Subject Chemistry

Paper No and Title 15:

Module No and 1: Introduction: Bioinorganic Chemistry Title Module Tag CHE_P15_M1

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

TABLE OF CONTENTS

1. Learning Outcomes 2. Introduction 3. Metal function in 4. Functions of metalloenzymes 5. Communication roles for metals in Biology 6. Interactions of metal ions and nucleic acids 7. Metal-Ion Transport and Storage 7.1 General aspects of storage and transport of metal-ions 7.2 : Function, Storage and Transport 7.2.1. Function 7.2.2. Iron storage 7.2.2.1. Ferritin 7.2.2.2. Hemosiderin 7.2.3. Transport of Iron: Transferrin 7.3 Calcium: Function, Storage and Transport 7.3.1. Function 7.3.2. Calcium Storage 7.3.3. Calcium Pump 7.4 : Function, Storage and Transport 7.4.1. Function 7.4.2. Storage of Copper 7.4.3. Copper transport 8. Summary

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

1. Learning Outcomes

After studying this module, you shall be able to

 Know the importance of inorganic elements.  Categorize the inorganic elements according to their roles in the biological system.  Learn the function of important elements such as Iron, Calcium and Copper.  Identify the general aspects of storage and transport of metal-ions.  Know the role of metals in medicine.

2. Introduction

Bioinorganic chemistry constitutes the discipline at the interface of the more classical areas of inorganic chemistry and biology. Although biology is generally related to organic chemistry, inorganic elements are also important to life processes. Table 1 lists the essential inorganic elements together with some of their known roles in biology. Bioinorganic chemists study these inorganic species according to their function in vivo.

Inorganic elements have also been artificially introduced into biological systems as probes of structure and function. Heavy metals such as and are used by X-ray crystallographers and electron microscopists to help elucidate the structures of macromolecules. Paramagnetic metal ions have been valuable in magnetic-resonance applications. Metal-containing compounds have been used not only as biological probes, but also as diagnostic and therapeutic pharmaceuticals. The mechanisms of action of platinum anticancer drugs, antiarthritic agents, and technetium radiopharmaceuticals are some currently active topics of investigation in bioinorganic chemistry.

Bioinorganic chemistry, thus has two major components: the study of naturally occurring inorganic elements in biology and the introduction of metals into biological systems as probes and drugs. Peripheral but essential aspects of the discipline include investigations of inorganic elements in nutrition, of the toxicity of inorganic species (including the ways in which such toxicities are overcome both by the natural systems and by human intervention), and storage and transport of metal-ions in biology.

Table 1. Essential inorganic elements and their role in biology.

Metal Function Charge carrier; osmotic balance Charge carrier; osmotic balance Structure; hydrolase; isomerase Calcium Structure: trigger; charge carrier Nitrogen fixation; oxidase Molybdenum Nitrogen fixation; oxidase; oxo transfer Tungsten Dehydrogenase

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

Manganese Photosynthesis; oxidase; structure Iron Oxidase; dioxygen tramped and storage; electron transfer: nitrogen fixation Oxidase; alkyl group transfer Hydrogenase; hydrolase Copper Oxidase; dioxygen transport: electron transfer Structure; hydrolase

3. Metal Function in Metalloproteins

Metals are commonly found as natural constituents of proteins. Nature has learned to use the special properties of metal ions to perform a wide variety of specific functions associated with life processes. Metalloproteins that perform a catalytic function are called metalloenzymes.

Metalloproteins are a class of biologically important macromolecules and account for nearly half of all proteins in biology. They are responsible for performing some of the most difficult yet important functions, including photosynthesis, respiration, water oxidation, oxygen transport, electron transfer, oxygenation and nitrogen fixation.

4. Functions of Metalloenzyme

Metalloenzymes are a subclass of metalloproteins that perform specific catalytic functions. A net chemical transformation occurs in the molecule, termed a substrate, being acted upon by the metalloenzyme. Some remarkable transformations for which no simple analogues exist in small- molecule chemistry under comparable conditions includes the catalytic reduction of N2, to NH3, (nitrogen fixation), the oxidation of water to O2, and the reduction of gem-diols to monoalcohols (reduction of ribonucleotides).Classification of Metalloenzymes is done as per their function. Within each category there are usually several kinds of metal centers that can catalyze the required chemical transformation, a situation analogous to that already encountered for respiratory proteins. The reasons for this diversity are shrouded in evolutionary history, but most likely include bioavailability of a given element in the geosphere biosphere interface during the initial development of a metalloenzyme, as well as pressure to evolve multiple biochemical pathways to secure the viability of critical cellular functions.

5. Communication Roles for Metals in Biology

Metal ions are used in biology as triggers for specific cellular functions, and to regulate gene expression. Studies of these cellular communication roles are an exciting frontier area in bioinorganic chemistry. Magnetotactic bacteria use magnetite, Fe3O4, as an internal compass for navigation of the microorganisms. They orient on the Earth's magnetic pole and, when transported to the opposite hemisphere, become disoriented and swim upward. Some bees, homing pigeons, and even humans are also believed to use magnetite in their brains for orientation purposes. Alkali and alkaline earth ions, especially Na+, K+, and Ca2+, are used in biology to trigger cellular responses. The firing of neurons by the rapid influx of sodium ions across the cell membrane and CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

the regulation of intracellular functions of calcium-binding proteins such as calmodulin are two examples of the phenomenon. In fact, Ca2+ has been referred to as a "second messenger," since primary signals, such as the binding of hormones to the cell surface, are converted into changes in the intracellular concentration of this ion. Among the most recently discovered metal-ion classes in biology are the zinc fingers that occur in many proteins which regulate transcription.

6. Interactions of Metal Ions and Nucleic Acids

Metal ions also interact directly with DNA and RNA. Some of these interactions are rather nonspecific, for instance, the stabilization of nucleic-acid structures by Na+ and Mg2+ ions through electrostatic interactions that shield the charged phosphate groups from one another. Recently, more specific binding of metal ions to nucleic acids has been discovered. Thus, Mg2+ and other divalent metal ions serve as cofactors for activating catalytic RNA molecules; and monovalent cations such as K+ stabilize the structure of telomeres, units that terminate the DNA double helix at the ends of chromosomes. The relative stability of these structures may be dictated by the concentrations Na+ and K+ in the cell. Finally, some inorganic-based drugs such as cisplatin act by coordinating directly to DNA and metal complexes have been used as cleaving agents to probe, the tertiary structures of nucleic acids.

7. Metal-Ion Transport and Storage

How do metal ions get into cells and how are they stored? This topic is an active area of investigation in bioinorganic chemistry, although it cannot be classified with the others according to metal function. The most thoroughly studied metal in this respect is iron. Iron enters bacterial cells following by low-molecular-weight compounds called siderophores that are excreted by the bacteria. In mammals, iron is bound and transported by the serum protein transferrin, and it is stored by terrain in most life forms. The nearly spherical, hollow shell of this latter protein has the capacity to bind up to 4,500 Fe3+, ions. Details about how iron is passed among these protein systems are incomplete and under active investigation. Copper is transported by the serum protein , and another such protein, albumin, is also known to bind and transport metal ions. Metallothionein is a cysteine-rich protein that is expressed in large amounts when excess quantities of certain metal ions, including toxic ones such as Cd2+ or Pb2+, are present in cells. Metallothionein thus serves a protective role and may also be involved in the control of metal transport, storage, and concentration under more normal conditions.

7.1. General aspects of storage and transport of metal-ions

 Charged Ions must pass through a Hydrophobic Membrane  Neutral gases (O2, CO2) and low charge density ions (anions) can move directly through the membrane  High charge density cations require help  Once inside the cell, metal ions must be transported to the location of their use, then released or stored for later  Release from ligand is often not trivial  Storage requires additional molecules CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

7.2. Iron: Storage and transport Iron is essential in small amounts for both plant and animal life. However, it is toxic in large quantities.  Biologically iron is the most important transition element and it is involved in several different processes:  As an oxygen carrier in the blood of mammals, birds and fish (Hb).  For oxygen storage in muscles tissue (Mb).  As an electron carrier in plants, animals and bacteria (cytochromes) and for electron transfer in plants and bacteria (ferredoxins).  For storage and transport of Fe in animals (ferretin and transferrin).  As nitrogenase (the enzyme in dinitrogen fixing bacteria).  As a number of other enzymes: aldehyde oxidase, catalase and peroxidase and succinic dehydrogenase.  Iron enters bacterial cell following chelation by low-molecular-weight components called siderophores that are excreted by bacteria in most life forms.  In mammals, iron is bound and transported by serum protein transferrin, and it is stored by ferritin. 7.2. 1. Iron storage Iron which has been released into the cell must either be used immediately for biosynthesis or stored in a safe form.  In humans, iron is stored mainly in the bone marrow, spleen and liver.  About 10% of all the iron in the body is in storage.  Two proteins are involved in iron storage:- . Ferritin . Haemosiderin.

7.2.1.1. Ferritin

 Ferritin is a huge hollow spherical protein, with a wall mostly made up of α-helical peptide chains. (Figure 2)  Fe2+ is oxidized and transported into the ferritin interior and deposited as an iron mineral core, traditionally described as ferrihydrite, which is attached to the inner wall of the sphere. Up to 4500 Fe can be stored although the normal value is about 2000.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

Figure 2. Ferritin

7.2.1.2. Hemosiderin  Hemosiderin is another storage form for iron in organism, in particular during iron overloading.  Hemosiderin was first isolated from horse spleen in 1929.  Although the structures of the iron cores of ferritin and hemosiderin are similar, the protein component of hemosiderin is largely unknown.  The iron/protein ratio in hemosiderin is even higher than in ferritin and is assumed that this insoluble species is formed via lysosomal decomposition of ferritin.  Protease in the lysosome degrade the protein shell of ferritin, the released iron core dissociates and reforms as the amorphous hemosiderin.

7.2.2. Transport of iron: Transferrin  Transferrins are iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids. (Figure 3)  Transferrin binds iron very tightly, but reversibly.  Contains two specific high-affinity Fe (III) binding sites.  The affinity of transferrin for Fe (III) is extremely high (1023 M−1 at pH 7.4) but decreases progressively with decreasing pH below neutrality.  Transferrin without iron known as “ apotransferrin ”.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

Figure 3. Transferrin  Once carbonate binds in this site, the metal binding site is fully organised, so that the Fe (III) can now coordinate in an octahedral, oxygen rich environment of ligands favourable for such a relative hard metal ion.

7.3. Calcium 7.3.1. Function: Signal pathways (Ex: Muscle Contraction) Skeletal Material  Concentration: Outside of Cell [Ca2+] = 0.001 M Inside Cell [Ca2+] = 10-7 M  Ca2+-ATPase in Cell Membrane controls concentration

7.3.2. Calcium storage  CaCO3 in a protein matrix makes up egg shells and coral skeletons  Calcium Hydroxyapatite in a collagen framework is the stored form of Ca2+ in bones and teeth: Ca10(PO4)6(OH)2  Collagen: triple helix fibrous protein  Hydroxyapatite crystallizes around the collagen When needed, Ca2+ can be released and reabsorbed

7.3.3. Calcium pump  Calcium pumps are ATPases that transport ions using energy obtained from the hydrolysis of ATP across membranes.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

 Calcium ATPases are members of the P-type family of ion pumps, which are responsible for the ATP dependent active transport of ions across a wide variety of cellular membranes.  The protein is far less stable when these calcium ions are removed. It was solved by adding a drug molecule that binds near the calcium-binding site and freezes the protein into a stable, but non-functioning form.  The calcium pump is an amazing machine with several moving parts. It is found in the membrane.  It has a big domain poking out on the outside of the sarcoplasmic reticulum, and a region that is embedded in the membrane, forming a tunnel to the other side.  When an ATP breaks, calcium pump transfers two Ca2+ ions through the membrane and two or three hydrogen ions back in opposite direction.  In this process of pumping, a phosphate is transferred from the ATP to a special aspartate amino acid in the pump, number 351.  The switching is controlled by large motions of the ATP-binding domains, which push and pull on the protein surrounding the tunnel, opening and closing it appropriately.  Permanent activation of the pump, by calmodulin, is physiologically as harmful to cells as its absence.  The concept is now emerging that the global control of cell Ca2+ may not be the main function of the pump; in some cell types, it could even be irrelevant.  The main pump role would be the regulation of Ca2+ in cell microdomains in which the pump co-segregates with partners that modulate the Ca2+ message and transduce it to important cell functions.

7.4. Copper 7.4.1. Function O2 transport (hemocyanin in crustacean and mollusks) O2 activation (Cu oxidases) electron transfer (plastocyanin)  Availability Third most abundant transition metal ion in organisms 300 mg in a human body -19 2+ -5 Ksp(Cu(OH)2) = 2.6 x 10 [Cu ] = 2.6 x 10 Solubility means less specialized transport and storage

7.4.2. Storage of Copper  After its hepatic uptake, copper may be stored within hepatocytes, secreted into plasma, or excreted in bile.  The biliary route represents the major excretory pathway of copper and largely accounts for its hepatic turnover.  Copper retamed by hepatocytes is mostly bound to specific metal-binding proteins, primarily metallothionein, or incorporated into several cuproenzymes.  Impairments of homeostatic mechanisms in brain copper metabolism have been associated with neurodegeneration in human disorders such as Menkes disease, Wilson's disease and Alzheimer's disease.

7.4.3. Copper transport

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

 Intakes of copper at doses that exceed physiologic demands are normally met with efficient homeostatic mechanisms.  Ceruloplasmin, albumin, and transcuprein, and to a lesser extent certain amino acids, are major copper-transporting constituents in circulating plasma.  Studies with radioactive copper have established that after intestinal absorption copper appears in the portal bloodstream bound to albumin and, to a much smaller extent, amino acids. This copper is rapidly transported to the liver.  Considering the number of copper atoms massed within its structure, ceruloplasmin can be considered to be an effective vehicle for shuttling copper out of the liver and delivering clusters of copper atoms into a variety of nonhepatic cells and tissues.

8. Metals in medicine

Many first encounter the use of metals in medicine as well as metal toxicity through literature. The use of iron and copper can be traced to the ancient Greeks and Hebrews through their writings. Lewis Carroll's Mad Hatter suffered from mercury poisoning. Among metal ions commonly used over the centuries were Hg2+ for the treatment of syphilis. Mg2+ for intestinal disorders, and Fe2+ for anemia. These early examples represent crude approaches, the refinement of which did not, until recently, begin to match in sophistication or efficacy the contributions of organic chemists, who introduced sulfa drugs, penicillin, and mechanism-based inhibitors such as metho-trexate.

One of the leading anticancer drugs is cis-[Pt(NH3)2Cl2]. Cisplatin administered by intravenous injection for the treatment of testicular, ovarian, and head and neck tumors. Cisplatin cures testicular cancer, when diagnosed early enough, in more than 90 percent of the cases. Auranofin, [Au(PEt3)(ttag)], where ttag is tetra-O-acetylthioglucose, is the first orally administered drug for the treatment of rheumatoid arthritis. It is an important member of the class of antiarthritic gold + agents, the others of which are injected. The third example is [Tc(CNR)6] , M which the technetium is supplied as its 99mTc radioisotope. This class of complexes is selectively taken up by myocardial tissue and has proved to be excellent for imaging the heart. Platinum, gold, and technetium, three nonessential transition elements, have found a place in medicine. Given the range and variety of inorganic compounds, the potential applications of inorganic chemistry to improving human health are boundless. This aspect of bioinorganic chemistry, now in its infancy, seems destined to be an area of rapid and significant growth.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

Figure 4. Metals in Medicine: Therapy and Diagnostic

Anticancer Therapeutics  Cisplatin was the first metal-based medicinal agent to enter into worldwide clinical use for the treatment of cancer. Figure 5 displays various platinum based anticancer agents. 2+ 2+  The Pt of the {Pt(NH3)2} unit binds covalently to deoxyribonucleic acid (DNA), more specifically, to the N-7 of either guanine (G) or adenine (A) in the dinucleotide sequences GG and AG to form interstrand cross-links and 1,2- or 1,3-intrastrand cross-links.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

Figure 5. Recently discovered Platinum based anticancer agent

Figure 5 represents some of the antimicrobial and anti-parasitic metallodrugs

One drug Antimony-based The history of ions are that is still used drugs have been bismuth drugs is incorporated into against prescribed against closely connected to surgical wound trypanosomiasis cutaneous and gastrointestinal dressing cloths (sleeping mucocutaneous disorders, but (e.g.,Acticoat) and sickness)today, leishmaniasis (skin bismuth is also catheters (e.g., despite its severe ulcers) coadministered in SilverSoaker) for side effect of the fight against the infection prevention encephalopathy, is bacterium or into textiles for melarsoprol. Helicobacter pylori the treatment of (H. pylori). acute neurodermitis.

Figure 6. Antimicrobial and Antiparasitic Metallodrugs

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry

9. Summary

 Bioinorganic chemistry includes the study of both natural phenomena such as the behavior of metalloproteins as well as artificially introduced metals, including those that are non-essential, in medicine and toxicology.  Metal ions are used in biology as triggers for specific cellular functions, and to regulate gene expression.  In mammals, iron is bound and transported by serum protein transferrin, and it is stored by ferritin.  Hemosiderin is another storage form for iron in organism, in particular during iron overloading.  Calcium pumps are ATPases that transport ions across membranes using energy obtained from the hydrolysis of ATP.  Major cooper-transporting constituents in circulating plasma are Ceruloplasmin, albumin, and transcuprein, and to a lesser extent certain amino acids.  Metal coordination compounds in therapy has open an array of possibilities, which traditional organic or biological molecules cannot fulfill any longer due to growing drug resistance.  Metallodrugs hold still tremendous potential to help mankind overcome drug resistance and to find new cures in medicine.

CHEMISTRY Paper No. 15: Bioinorganic Chemistry Module No. 1: Introduction: Bioinorganic Chemistry