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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