Folates are essential for life.1
Mammals obtain folates from their diet, whereas most microorganisms must synthesize folates de novo
. Therefore, the folate biosynthetic pathway is an ideal target for antimicrobial agents. Inhibitors of folate enzymes dihydropteroate synthase and dihydrofolate reductase are currently used as antibiotics.2
However, the resistance to these and other antimicrobial agents has increased dramatically during the past two decades. The crisis is further aggravated by the fact that most of new antibiotics are chemical modifications of the chemical structures of existing drugs. These compounds act against old targets and are therefore less effective tackling the widespread antibiotic resistance problem. Thus, new antimicrobial targets and agents are urgently needed. Dihydroneopterin aldolase (DHNA) is a folate pathway enzyme and is not a target for existing antibiotics. Therefore, DHNA is a new molecular target in a biosynthetic pathway proven to be susceptible to antimicrobial agents.
DHNA catalyzes two reactions, the epimerization of 7,8-dihydroneopterin (DHNP) to 7,8-dihydromonapterin (DHMP)3
and the conversion of DHNP or DHMP to 6-hydroxymethyl-7,8-dihydropterin (HP) with the generation of glycoaldehyde (GA)4
as shown in . DHNA is a unique aldolase; it requires neither metal ions nor the formation of a Schiff base between the enzyme and the substrate.4
Although the epimerase reaction catalyzed by DHNA is very similar to the reaction catalyzed by L-ribulose-5-phosphate 4-epimerase,5
DHNA is different in structure and has no apparent sequence identity with other epimerases. While other aldolases and epimerases catalyze one type of reaction, DHNA catalyzes both. Therefore, it is of great interest to elucidate the structure and catalytic mechanism of DHNA.
Figure 1 (a) The aldolase and epimerase reactions catalyzed by DHNA. (b) Chemical structures of NP and MP. (c) Two views (side, on the left; top, on the right) of the SaDHNA●HP octamer (PDB entry 2DHN). The hollow cylinder has the crystallographic 422 (more ...)
To date, crystal structures have been determined for DHNA enzymes from Staphylococcus aureus (SaDHNA),6,7
Mycobacterium tuberculosis (MtDHNA),9
and Streptococcus Pneumoniae
The active site of the enzyme has been identified by the crystal structures of enzyme-product complexes SaDHNA●HP (PDB entry 2DHN)6
and MtDHNA●HP (PDB entry 1NBU).9
An active site lysine residue (K100) has been proposed to function as the general base and a bound water as a proton donor, leading to the first reaction scheme predicted for DHNA.6
Recently, our biochemical and biophysical studies have yielded further insights into the mechanism of DHNA, including the important roles of active site glutamate (E22) and tyrosine (Y54) residues,11,12
and the reversible nature of DHNA-catalyzed epimerization reaction.13
Here, we present two crystal structures, which make it possible to derive the critical interactions between DHNA and the trihydroxypropyl moiety of the substrate, providing further structural insights into the mechanism of DHNA-catalyzed reactions.