Page 14 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

110 Bioenergetics

C. ATP Synthesis when the proton potential is low, ATP synthases should
hydrolyze ATP and cause the pumping of protons across
ATP synthesis in chloroplasts is called photophosphory-
the membrane in the direction opposite that which occurs
lation and is similar to oxidative phosphorylation in mi-
during ATP synthesis. ATP-dependent proton transport by
tochondria. The light-driven transport of electrons from
the ATP synthase is of physiological significance in E. coli
+
water to NADP is coupled to the translocation of pro-
under anaerobic conditions in that it generates the electro-
tons from the stroma across the thylakoid membrane (the
chemical proton potential across the plasma membrane of
green,energy-convertingmembrane)intothelumen.Elec-
the bacterium. This potential is used for the active uptake
−
+
tron transport from Q to P700 is exergonic. Part of the
of some carbohydrates and amino acids.
energy released by electron transport is conserved by the
In contrast, ATP hydrolysis by the chloroplast ATP syn-
formation of an electrochemical proton gradient. The cy-
thase in the dark has no physiological role and would be
tochrome b 6 f complex of chloroplasts functions not only
wasteful. In fact, the rate of ATP hydrolysis by the ATP
in electron transport, but also in proton translocation.
synthase in thylakoids in the dark is less than 1% of the
The active site of the oxygen-evolving enzyme is ar-
rate of ATP synthesis in the light. Remarkably, within
ranged so that the protons formed during water oxidation
10–20 msec after the initiation of illumination, ATP syn-
are released into the thylakoid lumen. These protons con-
thesis reaches its steady-state rate. Thus, the activity of
tribute to the electrochemical proton potential. The thy-
the chloroplast ATP synthase is switched on in the light
lakoid membrane contains a proteinthatfunctionsto trans-
and off in the dark. In addition to being the driving force
−
port Cl across the membrane. Proton accumulation in the
for ATP synthesis, the electrochemical proton potential
thylakoid lumen is electrically balanced in large part by
is involved in switching the enzyme on. Structural per-
−
Cl uptake. As a result, thylakoids accumulate HCl and
turbations of the enzyme induced by the proton potential
the membrane potential across the membrane is low. The
overcome inhibitory interactions with bound ADP as well
pH inside the lumen during steady-state photosynthesis is
as with a polypeptide subunit of the synthase. An addi-
about 5.0.
tional regulatory mechanism that is unique to the chloro-
One of the earliest experiments that supported the hy-
plast ATP synthase is reductive activation. Reduction of a
pothesis that ATP synthesis and electron transport were
disulfide bond in a subunit of the chloroplast ATP synthase
linked by the electrochemical proton potential was car-
to a dithiol enhances the rate of ATP synthesis, especially
ried out with isolated thylakoid membranes. Thylakoid
at physiological values of the proton potential. The elec-
membranes were placed in a buffer at pH 4.0 and after a
trons for this reduction are derived from the chloroplast
few seconds the pH was rapidly increased to 8.0, which
electron transport chain.
resulted in the formation of a proton activity gradient. This
artificially formed gradient was shown to drive the syn-
thesis of ATP from ADP and P i . The experiments were III. ORIGIN OF MITOCHONDRIA
carried out in the dark so that the possibility that electron AND CHLOROPLASTS
transport contributed to the ATP synthesis was excluded.
Thus, a proton activity gradient was proven capable of
In animal, yeast, and fungal cells, DNA is present in two
driving ATP synthesis.
organelles, the nucleus and the mitochondria. In plant and
The thylakoid membrane enzyme that couples ATP syn-
algal cells, DNA is present in plastids (of which chloro-
thesis to the flow of protons down their electrochemi-
plasts are one example) as well as in mitochondria and the
cal gradient is called the chloroplast ATP synthase (see
nucleus. Unlike the DNA in the nucleus, which is pack-
Fig. 10). This enzyme has remarkable similarities to ATP
aged into chromosomes, plastid DNA and mitochondrial
synthases in mitochondria and certain bacteria. For exam-
DNA are circular and thus resemble the DNA in prokary-
ple, the β subunits of the chloroplast ATP synthase have
otes (e.g., bacteria).
76% amino acid sequence identity with the β subunits of
Mitochondrial DNA is small and codes for relatively
the ATP synthase of the bacterium E. coli.
few mitochondrial proteins. Although mitochondria con-
The reaction catalyzed by ATP synthases is
tain their own protein synthesis machinery, the majority
+
+
+
nH + ADP + P i + H → nH + ATP + H 2 O, (11) of the hundreds of mitochondrial proteins are coded for by
a
b
nuclear genes. These proteins are synthesized in the cyto-
where n is the number of protons translocated per ATP plasm and imported into the mitochondria. Plastid DNA
synthesized, probably three or four, and a and b refer to is somewhat larger than that of the mitochondrion and
the opposite sides of the coupling membrane. Provided contains the genetic information for more chloroplast pro-
the electrochemical proton potential is high, the reaction teins. However, as is the case for mitochondria, most of
is poised in the direction of ATP synthesis. In principle, the proteins in a chloroplast are coded by nuclear genes