Editing Dihydrofolate reductase (section)

=== R67 DHFR ===
{{Infobox protein family
| Symbol = DHFR_2
| Name = R67 dihydrofolate reductase
| image = PDB 2rk1 EBI.png
| width = 
| caption = R67 in complex with DHF and NADP+, monomer. PDB entry {{PDBe|2rk1}}.
| Pfam = PF06442
| Pfam_clan =  
| InterPro = IPR009159
| SMART = 
| PROSITE = 
| MEROPS = 
| SCOP = 1vif
| TCDB = 
| OPM family = 
| OPM protein = 
| CAZy = 
| CDD = 
}}
Due to its unique structure and catalytic features, R67 DHFR is widely studied. R67 DHFR is a type II R-plasmid-encoded DHFR without geneticay or structural relation to the E. coli chromosomal DHFR. It is a homotetramer that possesses the 222 symmetry with a single active site pore that is exposed to solvent.<ref>{{cite journal | vauthors = Narayana N, Matthews DA, Howell EE, Nguyen-huu X | title = A plasmid-encoded dihydrofolate reductase from trimethoprim-resistant bacteria has a novel D2-symmetric active site | journal = Nature Structural Biology | volume = 2 | issue = 11 | pages = 1018–25 | date = November 1995 | pmid = 7583655 | doi=10.1038/nsb1195-1018| s2cid = 11914241 }}</ref> This symmetry of active site results in the different binding mode of the enzyme: It can bind with two dihydrofolate (DHF) molecules with positive cooperativity or two NADPH molecules with negative cooperativity, or one substrate plus one, but only the latter one has the catalytical activity.<ref>{{cite journal | vauthors = Bradrick TD, Beechem JM, Howell EE | title = Unusual binding stoichiometries and cooperativity are observed during binary and ternary complex formation in the single active pore of R67 dihydrofolate reductase, a D2 symmetric protein | journal = Biochemistry | volume = 35 | issue = 35 | pages = 11414–24 | date = September 1996 | pmid = 8784197 | doi = 10.1021/bi960205d }}</ref> Compare with E. coli chromosomal DHFR, it has higher K<sub>m</sub> in binding dihydrofolate (DHF) and NADPH. The much lower catalytical kinetics show that hydride transfer is the rate determine step rather than product (THF) release.<ref>{{cite journal | vauthors = Park H, Zhuang P, Nichols R, Howell EE | title = Mechanistic studies of R67 dihydrofolate reductase. Effects of pH and an H62C mutation | journal = The Journal of Biological Chemistry | volume = 272 | issue = 4 | pages = 2252–8 | date = January 1997 | pmid = 8999931 | doi = 10.1074/jbc.272.4.2252 | doi-access = free }}</ref>

In the R67 DHFR structure, the homotetramer forms an active site pore. In the catalytical process, DHF and NADPH enters into the pore from opposite position. The π-π stacking interaction between NADPH's nicotinamide ring and DHF's pteridine ring tightly connect two reactants in the active site. However, the flexibility of p-aminobenzoylglutamate tail of DHF was observed upon binding which can promote the formation of the [[transition state]].<ref>{{cite journal | vauthors = Kamath G, Howell EE, Agarwal PK | title = The tail wagging the dog: insights into catalysis in R67 dihydrofolate reductase | journal = Biochemistry | volume = 49 | issue = 42 | pages = 9078–88 | date = October 2010 | pmid = 20795731 | doi = 10.1021/bi1007222 }}</ref>

<gallery class="skin-invert-image" mode=packed>
File:Reaction Kinetics comparison between EcDHFR and R67 DHFR.png|Reaction Kinetics comparison between ''E. coli'' DHFR (EcDHFR) and R67 DHFR
File:Structure difference of substrate binding in E. coli and R67 DHFR.png|Structure difference of substrate binding in EcDHFR and R67 DHFR
</gallery>