Principles of Bioinorganic Chemistry - 2003

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Principles of Bioinorganic Chemistry - 2003
The grade for this course will be determined by a
term exam (35), a written research paper with
oral presentation (45), problem sets (12) and
classroom participation (8). The oral
presentations will be held in research conference
style at MIT's Endicott House estate in Dedham,
MA, on Saturday, October 18. Please reserve the
date for there are no excused absences. Papers
will be due approximately one week earlier. WEB
SITE web.mit.edu/5.062/www/
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The Major Metal Units in ET Proteins (1)
Iron-Sulfur Clusters
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Properties of Iron-Sulfur Clusters
(A) Rubredoxin FeS, 2.25 - 2.30 Å in
oxidized (FeIII) and reduced (FeII) states
Reduction potentials - 50 to 50 mV (B)
2Fe-2S Ferredoxins (Fd)
Reminder eo -RT/nF lnQ pH, where Q
Mn/Mn-1 Thus, at pH 7, the biological
H2/2H standard couple is - 420 mV.
FeII FeII
FeII FeIII
FeIII FeIII
oxidized
reduced
mixed-valent
all physiological uses
Reduction potentials -490 to - 280 mV
(C) 3Fe-4S Ferredoxins (cube missing a corner)
FeIII 3S4
FeIII 2 FeII S4
Reduction potentials -700 to - 100 mV
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Properties of Iron-Sulfur Clusters, contd
(D) 4Fe-4S Ferredoxins and High-potential Iron
Proteins (HiPIPs)
The three state hypothesis
FeII3 FeIII
FeII2 FeIII2
FeII FeIII3
HiPIP
Ferredoxin
Reduction potentials -650 to - 280 mV (Fd)
350 mV (HiPIP)
minimal reorganizational energy
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The Physical Properties of Iron-Sulfur Clusters
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Structure of an 8Fe-8S Ferredoxin
Primary structure (sequence) does not dictate
the tertiary structure of a metalloprotein, as
revealed by this 8-iron ferredoxin crystal
structure.
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The Major Metal Units in ET Proteins (2)
Blue Copper and CuA
Depicted at the right are the three copper sites
in the enzyme ascorbate oxidase. Type 1, or blue,
copper is the ET center. Below is depicted CuA .
Blue Copper
CuA
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The Physical Properties of Blue Copper Centers
The deep sky blue color of these proteins
facilitated their purification on columns the
optical band is CuS C.T.
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Structure of Poplar Plastocyanin
The copper(II) thiolate center is difficult to
model.
The oxidized, reduced and apo plastocyanin
structures are nearly identical.
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EPR Spectra Distinguish the Three Types of Copper
Found in Metalloproteins
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CuA Model Chemistry Reversible 1-Electron
Transfer
These complexes demonstrate that constrained
dicopper(I/II) units afford good 1-electron
reversible transfer centers and display the
possible environments that could be encountered
in biology.
LeCloux
Chuan He
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The Major Metal Units in ET Proteins (3)
Cytochrome c from tuna showing coordination of
the iron porphyrin group by the protein side
chains from Met (left) and His (right) residues.
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Electronic Properties of Low-Spin
Metalloporphyrins
Note again, minimal reorganization energy upon
electron transfer
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Long-Distance Electron Transfer in Proteins
Three ways to measure
1. Self-exchange 2. Artificial donor-acceptor
pairs 3. Study of natural protein redox pairs
RedAz
OxAz
OxAz
RedAz
CuI CuII
CuII CuI
for azurin
k 1.3 x 106 M-1 s-1
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Artificial Donor-Acceptor Pairs
Cytochrome c Fe---Ru, 12 Å