TY - JOUR
T1 - Iron-Blocking the High-Affinity Mn-Binding Site in Photosystem II Facilitates Identification of the Type of Hydrogen Bond Participating in Proton-Coupled Electron Transport via Yz
AU - Semin, Boris K.
AU - Lovyagina, Elena R.
AU - Timofeev, Kirill N.
AU - Ivanov, Ilya I.
AU - Rubin, Andrei B.
AU - Seibert, Michael
PY - 2005
Y1 - 2005
N2 - Incubation of Mn-depleted PSII membranes [PSII(-Mn)] with Fe(II) is accompanied by the blocking of YZ. at the high-affinity Mn-binding site to exogenous electron donors [Semin et al. (2002) Biochemistry 41, 5854-5864] and a shift of the pKapp of the hydrogen bond partner for YZ (base B) from 7.1 to 6.1 [Semin, B. K., and Seibert, M. (2004) Biochemistry 43, 6772-6782]. Here we calculate activation energies (E a) for YZ. reduction in PSII(-Mn) and Fe-blocked PSII(-Mn) samples [PSII(-Mn, +Fe)] from temperature dependencies of the rate constants of the fast and slow components of the flash-probe fluorescence decay kinetics. At pH < pKapp (e.g., 5.5), the decays are fit with one (fast) component in both types of samples, and Ea is equal to 42.2 ± 2.9 kJ/mol in PSII(-Mn) and 46.4 ± 3.3 kJ/mol in PSII(-Mn, +Fe) membranes. At pH > pKapp, the decay kinetics exhibit an additional slow component in PSII(-Mn, +Fe) membranes (Ea = 36.1 ± 7.5 kJ/mol), which is much lower than the Ea of the corresponding component observed for YZ. reduction in PSII(-Mn) samples (48.1 ± 1.7 kJ/mol). We suggest that the above difference results from the formation of a strong low barrier hydrogen bond (LBHB) between YZ and base B in PSII(-Mn, +Fe) samples. To confirm this, Fe-blocking was performed in D2O to insert D+, which has an energetic barrier distinct from H+, into the LBHB. Measurement of the pH effects on the rates of YZ. reduction in PSII(-Mn, +Fe) samples blocked in D2O shows a shift of the pKapp from 6.1 to 7.6, and an increase in the Ea of the slow component. This approach was also used to measure the stability of the YZ. EPR signal at various temperatures in both kinds of membranes. In PSII-(-Mn) membranes, the freeze-trapped YZ. radical is stable below 190 K, but half of the YZ. EPR signal disappears after a 1-min incubation when the sample is warmed to 253 K. In PSII(-Mn, +Fe) samples, the trapped YZ. radical is unstable at a much lower temperature (77 K), However, the insertion of D + into the hydrogen bond between YZ and base B during the blocking process increases the temperature stability of the YZ . EPR signal at 77 K. Again, these results indicate that Fe-blocking involves YZ in the formation of a LBHB, which in turn is consistent with the suggested existence of a LBHB between YZ and base B in intact PSII membranes [Zhang, C., and Styring, S. (2003) Biochemistry 42, 8066-8076].
AB - Incubation of Mn-depleted PSII membranes [PSII(-Mn)] with Fe(II) is accompanied by the blocking of YZ. at the high-affinity Mn-binding site to exogenous electron donors [Semin et al. (2002) Biochemistry 41, 5854-5864] and a shift of the pKapp of the hydrogen bond partner for YZ (base B) from 7.1 to 6.1 [Semin, B. K., and Seibert, M. (2004) Biochemistry 43, 6772-6782]. Here we calculate activation energies (E a) for YZ. reduction in PSII(-Mn) and Fe-blocked PSII(-Mn) samples [PSII(-Mn, +Fe)] from temperature dependencies of the rate constants of the fast and slow components of the flash-probe fluorescence decay kinetics. At pH < pKapp (e.g., 5.5), the decays are fit with one (fast) component in both types of samples, and Ea is equal to 42.2 ± 2.9 kJ/mol in PSII(-Mn) and 46.4 ± 3.3 kJ/mol in PSII(-Mn, +Fe) membranes. At pH > pKapp, the decay kinetics exhibit an additional slow component in PSII(-Mn, +Fe) membranes (Ea = 36.1 ± 7.5 kJ/mol), which is much lower than the Ea of the corresponding component observed for YZ. reduction in PSII(-Mn) samples (48.1 ± 1.7 kJ/mol). We suggest that the above difference results from the formation of a strong low barrier hydrogen bond (LBHB) between YZ and base B in PSII(-Mn, +Fe) samples. To confirm this, Fe-blocking was performed in D2O to insert D+, which has an energetic barrier distinct from H+, into the LBHB. Measurement of the pH effects on the rates of YZ. reduction in PSII(-Mn, +Fe) samples blocked in D2O shows a shift of the pKapp from 6.1 to 7.6, and an increase in the Ea of the slow component. This approach was also used to measure the stability of the YZ. EPR signal at various temperatures in both kinds of membranes. In PSII-(-Mn) membranes, the freeze-trapped YZ. radical is stable below 190 K, but half of the YZ. EPR signal disappears after a 1-min incubation when the sample is warmed to 253 K. In PSII(-Mn, +Fe) samples, the trapped YZ. radical is unstable at a much lower temperature (77 K), However, the insertion of D + into the hydrogen bond between YZ and base B during the blocking process increases the temperature stability of the YZ . EPR signal at 77 K. Again, these results indicate that Fe-blocking involves YZ in the formation of a LBHB, which in turn is consistent with the suggested existence of a LBHB between YZ and base B in intact PSII membranes [Zhang, C., and Styring, S. (2003) Biochemistry 42, 8066-8076].
UR - http://www.scopus.com/inward/record.url?scp=22244473971&partnerID=8YFLogxK
U2 - 10.1021/bi047618w
DO - 10.1021/bi047618w
M3 - Article
C2 - 16008359
AN - SCOPUS:22244473971
SN - 0006-2960
VL - 44
SP - 9746
EP - 9757
JO - Biochemistry
JF - Biochemistry
IS - 28
ER -