Principles of Bioinorganic Chemistry - 2004

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Principles of Bioinorganic Chemistry - 2004
The grade for this course will be determined by a
term exam (35), a written research paper with
oral presentation (55), and problem sets (10).
The oral presentations will be held in research
conference style at an all-day symposium at MIT
on Saturday, October 30th. Please reserve the
date for there are no excused absences. Papers
are due October 28th. Problem sets are due one
week after their assigned date. Recitations are
held at 5 PM on Mondays. WEB SITE
web.mit.edu/5.062/www/
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Control and Use of Metal Ion Concentrations
PRINCIPLES

• Homeostasis maintain M in proper range
• Detoxification remove excess and/or unnatural
  metal ions
• Extracellular carriers
• Passive transport
• Ion channels/pumps
• Metalloregulation

• Binding and release of metal ions to receptors
  controlled by pH and redox changes
• Ion concentration gradients - used to transmit
  energy and information

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Note hinge motion that accompanies iron/carbonate
binding
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Transferrin and Structural Changes on Fe Binding
Baker, Anderson, and Baker, PNAS, 2003, 100, 3579.
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Various Anions Can Bind Transferrin
Nomenclature Fbp, ferric binding proteins n,
for Neisseria meningitidis Iron must bind as
Fe(III), or the ferric state. If reduced, a
bacterial reductase must be involved, thus
affording control of iron binding and uptake in
the organism (see E1/2 values in the table above.
Crumbliss, et al. PNAS, 2003, 100, 3659.
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Mechanism of Transferrin Uptake and Iron Release
in Cells by Receptor-Mediated Endocytosis
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Metal Regulation of Gene Expression
PRINCIPLES

• Metal-mediated protein structure changes affect
  transcription
• Metal-mediated protein structure changes affect
  translation
• Apo vs holo metalloproteins bind DNA/RNA
  differently
• Metalloregulatory protein is the sensor -
  inorganic chemistry
• Metal-induced protein structure changes also
  activate enzymes
• Metal-induced protein structure changes are
  metal-specific

ILLUSTRATIONS

• Iron regulatory proteins (IRPs) control Ft and
  Tf translation
• Regulation of a toxic metal, mercury
• Zinc finger proteins control transcription
• Ca2, a second messenger and sentinel at the
  synapse

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Regulation of Iron Levels in Cells
The Players

• Ferritin, the iron storage protein 24-subunits,
  175 aa each has cubic symmetry apoFt can house
  1000 iron atoms in its central core a
  ferroxidase center loads the iron into the
  protein
• Transferrin, the uptake protein, discussed
  previously

Metalloregulation

• In bacteria, occurs at the transcriptional level
• In mammals, the synthesis of apoferritin and of
  the transferrin receptor are regulated at the
  level of translation, not transcription

Central dogma of molecular biology DNA
mRNA Protein
transcription
translation
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Ferritin Subunit and Channel Structure
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Ferroxidase Center Loads Fe into ApoFt
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Mixed-valent polyiron oxo cluster prepared as a
model for ferritin core formation intermediates.
Taft, et al., Science 1993, 259, 1302
Overall formula Fe12O2 (OCH3)18(O2CCH3)
6(CH3OH)n
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Reminder Apo (left) and Holo (right) Forms of
Transferrin Only Iron-Loaded Transferrin Binds to
the Receptor
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Metalloregulation of Iron Uptake and Storage
Bacteria A single protein, Fur (for iron uptake
regulator), controls the transcription of genes
involved in siderophore biosynthesis. Fur is a
dimer with subunits of Mr 17 kDa. At high iron
levels, the Fur protein has bound metal and
interacts specifically with DNA repressing
transcription. Mammals Expression of ferritin
and the transferrin receptor is regulated at the
translational level.
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Components of the Metalloregulatory System
IRP
Iron-responsive protein (IRP)
Stem-loop structure in the mRNA
IRP
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Regulation events High Fe, low TfR, high Ft Low
Fe, high TfR, low Ft
Fe
IRP
Message translated
Message degraded
Ferritin
Transferrin
IRP
Message blocked
Message translated
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IRP1 is the Cytosolic Aconitase Contains an Fe4S4
Cluster
Cluster assembled in protein, which then
dissociates from mRNA
Apoprotein stays associated with mRNA
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Regulation of a Toxic Metal, Mercury
The problem Mercury in the environment of
industrial plants is converted by bacterial to
harmful organomercury compounds. Fish and other
plant and animal life assimilate the mercury
which ultimately enters the human food chain.
Bacteria defend themselves against the mercury
by using the proteins listed below. The
players Organomercurial lyase Mercuric ion
reductase MerR, the intracellular mercuric ion
sensor The implications Transcription of the
genes encoding the proteins is controlled by
MerR in response to mercury levels
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Postulated Mechanism for Organomercurial Lyase
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MerR and Mercuric Ion Reductase Properties
Reductase no structural or detailed mechanistic
information
MerR EXAFS spectroscopy and chemical
modification experiments indicate that Hg-MerR
has a 3-coordinate, Hg(S-Cys)3 environment with
an average HgS distance of 2.43 Å. This unusual
tridentate heavy metal receptor site is
consistent with the thermodynamic stability of
Hg(SR) 3- complexes and may account both for
the high affinity of the Hg(II) binding and
for the selectivity for Hg(II) over other soft
metal ions that prefer tetrahedral metal-thiolate
coordination.
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Effect of Hg2 on Transcription Activity