Calcium binding studies of photosystem II using a calcium-selective electrode.

The identification of Ca2+ as a cofactor in photosynthetic O2 evolution has encouraged research into the role of Ca2+ in photosystem II (PSII). Previous methods used to identify the number of binding sites and their affinities were not able to measure Ca2+ binding at thermodynamic equilibrium. We introduce the use of a Ca2(+)-selective electrode to study equilibrium binding of Ca2+ to PSII. The number and affinities of binding sites were determined via Scatchard analysis on a series of PSII membrane preparations progressively depleted of the extrinsic polypeptides and Mn. Untreated PSII membranes bound approximately 4 Ca2+ per PSII with high affinity (K = 1.8 microM) and a larger number of Ca2+ with lower affinity. The high-affinity sites are assigned to divalent cation-binding sites on the light-harvesting complex II that are involved in membrane stacking, and the lower-affinity sites are attributed to nonspecific surface-binding sites. These sites were also observed in all of the extrinsic polypeptide- and Mn-depleted preparations. Depletion of the extrinsic polypeptides and/or Mn exposed additional very high-affinity Ca2(+)-binding sites which were not in equilibrium with free Ca2+ in untreated PSII, owing to the diffusion barrier created by the extrinsic polypeptides. Ca2(+)-depleted PSII membranes lacking the 23 and 17 kDa extrinsic proteins bound an additional 2.5 Ca2+ per PSII with K = 0.15 microM. This number of very high-affinity Ca2(+)-binding sites agrees with the previous work of Cheniae and co-workers [Kalosaka, K., et al. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., Ed.) pp 721-724, Kluwer, Dordrecht, The Netherlands] whose procedure for Ca2+ depletion was used. Further depletion of the 33 kDa extrinsic protein yielded a sample that bound only 0.7 very high-affinity Ca2+ per PSII with K = 0.19 microM. The loss of 2 very high-affinity Ca2(+)-binding sites upon depletion of the 33 kDa extrinsic protein could be due to a structural change of the O2-evolving complex which lost 2-3 of the 4 Mn ions in this sample. Finally, PSII membranes depleted of Mn and the 33, 23, and 17 kDa extrinsic proteins bound approximately 4 very high-affinity Ca2+ per PSII with K = 0.08 microM. These sites are assigned to Ca2+ binding to the vacant Mn sites.

X-ray spectroscopy of the iron site in soybean lipoxygenase-1: changes in coordination upon oxidation or addition of methanol.

Iron K-edge X-ray spectroscopy (XANES and EXAFS) was used to study iron coordination in frozen solutions of soybean lipoxygenase-1 (SLO). The intensity of the 1s-->3d pre-edge transition of native iron(II) lipoxygenase is greater than what was found for six-coordinate high-spin iron(II) model complexes, but comparable to that of a five-coordinate model. This and a relatively short average bond length determined by EXAFS (2.13 A) indicate that the native lipoxygenase in our frozen samples is five-coordinate, excluding possible bonds longer than 2.5 A. The coordination of the iron(II) in native lipoxygenase changes when methanol (as low as 0.1%) or glycerol (20%) is added to the buffer prior to freezing. The addition of methanol diminishes the pre-edge transition and increases EXAFS-derived bond lengths by 0.04 A, indicating a change to six-coordination. The small pre-edge feature in active iron(III) lipoxygenase suggests six-coordination. EXAFS indicates a short, 1.88 A Fe-O bond, which, given other spectroscopic and crystallographic evidence, is assigned to coordinated hydroxide. The average of the remaining bond lengths is 2.11 A. The iron coordination in iron(III) lipoxygenase is less affected by the presence of alcohols than is the site in the iron(II) enzyme. Bond valence sums indicate that the bond lengths for lipoxygenase derived from our EXAFS analyses are comparable to those of crystallographically characterized model complexes. The flexibility of the coordination number in SLON (native SLO) and the presence of an [FeIIIOH]2+ unit in SLOA (active SLO) are of possible mechanistic importance.