DHFR/TMP

Fig. 11 Comparison of experimental STDs (shaded circles) and calculated STD values (solid line) from CORCEMA-ST method for the crystal structure of chicken liver DHFR/TMP complex. The optimized parameters are tl =0.101 ns tp = 20.43 ns t i = 0.81 ps Tm2 = 3.04 ps Tm3 = 3.26 ps leakage factor = 0.065 and NOE R-factor = 0.076. Reprinted with permission from [75] 2005, American Chemical Society...

The CORCEMA-ST calculations on the initial bovine DHFR/TMP model already yielded a very low NOE R-factor of 0.076, and this was further improved (NOE R-factor = 0.055) with a very shght refinement of two torsion angles (ti... 

Streptococcus pneumoniae DHFR inhibitor, (R)-enantiomer, iCso = 9.8 niVI (S. pn. DHFR), iCgo = 2.8 nM (TMP-resistant S. pn. DHFR)... 

A highly unusual feature of DHFR in Apicomplexa and Kinetoplastida is its association with thymidylate synthase in the same protein. DHFR activity is always located at the amino terminal portion, while the thymidylate synthase activity resides in the carboxyl terminal. The two enzyme functions do not appear to be interdependent eg, the DHFR portion of the P falciparum enzyme molecule was found to function normally in the absence of the thymidylate synthase portion. It is likely that since the protozoan parasites do not perform de novo synthesis of purine nucleotides, the primary function of the tetrahydrofolate produced by DHFR is to provide 5,10-methylenetetrahydrofolate only for the thymidylate synthase-catalyzed reaction. Physical association of the two enzymes may improve efficiency of TMP synthesis. If an effective means of disrupting the coordination between the two activities can be developed, this bifunctional protein may qualify as a target for antiparasitic therapy. 

Many drugs have been developed to intervene in the synthesis of TMP (Figure 6.24). For example, methotrexate (6.66) inhibits DHFR by blocking the binding site of DHF. Another antimetabolite, 5-fluorouracil (5-FU, 6.67), is converted in the body to 5-fluoro-2 -deoxyuridine 5 -monophosphate (F-dUMP, 6.68), a potent inhibitor of TS.26... 

The donation of the methyl group from N5,N10-methylene tetrahydrofolate leads to the oxidation of the cofactor to dihydrofolate. This points to the importance of dihydrofolate reductase (DHFR) in the functioning of thymidylate synthase. Thus, synthesis of TMP requires a supply of both methyl groups—for example, from serine— and reducing equivalents. 

A classic example of a drug that works by species-specific protein inhibition is trimethoprim (TMP). Because this drug binds to bacterial dihydrofolate reductase (DHFR) 10 moie tightly than to the mammalian enzyme, there is a therapeutic concentration in which the drug can be used as an antibacterial with little deleterious consequences for a mammalian host. 

Figure 28-24 Dihydrofolate reductase (DHFR) has been a popular target for drug design. Methotrexate (MTX). 12. and trimethoprim (TMP). 13. resemble folic acid. 14. the natural substrate.

NATA, N-L-acetyltryptophanamide BSND, benzenesulfonamide BSFN, benzenesulfonate Sta, statine BZD, benzamidine CFM, coformycin MTX, methotrexate TMP, trimethoprim PT, pterin NPT, N5-deazapterin THP, tetrahydropyrrole Bz, benzene ADA, adenosine deaminase HCAIl, human carbonic anhydrase II TS, thymidyiate synifaase DHFR, dihydrofolate reductase MBP, mannose bindnig protein HEI, hydroxyethylene inhibitor HPR, (6R)-6-hydroxy-l,6 dihydropurine riboside (R),R configuration (S),S configuration NA, not available or not applicable. 

Figure 10 Chicken DHFR with pseudo TMP. Image generated on Silicon Graphics 02 workstation, circa 1997, using a raster-based image representation created with the Conic delegate from Midas-Plus ...

Figure 9 Chicken DHFR with TMP. Image generated on an Evans and Sutherland Picture System 2 with CSM color display, circa 1984, using dots and line drawings for representation and created with the MIDAS software package ...

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