. The OPRT structure consists of an obligate dimer, conserved throughout the phosphoribosyltransferase family. Here, we describe the structural characterization of Burkholderia cenocepacia OPRT (BcOPRT), both by X-ray crystallography and Cryo electron microscopy (Cryo-EM). While the known dimer is present in the structure of BcOPRT, a putative hexameric form was also observed by multiple methods. Analyses by chromatography, Cryo-EM, and kinetics indicate that both dimeric and hexameric forms of this enzyme are present together in solution. Comparison of the kinetics of the native protein and two variants, which were specifically designed to prevent hexamerization, reveal that only the hexameric form is enzymatically active. Collectively, these data suggest that BcOPRT may use oligomerization to control overall enzymatic activity, thus contributing to the local regulation of pyrimidine biosynthesis in this organism. 

Burkholderia cenocepaciais an opportunistic human pathogen that can cause lethal infections in immunocompromised individuals, particularly those with cystic fibrosis. As such, there is a critical need to identify and characterize the structure and function of enzymes that participate in the metabolic pathways of this bacterium. Here, the high-resolution X-ray crystal structure of a short-chain dehydrogenase reductase (SDR) fromB. cenocepaciaJ2315 (BcSDR) in complex with the coenzyme NADP⁺and a benzoic acid ligand is presented. This protein has the conserved Rossmann fold of the SDR superfamily and the characteristic TGxxxGxG motif of the classical SDR subfamily. However, unlike classical SDRs, the active site of BcSDR has a leucine residue in place of the highly conserved and catalytically important tyrosine residue. Sequence analysis confirms that this leucine residue is conserved in this SDR across the Burkholderiales order. This suggests that BcSDR is more appropriately classified into the divergent SDR subfamily. In addition, this enzyme would necessarily employ a different enzyme mechanism to that proposed as a general mechanism for most SDRs. 

Thiamin and its phosphate derivatives are ubiquitous molecules involved as essential cofactors in many cellular processes. The de novo biosynthesis of thiamin employs the parallel synthesis of 4-methyl-5-(2-hydroxyethyl)thiazole (THZ-P) and 4-amino-2-methyl-5(diphosphooxymethyl) pyrimidine (HMP) pyrophosphate (HMP-PP), which are coupled to generate thiamin phosphate. Most organisms that can biosynthesize thiamin employ a kinase (HMPK or ThiD) to generate HMP-PP. In nearly all cases, this enzyme is bifunctional and can also salvage free HMP, producing HMP-P, the monophosphate precursor of HMP-PP. Here we present high-resolution crystal structures of an HMPK from Acinetobacter baumannii (AbHMPK), both unliganded and with pyridoxal 5-phosphate (PLP) noncovalently bound. Despite the similarity between HMPK and pyridoxal kinase enzymes, our kinetics analysis indicates that AbHMPK accepts HMP exclusively as a substrate and cannot turn over pyridoxal, pyridoxamine, or pyridoxine nor does it display phosphatase activity. PLP does, however, act as a weak inhibitor of AbHMPK with an IC50 of 768 μM. Surprisingly, unlike other HMPKs, AbHMPK catalyzes only the phosphorylation of HMP and does not generate the diphosphate HMP-PP. This suggests that an additional kinase is present in A. baumannii, or an alternative mechanism is in operation to complete the biosynthesis of thiamin. 

Agmatine amidinohydrolase, or agmatinase, catalyzes the conversion of agmatine to putrescine and urea. This enzyme is found broadly across kingdoms of life and plays a critical role in polyamine biosynthesis and the regulation of agmatine concentrations. Here we describe the high-resolution X-ray crystal structure of the E . coli agmatinase, SPEB. The data showed a relatively high degree of pseudomerohedral twinning, was ultimately indexed in the P 3 1 space group and led to a final model with eighteen chains, corresponding to three full hexamers in the asymmetric unit. There was a solvent content of 38.5% and refined R/R free values of 0.166/0.216. The protein has the conserved fold characteristic of the agmatine ureohydrolase family and displayed a high degree of structural similarity among individual protomers. Two distinct peaks of electron density were observed in the active site of most of the eighteen chains of SPEB. As the activity of this protein is known to be dependent upon manganese and the fold is similar to other dinuclear metallohydrolases, these peaks were modeled as manganese ions. The orientation of the conserved active site residues, in particular those amino acids that participate in binding the metal ions and a pair of acidic residues (D153 and E274 in SPEB) that play a role in catalysis, are similar to other agmatinase and arginase enzymes and is consistent with a hydrolytic mechanism that proceeds via a metal-activated hydroxide ion. 

The flavin cofactor performs many functions in the cell based on the ability of the isoalloxazine ring to undergo one- or two-electron reduction and form covalent adducts with reactants such as amino acids. In addition, the strong visible absorption of the cofactor is also the basis for flavin-dependent photoreceptors. Vibrational spectroscopy is uniquely suited to studying the mechanism of flavoproteins since the frequency of the vibrational modes is very sensitive to the electronic structure and environment of the isoalloxazine ring. This chapter describes the mechanistic information that can be gained using vibrational spectroscopy as well experimental challenges and approaches that are used to obtain and interpret the complex data contained in a vibrational spectrum.