Transcript The Modified Q-cycle

SBS-922 Membrane Proteins
Mitochondria and respiratory chains
John F. Allen
School of Biological and Chemical Sciences, Queen Mary, University of
London
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The chloroplast homologue of
respiratory complex III:
The cytochrome b6f complex
Chloroplast stroma
Light
NADP+
Light
FNR
Fd
n-side
Cyt b6f
PS II
PQ
p-side
PS I
2H2O
O2 + 4H+
2H+
PC, Cyt c6
Chloroplast lumen
Peter Mitchell c. 1943 (adapted from Mitchell 1981). A young Peter
Mitchell in the Department of Biochemistry at Cambridge. Left to right
are Joan Keilin, Jim Danielli, Peter Mitchell, Mary Danielli. The ideas
of David Keilin on the cytochromes and Jim Danielli on the lipid
bilayer were seminal in the development of MitchellХs views on
chemiosmosis and vectorial metabolism.
Crofts, A. R. (2004) The Q-cycle, - a personal perspective. Photosynth. Res.
MitchellХsproton pumping loops (Mitchell 1961, 1966).
Crofts, A. R. (2004) The Q-cycle, - a personal perspective. Photosynth. Res.
MitchellХsoriginal Q-cyc le (Mitchell 1975a).
The Modified Q-cycle.
The experiments from
the Crofts lab in the
early 1980’s provided
severe constraints that
limited the types of
plausible Q-cycle
model. This version is
essentially the same as
that proposed by Crofts
et al 1982, and
reviewed by Crofts
(1986)
Crofts, A. R. (2004) The Qcycle, - a personal perspective.
Photosynth. Res.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
http://jfa.bio.qmul.ac.uk/lectures
Why do mitochondria and chloroplasts have genome?
Typical prokaryotic (left) and eukaryotic (right) cells.
W. Ford Doolittle Nature 392, 15-16, 1998
The endosymbiont hypothesis for the origin of mitochondria.
W. Ford Doolittle Nature 392, 15-16, 1998
Problem
Why Do Mitochondria and Chloroplasts Have Their Own
Genetic Systems?
Why do mitochondria and chloroplasts require their own
separate genetic systems when other organelles that
share the same cytoplasm, such as peroxisomes and
lysosomes, do not? …. The reason for such a costly
arrangement is not clear, and the hope that the nucleotide
sequences of mitochondrial and chloroplast genomes
would provide the answer has proved unfounded. We
cannot think of compelling reasons why the proteins
made in mitochondria and chloroplasts should be made
there rather than in the cytosol.
Molecular Biology of the Cell
© 1994 Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson
Molecular Biology of the Cell, 3rd edn. Garland Publishing
Why Do Mitochondria and Chloroplasts Have Their Own Genetic Systems?
Proposed solutions (hypotheses)
There is no reason. “That’s just how it is”. (Anon)
The “Lock-in” hypothesis. (Bogorad, 1975). In order for core components of multisubunit
complexes to be synthesised, de novo, in the correct compartment.
The evolutionary process of transfer of genes from organelle to nucleus is still incomplete.
E.g. Herrmann and Westhoff, 2001: The partite plant genome is not in a phylogenetic
equilibrium. All available data suggest that the ultimate aim of genome restructuring in the plant
cell, as in the eukaryotic cell in general, is the elimination of genome compartmentation while
retaining physiological compartmentation.
The frozen accident. The evolutionary process of gene transfer was underway when something
happened that stopped it. E.g. von Heijne, 1986.
It’s all a question of hydrophobicity. The five-helix rule. (Anon)
Some proteins (with co-factors) cannot be imported. (Anon)
Co-location for Redox Regulation - CORR (Allen 1993, 2003 et seq.)
Bioenergetic organelle
Endosymbiont
Bacterium
Proposed solution (hypothesis)
Why Mitochondria and Chloroplasts Have
Their Own Genetic Systems
Co-location for Redox Regulation - CORR
Vectorial electron and proton transfer exerts regulatory control over expression of
genes encoding proteins directly involved in, or affecting, redox poise.
This regulatory coupling requires co-location of such genes with their gene
products; is indispensable; and operated continuously throughout the transition
from prokaryote to eukaryotic organelle.
Organelles “make their own decisions” on the basis of environmental changes
affecting redox state.
Allen, J. F. (1993) J. Theor. Biol. 165, 609-631
Allen, J. F. (2003) Phil. Trans. R. Soc. B458, 19-38
Prediction
Explanation of previous knowledge
Distribution of genes for components
of oxidative phosphorylation between
mitochondria and the cell nucleus
Redox regulation
Redox regulation
Inter-membrane space
I
II
III
IV
ATPase
Mitochondrial matrix
Inter-membrane space
I
II
H+
III
IV
H+
ATPase
H+
H+
NADH
O2
NAD+
succinate fumarate
H2O
ADP
ATP
Mitochondrial matrix
Redox regulation
The end. Fin. Really. Thank you for listening.