Role of Bicarbonate and of Manganese in Photosystem Ii Reactions of Photosynthesis

Khanna, Rita

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Description

Title

Role of Bicarbonate and of Manganese in Photosystem Ii Reactions of Photosynthesis

Author(s)

Khanna, Rita

Issue Date

1980

Department of Study

Biology

Discipline

Biology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Biology, General

Language

eng

Abstract

• This thesis examines some of the reactions associated with Photosystem II. On the reducing side of Photosystem II, the role of bicarbonate ion on electron transport from Photosystem II to Photosystem I was established and on the oxidizing side studies were carried out to examine the involvement of manganese in oxygen evolution.
• When bicarbonate is added to thylakoid membranes, previously depleted of bicarbonate, a large increase is observed (4-10 fold) in the Hill reaction as measured by oxygen evolution. In order to establish the role of bicarbonate in this reaction, the bicarbonate effect was characterized and the site of its action established. The major site of bicarbonate action in the electron flow pathway from water to "X" (the electron acceptor of Photosystem I) is between R (a two electron acceptor of Photosystem II) and PQ (plastoquinone). In bicarbonate depleted thylakoids, the reduction of PQ by R('2-) is slowed down from a t(,1/2) (TURN) 0.6 ms to a t(,1/2) (TURN) 100-200 ms. This effect if fully reversible upon bicarbonate addition. Furthermore, bicarbonate depletion of thylakoids leads to (a) a loss of proton translocation at the PQ level, without any significant change in the protons released from water; (b) a change in binding affinity of atrazine to bicarbonate depleted by thylakoid membranes; and (c) a lack of bicarbonate stimulation in trypsin treated membranes.
• Manganese is known to be required for oxygen evolution and is believed to be an integral part of the oxygen evolving system. Water proton relaxation rate (transverse relaxation rate, T('-1)) measurements were used to monitor the amount and oxidation state of Mn bound to thylakoid membranes. Once released, Mn(II) becomes ESR detectable as characterized by the appearance of six-line pattern typical for free Mn(II). Parallel measurements of T('-1), ESR and O(,2) evolution activity were used to study the role of Mn in oxygen evolution. It is shown that T('-1) monitors both functional and non-functional pools of Mn(II). Under extreme conditions, Cu may also become accessible to water and influence T('-1). Some of the important conclusions were: (a) the reductants (TPB, NH(,2)OH and H(,2)O(,2)) that reduce the S states of the O(,2) evolving system cause an increase in T('-1) suggesting a conversion of higher oxidation states of Mn to Mn(II); (b) a good correlation between functional Mn and T('-1) is observed when thylakoids are aged at 35(DEGREES)C as there is a parallel decrease in O(,2) evolution activity, T('-1) and the content of bound Mn; (c) a relationship between non-functional Mn and T('-1) is suggested by the observation that low concentrations of MgCl(,2) release Mn without affecting O(,2) evolution; (d) a contribution from other paramagnetic species (e.g., copper) or a change in the accessibility of Mn(II) to water may arise when thylakoid membranes are heated to 40-50(DEGREES)C or suspended in media at very low and high pHs; and (e) the isolated light harvesting complex contains about one-third of the Mn in thylakoid membranes; this may represent the tightly bound pool of Mn in thylakoids.

Graduation Semester

1980

Type of Resource

text

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