The Mechanism of Chloride Activation of Oxygen Evolution in Spinach Photosystem Ii

The mechanism by which ${\rm Cl}\sp-$ activates the oxygen-evolving complex (OEC) of Photosystem II (PS II) was studied by $\sp{35}{\rm Cl}$-NMR and steady-state measurements of oxygen evolution. This study represents the first attempt to directly monitor ${\rm Cl}\sp-$ binding in spinach. At low light intensity, the ${\rm Cl}\sp-$ activation curve (Hill activity vs. (${\rm Cl}\sp-$)) shows three intermediary plateaus in the concentration range 0.1-10 mM ${\rm Cl}\sp-,$ indicating kinetic cooperativity with respect to ${\rm Cl}\sp-.$ The $\sp{35}{\rm Cl}$-NMR binding curve (excess linewidth vs. (${\rm Cl}\sp-$)) for ${\rm Cl}\sp-$ depleted thylakoids and PS II membranes is not a smoothly descending hyperbola. Instead, this curve shows four sharp increases in linewidth (linewidth maxima) in the concentration range 0.1-10 mM ${\rm Cl}\sp-.$ The presence of these linewidth maxima indicates that ${\rm Cl}\sp-$ addition exposes four ${\rm Cl}\sp-$ binding sites that were not previously accessible to exchange. The pH-dependence for the excess linewidth at 0.75 mM ${\rm Cl}\sp-$ shows a maximum at pH 6.0 and two smaller maxima at pH 5.4 and 6.5. Mild heating eliminates both the linewidth maxima and the plateaus in the ${\rm Cl}\sp-$ activation curve. Hydroxylamine treatment (1.5 mM) has little effect on ${\rm Cl}\sp-$ binding, indicating that Mn is not involved. Tris-washing eliminates nearly all of the high-affinity ${\rm Cl}\sp-$ binding. Measurements of PS II membranes washed with 1.0 M NaCl or ${\rm CaCl}\sb2$ indicate that the native ${\rm Cl}\sp-$ binding mechanism requires the presence of the 33 kD extrinsic polypeptide, but that the function of the 18 kD and 24 kD extrinsic polypeptides can be replaced by 2.0 mM ${\rm Ca}\sp{2+}.$ Chloride binding is still observed after removal of all three extrinsic polypeptides, but the $\sp{35}{\rm Cl}$-NMR binding curve shows only a single, broad linewidth maximum at about 0.5 mM ${\rm Cl}\sp-.$ A hypothetical model is proposed to explain these results. This model involves ${\rm Cl}\sp-$ binding at two types of sites: (1) an intrinsic site (composed of three histidines) on the D1/D2 proteins, and (2) a set of four extrinsic sites (composed of lysines and arginines) on the 33 kD polypeptide. Possible amino acid ligands for the tetra-nuclear Mn cluster (on D1/D2) are also described.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Biochemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Coleman, William Joseph

• Chemistry, Biochemistry

Language dc:language

eng

Identifier

(UMI)AAI8721614

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/77685