Editing Dihydrofolate reductase (section)

=== Conformational changes of DHFR ===
[[File:Conformational changes during the DHFR catalytic cycle.png|thumb|The closed structure is depicted in red and the occluded structure is depicted in green in the catalytic scheme. In the structure, DHF and THF are colored red, NADPH is colored yellow, and Met20 residue is colored blue.]]
The catalytic cycle of the reaction catalyzed by DHFR incorporates five important intermediate: holoenzyme (E:NADPH), Michaelis complex (E:NADPH:DHF), ternary product complex (E:NADP<sup href="NADP+">+</sup>:THF), tetrahydrofolate binary complex (E:THF), and THF‚NADPH complex (E:NADPH:THF). The product (THF) dissociation step from E:NADPH:THF to E:NADPH is the rate determining step during steady-state turnover.<ref name="Fierke_1987">{{cite journal | vauthors = Fierke CA, Johnson KA, Benkovic SJ | title = Construction and evaluation of the kinetic scheme associated with dihydrofolate reductase from ''Escherichia coli'' | journal = Biochemistry | volume = 26 | issue = 13 | pages = 4085–92 | date = June 1987 | pmid = 3307916 | doi = 10.1021/bi00387a052 }}</ref>

Conformational changes are critical in DHFR's catalytic mechanism.<ref>{{cite journal | vauthors = Antikainen NM, Smiley RD, Benkovic SJ, Hammes GG | title = Conformation coupled enzyme catalysis: single-molecule and transient kinetics investigation of dihydrofolate reductase | journal = Biochemistry | volume = 44 | issue = 51 | pages = 16835–43 | date = December 2005 | pmid = 16363797 | doi = 10.1021/bi051378i }}</ref> The Met20 loop of DHFR is able to open, close or occlude the active site.<ref name="Sawaya_1997">{{cite journal | vauthors = Sawaya MR, Kraut J | title = Loop and subdomain movements in the mechanism of ''Escherichia coli'' dihydrofolate reductase: crystallographic evidence | journal = Biochemistry | volume = 36 | issue = 3 | pages = 586–603 | date = January 1997 | pmid = 9012674 | doi = 10.1021/bi962337c }}</ref><ref name="Rod_2003">{{cite journal | vauthors = Rod TH, Brooks CL | title = How dihydrofolate reductase facilitates protonation of dihydrofolate | journal = Journal of the American Chemical Society | volume = 125 | issue = 29 | pages = 8718–9 | date = July 2003 | pmid = 12862454 | doi = 10.1021/ja035272r }}</ref> Correspondingly, three different conformations classified as the opened, closed and occluded states are assigned to Met20. In addition, an extra distorted conformation of Met20 was defined due to its indistinct characterization results.<ref name="Sawaya_1997" /> The Met20 loop is observed in its occluded conformation in the three product ligating intermediates, where the nicotinamide ring is occluded from the active site. This conformational feature accounts for the fact that the substitution of NADP<sup>+</sup> by NADPH is prior to product dissociation. Thus, the next round of reaction can occur upon the binding of substrate.<ref name="Fierke_1987" />