Download Lecture 15 – Biological Inorganic Chemistry

2P32 – Principles in Inorganic Chemistry
Dr. M. Pilkington
Lecture 15 – Biological Inorganic Chemistry
1. What is biological inorganic chemistry (biochemistry)
2. Functional roles of biological inorganic elements
3. Metal ions and proteins: binding, stability and folding
4. Vitamin B12 - Cobalt an essential element for life
5. Biomineralization
6. Metals in medicine
7. Poisoning by metals
1. What is biological inorganic chemistry (bioinorganic chemistry)?

An interdisciplinary research field at the interface of the more classical areas
of inorganic chemistry and biology/biochemistry.

Understanding the roles that metallic and nonmetallic elements
play
l in
i bi
biological
l i l systems
s st ms is the
th goall of
f bi
biological
l i l inorganic
i
i
(bioinorganic) chemistry.

There are two main fields of bioinorganic chemistry:
1.
Investigations of inorganic elements in processes e.g. nutrition, the toxicity of
inorganic species, including the ways in which such toxicities are overcome both
by natural systems and by human intervention, and of metal-ion transport and
storage in biology.
2.
The introduction of metals (metal complexes) into biological systems as probes
and drugs.
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
The familiar elements C, H, N, O, P and S, the big six, which are well covered in
biochemistry texts provide the major building blocks for cellular components
including
proteins,
nucleic
acids,
lipids-membranes,
polysaccharides
and
metabolites.

Despite this organic diversity,
diversity life cannot survive with only these principle
elements.

Inorganic elements are also essential to life processes - eleven elements of the
periodic table are required for all forms of life and an additional seven or eight
elements are used by organisms on our planet.

Blood known to contain iron since the 17th century.

Need for Zinc, 1896.
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Transition Elements Relevent to Bioinorganic Chemistry
The Biometals
Why does biology utilize transition metals ?
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2.
Functional roles of selected biological inorganic elements

Charge balance and electrolytic conductivity: Na, K, Cl

Structure and templating: Ca, Zn, Si, S

Signaling: Ca, B, NO

Bronstead Acid-Base Buffering: P, Si, C

Lewis Acid-Base
Acid Base Catalysis: Zn,
Zn Fe
Fe, Ni
Ni, Mn

Electron Transfer: Fe, Cu,

Group Transfer (e.g. CH3, O, S): V, Fe, Co, Ni, Cu, Mo, W

Redox Catalysis: V, Mn, Fe, Co, Ni, Cu, W, S, Se

Energy Storage: H, P, S, Na, K, Fe

Biomineralization: Ca, Mg, Fe, Si, Sr, Cu, P

Owing to the great advances in research in biological inorganic chemistry we now know the
structures of
f many components of
f the
h systems that
h biology
b l
has
h adapted
d
d through
h
h evolution
l
to perform these essential functions. Many relationships between structure and function
have been elucidated.

Biological inorganic chemistry has also profoundly impacted both environmental science and
medicine
Selected metal ions and their function together with typical
deficiency symptoms
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3.
Metal ions and proteins: binding, stability and folding

Life has evolved with the minerals of the Earths crust and the ions in the
Earths waters.

Therefore it is not surprising that living beings have evolved the capability to
use inorganic elements for key biological processes and to defend themselves
from poisoning by other elements.
elements

Some metal ions, when associated with polypeptides, can help catalyze unique
chemical reactions and perform specific physiological functions. We call such
metal ions “metal cofactors”.

Amino acids and proteins alone are not sufficient to perform all the
reactions needed for life. For example, the Fe3+/Fe2+ and Cu2+/Cu+ redox
couples play critical roles as cofactors for electron transfer reactions in the
catalysis of redox reactions.
reactions

The Fe2+ ion can reversibly bind dioxygen (O2) if a coordination site is available.

In the periodic table those metal ions essential for life are highlighted in
green. Some of these e.g. Fe, Cu and Zn are strongly associated with proteins
and form the so-called metalloproteins.
For example, ferritin the metalloprotein that stores iron in the body.



In mammals iron is bound and transported by the serum protein transferrin, and
it is stored by ferritin in most life forms.
Ferritin is a spherical molecule with an outer coat of protein and an inner core of
hydrous ferric oxide [FeO3(H2O)n].
As many as 4500 atoms of Fe can be stored in a single ferritin molecule.
A three-dimensional representation
showing ferritin, the iron-storage
protein in the body.
Ferritin has a spherical shape, and iron
(brown) is stored as a mineral inside the
sphere.
We do not yet fully understand the
control of Fe loading during abundance
and mobilization during scarcity.
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
Some metal ions are found deeply buried within proteins.

Such metal ions are often “structural” in function.

Their interaction with the protein helps insure the optimal protein structure and
contributes to the stability and appropriate acid-base behavior necessary for
the physiological function.

For example,
example the Zn2+ ions in Zn fingers which are transcription factors are
necessary for the adoption of the proper shape of the protein, which allows it to
interact with DNA. It is not currently known if the zinc ion plays more than a
structural role in this proteins i.e. if the Zn2+ concentrations are also used in
some manner to regulate gene expression.
Structure of the first zinc finger. Residues 13, 15,
16 and 19 are implicated in DNA recognition, in
this case the base triplet GCG. The zinc ion is in
the lower right portion of the structure and is
chelated by two cysteines and two histidines.



In multicellular organisms, sodium and calcium are found mostly outside the
cellular compartment (extracellular), while potassium and magnesium are largely
intracellular.
Calcium and magnesium are often metal activators in proteins to which they bind
with relatively low affinity.
Under appropriate circumstances, these metal ions induce conformational
changes in the protein upon binding and in doing so they may transmit a signal e.g.
the firing of neurons by rapid influx of sodium ions across a cell membrane

Or the regulation of intracellular functions by calcium binding proteins such as
calmodulin.
Structure of calmodulin where all four sites
are occupied with calcium ions, and the linker
has formed a long alpha helix, separating the
two calcium-binding domains.
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4. Vitamin B12 – cobalt an essential element for life.

Cobalt appears centrally in the periodic table, and with its neighbors, iron,
manganese, nickel and copper, has a vital role in a number of biochemical
metalloenzyme reactions.

Vitamin B12, also called cobalamin,, is a water-soluble vitamin with a key
y role in
the normal functioning of the brain and nervous system, and for the formation
of blood.

It contains the biochemically rare element cobalt.

It is one of the eight B vitamins.

It is normally involved in the metabolism of every cell
of the human body, especially affecting DNA synthesis
and regulation, but also fatty acid synthesis and
energy production.
Vitamin B12 has a porphryin core:
The macrocycle has 26 π electrons in total.
 Porphyrins are heterocyclic macrocycles composed of four modified pyrrole
subunits interconnected at their α carbon atoms via methine bridges.
 Porphyrins are aromatic. That is, they obey Hückel's rule for aromaticity,
possessing 4n+2 π electrons (n=4 for the shortest cyclic path) delocalized over
the macrocycle.
 Thus porphyrin macrocycles are highly conjugated systems. As a consequence,
they typically have very intense absorption bands in the visible region.
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
One of the best-known porphyrins is heme, the pigment in red blood
cells; heme is a cofactor of the protein hemoglobin.

Vitamin B12 was discovered from its relationship to the disease pernicious
anemia, which is an autoimmune disease in which parietal cells of the stomach
responsible for secreting intrinsic factor are destroyed. Intrinsic factor is
crucial for the normal absorption of B12, so a lack of intrinsic factor, as seen in
pernicious anemia, causes a vitamin B12 deficiency.
In pernicious anemia, the body does not make enough
red blood cells.
Medical uses of Vitamin B12

Vitamin B12 is used to treat vitamin B12 deficiency, cyanide
poisoning, and hereditary deficiency of transcobalamin II.
It is given as part of the Schilling test for detecting
pernicious anemia.
High vitamin B12 level in elderly individuals may protect against
brain atrophy or shrinkage associated with Alzheimer's disease
and impaired cognitive function
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
Small red crystals of Vitamin B12 were then grown by Lester Smith and given to
Dorothy Hodgkin for X-ray crystal structure analysis. All that was known at this
stage was that the approximate empirical formula was:
C61-64H84-90N14O13-14PCo.

A crystal
t l structure
t
t
on a molecule
l
l of
f this
thi size
i
and
d complexity
l it had
h d never been
b
attempted before, it was a huge and complex task, since crystal structure
determinations were not the routine tasks that they are today, and the
techniques were still being developed, both the X-ray and the computer
equipment were tedious and difficult to use. Thus the X-ray crystal structure
which emerged from this study between 1950 the early 1960’s was the first
determination of a chemical formula by X
X-ray
ray diffraction,
diffraction and the first
determination of the structure of a metalloenzyme.

This achievement is recognized as the birth of Biochemistry.
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
It
was
Hodgkin
a
triumph
and
her
crystallography
for
Dorothy
Oxford
group,
X-ray
inspiring
many young crystallographers, and
pointing them to biochemistry as
an exciting new subject for their
endeavors.

The structure work also caused Woodward (at Harvard) and Eschenmoser (at
the Swiss Federal Institute of Technology) to start synthetic work on Vitamin
B12.

The synthesis took 11 more years,
years and involved more than 90 separate reactions
performed by over 100 co-workers.

The stereochemical puzzles involved in the synthesis led to the WoodwardHoffman rules.


This all adds up to three Nobel prizes in chemistry and one in medicine!
Vitamin
B12
is
a
metalloezyme,
about
40%
of
metalloproteins
are
metalloenzymes.

Metalloenzyme- metal ions main role is to function in enzymatic reactions.
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Examples of enzymes classified by metal centres
5. Biomineralization

The biological process that give rise to bones, shells and teeth is called biomineralization.

Over the last two decades, the study of biomineralization has shifted more toward a more
chemical perspective and in doing so has become established as a new branch of bioinorganic
chemistry that represents the length scale and interplay between biological processes and
inorganic chemistry.
chemistry

The research aims of biomineralization include the structural and compositional
characterization
of
biominerals,
understanding
the
functional
properties
of
biominerals, and elucidation of the processes through which organic macromolecules and
organic structures control the synthesis, construction and organization of inorganic
mineral-based materials. Two examples of biominerals are:
1
1.
C l i
Calcium
Bi i
Biominerals
l – shells
h ll and
d mineralized
i
li d tissues,
ti
such
h as bone
b
and
d teeth
t th are composed
d
of calcium carbonate or calcium phosphate minerals, combined with a complex organic
macromolecular
matrix
of
proteins,
polysacharides
and
lipids.
Calcium
carbonate
biominerals such as calcite and aragonite are used for structural support.
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Calcium Biominerals

The mother of pearl layer of seashells is
a
laminate
of
0.5-m
thick
calcium
carbonate (aragonite) polygonal tablets
sandwiched between thin 30–nm sheets if
a protein-polysacharide organic matrix.

The matrix plays a key spatial role in
limiting the thickness of the crystals and
is
structurally
important
in
the
mechanical “design” of the shell.

Biominerals
also
have
some
unusual
functions. For example crystals of calcite
are used as gravity sensors in a wide
range of animals.
animals

The optical properties of calcite are
exploited in the lenses of the compound
eyes of extinct creatures called trilobites
which are preserved as fossils.

Trilobite – extinct arthropods that dissapeared about 250 million years
ago.
The name trilobite mans three lobed since they are made up of three
body sections, a longitudinal lobe, a central axis lobe and two
symmetrical pleural lobes that flank the axis
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Bone and teeth are made from calcium phosphate in the form of a mineral

hydroxyapatite (HAP).
The structure and mechanical properties of bone are derived from the

organized mineralization of HAP within a fibrous matrix of a structural protein,
collagen along with proteins with sugar sidechains.
The distinction between an inorganic and a bioinorganic mineral is clearly seen

in bone, which is close to being described as a “living mineral” since it
undergoes continual growth, dissolution and remodeling.
2.
Iron Oxides – bioinorganic iron oxides are widespread and serve several
functions.

A mixed valence compound magnetite (Fe3O4) is of special biological relevance.

Magnetite is synthesized in a wide range of magnetotactic bacteria.

These organisms are aligned in the Earth’s magnetic field such that in the
northern hemisphere they swim downward (north seeking) toward the oxygen
depleted zone at the sediment-water interface of fresh water and marine
environments.
Magnetic Microbes- magnetotactic bacteria.
 Magnetotactic bacteria were discovered in 1975 by Richard P. Blakemore.
Blakemore noticed that some of the bacteria that he observed under a microscope
always moved to the same side of the slide.
 If he held a magnet near the slide, the bacteria would move towards the north end
of the magnet. These bacteria are able to do this because they make tiny, ironcontaining, magnetic particles.
 Each of these particles is a magnet with a north pole and a south pole. The bacteria
arrange these tiny magnets in a line to make one long magnet. They use this magnet
as a compass to align themselves to the earth's geomagnetic field.
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
Why would these bacteria need a compass? Like many other types of bacteria,
magnetotactic bacteria don't like oxygen very much. They will move away from
areas with high oxygen and toward areas with low or no oxygen. In an aquatic
environment, the level of oxygen decreases as one moves deeper into the water.
So, magnetotactic
g
bacteria like to live in the deeper
p parts
p
of their aquatic
q
environments. They use their magnetic compass to tell them which way is down.

Scientists are also interested in practical applications involving these magnetic
microbes.

While it isn't likely we'll be using these bacteria to stick notes to our
refrigerators, they could prove to be useful to humans.

The tiny magnets that these simple organisms make are far superior to those
produced by people. So, scientists and engineers are trying to develop ways to
use this magnetic material in places where tiny magnets are much better than
big magnets.
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6.




Metals in Medicine
The use of iron and copper can be traced to the ancient Greeks and Hebrews
through their writings.
Among metal ions commonly used over the centuries were Hg2+ for the
treatment of syphilis, Mg2+ for intestinal disorders, and Fe2+ for anemia.
It is seldom useful to describe elements as “toxic” or “nontoxic”. Even socalled
ll d toxic
t i compounds
d can usually
ll be
b tolerated
t l
t d in
i low
l
doses,
d
and
d may exhibit
hibit
therapeutic effects within narrow concentration ranges, and biochemically
essential elements can be toxic at high doses.
The Betrand diagram schematically summarizes this situation:
All things can be poisons
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Essential element dosage and physiological response
Metal homeostasis - trafficking, and sensing pathways that allow organisms to
maintain an appropriate (often narrow) intracellular concentration range of essential
transition metals.
Dose response – Non essential elements
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Recommended Daily Allowances for Inorganic Elements
in the Human Body

Some of the areas of medicinal inorganic chemistry are shown below:
.


Today inorganic chemistry is beginning to have a major impact in modern
medicine.
Both essential and nonessential metals can be used in therapy and diagnosis.
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
Compounds in current clinical use
are summarized to the right.

It is important to ask which parts
of the compound are essential for
activity the metal itself,
activity,
itself the
ligands or the intact complex of
metal plus at least some of the
ligands.

Many metallodrugs are “prodrugs”
they undergo ligand substitution
and/or
d/ redox
d reactions
i
b
before
f
they reach the target site.
Metallotherapeutics – cancer is one of the top three killers worldwide and is a
difficult disease to treat. It is hard to find drugs that are both effective and have
low toxicity to the human body as a whole.
Three important inorganic pharmaceuticals are:
1. Platinum
m drugs
g such as cisplatin
p
– an anticancer drug
g
There is a need to develop new Pt anticancer drugs because cisplatin is a very toxic
compound with severe side effects such as kidney poisoning. Activity is requires
against a wider range of cancer types such as lung, breast and colon cancers. Cancer
cells can also become resistant to cisplatin after repeated treatments. Other drugs
related to cisplatin that are approved for clinical use are carboplatin, and nedaplatin.
Oxaliplatin (trade name Eloxatin) was approved for clinical use in August 2002 for
use in the treatment of metastatic carcinoma of the colon or rectum as a combination
infusion with 5-fluorouracil and leuovorin.
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2.
Gold antiarthritic drugs e.g. Auranofin

Injectible Au(I) thiolate drugs and one oral Au(I) phosphine drug (Auranofin) are widely
used in clinics today for the treatment of difficult cases of rheumatoid arthritis. There is
also interest in the potential use of Au compounds for treating asthma, malaria, cancer and
HIV.

Auranofin is thought to deposit Au(I) in the lysosomes (intracellular compartments that
house destructive enzymes) and inhibit the enzymes that destroy joint tissues.

There is still much to learn - the cause of rheumatoid arthritis remains unknown to-date.
3.
Radiodiagnostic and radiopharmaceutical drugs e.g.Cardiolyte – a heart imaging agent

Radionucleotides are used for both imaging and therapy, 99mTc is used in > 85% of all
diagnostic scans in hospitals because of its ideal properties. i.e. it has a half life long
enough to allow accumulation in the target tissue, yet short enough to minimize the
radiation dose to the patient.

99mTc

Cardiolyte is a +vly charged 99mTc complex
complex. Hundreds of isonitrile complexes were
investigated to obtain the optimum balance between uptake and clearance in the heart
compared to other organs.
is a -emitting radionucleotide.

The six methoxy ligands are sequentially metabolized in the liver to hydroxyl groups.

This transformation turns the complex into increasingly hydrophilic species which are not
retained in myocardial tissue.
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Examples of Metal-based drugs
7. Poisoning by Metals

Hg2+, Pb2+ and Ti+ are poisonous by any dose.

Fe and Cu are poisonous in excess (they are excreted, excess only becomes a
problem in cases of genetic diseases that affect the excretion of excess.

Fe(toxicity)- inherited – thalassemia (treated with desferrioxamine)

Cu(toxicity) –inherited – Wilson’s disease (British Anti-lewisite helps with this
disease it can get rid of copper build up).

See the important ligands in the bioinorganic chemistry handout.
Cl
H
C
H
C
As
Cl
+ BAL
Cl
Lewisite (nerve gas)
shuts down respiritory
system
H
Cl
C
H
C
S
As
CH2OH
S
neutralizes the effect of Lewisite
and it can be excreted.
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