The Central Role of

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The Central Role of Protons in Establishing,
Regulating, Controlling and Limiting the
Energy Budget of Photosynthesis
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• The photosynthetic apparatus must
• Efficiently store solar energy in chemical bonds
• Provide the correct ratio of products (NADPH,
  ATP)
• Avoid over-excitation of the reaction centers
  which can lead to photodamage.

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An electrochemical gradient of protons is the
essential energetic intermediate in ATP synthesis
ADP Pi
NADPH
ATP
The energetic intermediate is an electrochemical
gradient of H.
In the formulation of Mitchell, Dp
-(D?H)/F DY - 59 mV(DpH) Dp is also called
pmf or proton-motive force.
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ADP Pi
NADPH
n
ATP
H/e-
The Ratio of NADPHATP produced by photosynthesis
is determined by 1) The ratio of H pumped per
e- transferred (H/e-) 2) The ratio of H
transferred through the ATP synthase ATP
produced (n)
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Chemiosmotic Coupling in Mitochondria
ATP synthesis (endergonic)
electron transfer (exergonic)
e-
O2
Cl-
SH2
ATP
ADP Pi
DY
ATP
Cl-
In this case, the pH of the matrix and IMS are
nearly the same, and essentially all pmf is
stored as DY.
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Cl-
ADP Pi
DY
ATP
Cl-
It is commonly believed that, in chloroplasts,
the Dy component collapses as Cl- ions move in
response to the electric field. Consequently, it
is widely thought that pmf in thylakoids is
stored solely as a DpH
Dp -(D?H)/F DY - 59 mV(DpH)
- 59 mV(DpH)
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The proton gradient also plays a central role in
the regulation of photosynthesis.
photochemistry
ADP Pi
ATP
heat
Xanthophyll cycle qE quenching
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• The relationships between the proton gradient and
  regulatory phenomena are fundamental to
  understanding how plants convert energy and
  respond to the environment.
• Several new discoveries have altered our view of
  these relationships
• Structure/function of
• cytochrome bc complexes
• ATP synthase
• The ability to probe key energy conserving
  reactions in vivo in the steady-state.

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Historical Perspective of the H/e-
1/3 reduction of ATP
Year
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Linear H/e- Electron Pool
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High-resolution structures provide an explanation
for the proton pumping activity of the
cytochrome bc1 (and related b6f complexes
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How large of a DpH is required to sustain
photosynthesis?
4

• Assuming that pmf is in the form of DpH and using
  a H/ATP of n4 1-5, the DpH required to sustain
  measured values of DGATP range between 1.7 and
  2.2, suggesting the minimal lumen pH required
  ranges from 5.4 to 5.9.
• If a steady state Dy component exists (DpH0.2 to
  0.7), see 6ab, this would increase the minimal
  pH range to 5.6 and 6.6.

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What is the value of n?
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Comparison of cytochrome f rereduction rates in
intact leaves from pea (squares), tobacco
(circles) and cucumber (triangles) with those
from isolated thylakoids suspended at different
pH values, adapted from12 (Inset). Observed
half-times ranged from 20ms to 28ms for the
entire range of light intensitities up to 2800
umoles m-2 s-1. Comparison to thylakoid data
(dotted lines in inset12) suggests that the
lumen pH is regulated so that it remains above
pH 6.
3b
f
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Upper and Lower Bounds of Lumen pH Estimated by
in situ Probes
PC2 Degrades
PSII2 OEC loses Ca2
VDE1
B6f3
ATP5 activation
qE quenching7
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Summary