Taxonomic distribution of proteins containing FolB domains

The tree below includes only several representative species and genera. The complete taxonomic breakdown of all proteins containing FolB domains can be accessed here. Click the counts or percentage values to display the corresponding proteins.

Biosynthesis of pteridines in Escherichia coli. Structural and mechanisticsimilarity of dihydroneopterin-triphosphate epimerase and dihydroneopterinaldolase.

Haussmann C, Rohdich F, Schmidt E, Bacher A, Richter G
J Biol Chem. 1998; 273: 17418-24

An open reading frame located at 69.0 kilobases on the Escherichia colichromosome was shown to code for dihydroneopterin aldolase, catalyzing theconversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin inthe biosynthetic pathway of tetrahydrofolate. The gene was subsequentlydesignated folB. The FolB protein shows 30% identity to the paralogousdihydroneopterin-triphosphate epimerase, which is specified by the folXgene located at 2427 kilobases on the E. coli chromosome. The folX andfolB gene products were both expressed to high yield in recombinant E.coli strains, and the recombinant proteins were purified to homogeneity.Both enzymes form homo-octamers. Aldolase can use L-threo-dihydroneopterinand D-erythro-dihydroneopterin as substrates for the formation of6-hydroxymethyldihydropterin, but it can also catalyze the epimerizationof carbon 2' of dihydroneopterin and dihydromonapterin at appreciablevelocity. Epimerase catalyzes the epimerization of carbon 2' in thetriphosphates of dihydroneopterin and dihydromonapterin. However, theenzyme can also catalyze the cleavage of the position 6 side chain ofseveral pteridine derivatives at a slow rate. Steady-state kineticparameters are reported for the various enzyme-catalyzed reactions. Wepropose that the polarization of the 2'-hydroxy group of the substratecould serve as the initial reaction step for the aldolase as well as forthe epimerase activity. A deletion mutant obtained by targeting the folXgene of E. coli has normal growth properties on complete medium as well ason minimal medium. Thus, the physiological role of the E. coli epimeraseremains unknown. The open reading frame ygiG of Hemophilus influenzaespecifies a protein with the catalytic properties of an aldolase. However,the genome of H. influenzae does not specify adihydroneopterin-triphosphate epimerase.

Single amino acid substitutions disrupt tetramer formation in thedihydroneopterin aldolase enzyme of Pneumocystis carinii.

Thomas MC, Ballantine SP, Bethell SS, Bains S, Kellam P, Delves CJ
Biochemistry. 1998; 37: 11629-36

In the opportunistic pathogen Pneumocystis carinii, dihydroneopterinaldolase function is expressed as the N-terminal portion of themultifunctional folic acid synthesis protein (Fas). This regionencompasses two domains, FasA and FasB, which are 27% amino acididentical. FasA and FasB also share significant amino acid sequencesimilarity with bacterial dihydroneopterin aldolases. In the presentstudy, this enzyme function has been overproduced as an independentmonofunctional activity in Escherichia coli. Recombinant FasAB-Met23(amino acids 23-290 of the predicted open reading frame) was purified andshown to contain dihydroneopterin aldolase activity. The nativeFasAB-Met23 is a tetramer of the 30-kDa subunit, demonstratingcharacteristics of an associating-dissociating equilibrium system in whichonly the multimeric form of the enzyme is active. Multiple sequencealignment of FasA and FasB with other dihydroneopterin aldolaseshighlights only three positions where the amino acid is invariable betweenall the predicted proteins. The role of these conserved amino acidresidues in enzyme function was investigated using site-directedmutagenesis. Mutant FasAB-Met23 species were overproduced and purified tonear homogeneity. Three FasA domain mutants and two FasB domain mutantshad little or no detectable dihydroneopterin aldolase activity,implicating both FasA and FasB in the catalytic mechanism. We show thateach mutant protein containing an inactivating amino acid substitution haslost its ability to form stable tetramers.