12

METALS IN BIOLOGICAL SYSTEM

Metal ions play essential roles in biological systems

12.1 INTRODUCTION

Metal ions have been associated with biological systems for billions of years. The metals are important ingredients in life because some are present in enzymes, in structural features like the Ca in bones, in transport systems like Fe in haemoglobin, in control systems like Na and K in nervous system, etc. For instance, the divalent Mg and Ca ions play important regulatory roles in cells. The divalent metals like Ca²⁺ and Mg²⁺ prevent cytotoxicity and in vivo antagonize Cd-induced carcinogenesis. Hence, metal ions play crucial roles in life processes, and they are also used as inorganic drugs for curing many diseases because of their biocompatibility.

Examples: The cisplatin (cis-[Pt(NH₃)₂]Cl₂) is the first member of a new class of protein anti-tumour drug belonging to metal co-ordination complexes that are being introduced in medicines. Hydroxylapatite [Ca₅(PO₄)₃OH] is used as a substantial material for bone healing and bone grafting.

From the chemical point of view, living organisms have the following twoa types of constituents:

1. Organic components: 90%, carbohydrates, proteins, etc.
2. Inorganic components: 10%, but it has vital activities than organic components.

On the basis of their utility and function in the living system, the elements can be divided into essential and non-essential elements. Of more than 115 known elements, 29 are known to be essential for the growth of at least one biological species and only 19 elements are essential to humans.

12.2 ESSENTIAL ELEMENTS

Essential elements have some unique chemical properties that an organism can use to its advantage and without which it cannot survive. In other words, an essential element is one whose absence results in abnormal biological function or development or even death, and hence, it must be required for life process.

Elements that are necessary to maintain the functions of a whole body are called essential elements. On the basis of the absolute amount present in the body, these essential elements are further divided into bulk elements, macrominerals and micro elements (trace elements).

Essential elements in mammals play vital roles as macrominerals, as structural components (Ca in bones), in transport system (haemoglobin), in control system (Na & K), as catalysts in group transfer reactions or redox reactions (enzymes), etc.

12.2.1 Bulk Elements

Bulk elements are those that are considered primary by most living organisms. They are oxygen, carbon, hydrogen, nitrogen, and sulphur. The building blocks of these compounds constitute our organs and muscles. These five elements also constitute the bulk of our diet, and for instance, more than 10 grams per day of these bulk elements are required for humans.

12.2.2 Macrominerals

Sodium, magnesium, potassium, calcium, chlorine, and phosphorus are referred to as macrominerals because they are essential ions in body fluids and form the major structural components of the body. In addition, phosphorus is a key constituent of both DNA and RNA—the genetic building blocks of living organisms. The six macrominerals present in the body are required in smaller amounts than the bulk elements, i.e., lower levels are required in the diet.

12.2.3 Micro Elements (Trace Elements)

Micro elements are those elements that are required in very small amounts, but these are very essential for maintaining the good health of the organism. These are also known as trace elements or oligo elements. Iron, copper, zinc, cobalt, manganese, molybdenum, iodine and fluoride are very important trace elements. Nickel, vanadium, chromium, selenium, tin and silicon are also required by the body but in very less amount.

Table 12.1 Essential elements

Table 12.2 Essential elements and their importance

Sodium, calcium and chlorine are known as extracellular fluids because they are located in blood plasma. Sodium and potassium play important roles in the maintaining of normal hydration and osmatic pressure. Potassium, magnesium and phosphate are known as intracellular fluids.

12.3 NON-ESSENTIAL ELEMENTS

These elements are not useful for biological activities. Their functions in biological systems are yet to be known are non-essential elements.

12.4 IMPORTANT METALS IN BIOLOGICAL SYSTEMS

12.4.1 Haemoglobin

Haemoglobin (Hb) is the red pigment mainly found in erythrocytes, i.e., red blood cells of human beings. The normal concentration of the Hb in Males is 14–16 g/dl and females 13–15g/dl.

Structure of Haemoglobin

Haemoglobin is a conjugated protein. It consists two parts:

1. Protein part is known as globin, and it is a polypeptide chain.
2. Non-protein part is known as haem, and it contains porphyrin ring chelated with central Fe metal atom.

Structure of Globin

Globin consists 4 polypeptide chains of two different primary structures. In these four polypeptide chains, two α-chains and two β-chains. Each α-chain contains 141 amino acids, whereas β-chain contains 146 amino acids. Therefore, haemoglobin has 574 amino acid residues. The four subunits of haemoglobin are held together by non-covalent interactions such as hydrophobic, ionic and hydrogen bonds. Subunits contain a haem group.

Structure of Porphyrin Ring

Porphyrin is a rigid, square planar molecule made of four pyrrole rings. (Pyrrole ring is a five-membered ring containing N atom.) If a metal atom enters into the ring, then it is called metalloporphyrin. When metal atom enters into the ring, it displaces two hydrogen atoms from the two nitrogen of 2 pyrrole rings.
The metal forms 2 covalent bonds with the 2 nitrogen atoms and 2 co-ordinate covalent bonds with another two nitrogen atoms of pyrrole ring.

Structure of porphyrin

If central metal atom of porphyrin is Fe, then it is haem, and if the central metal atom is magnesium, then it is chlorophyll.

Structure of Haem

The haem contains a porphyrin, namely protophyrine with iron as its centre. porphyrin consists of four pyrrole rings to which four methyl, two propinyl and two vinyl groups are attached. The iron atom is in ferrous state, it can form six co-ordinate sites. The Fe is centred and bonded by the four nitrogen atoms of porphyrine ring. The other two bonds are formed on either side of the planar porphyrin ring. On one side, iron binds with amino acid residue (histidine of the globin). On another side, the co-ordinated position of Fe⁺² is available to bind oxygen. Haemoglobin contains four haem groups and four polypeptide chains. In these four groups, two α, and two β. Haemoglobin is used for oxygen transport.

Structure of haemoglobin

Physiological importance of haemoglobin is uptake of O₂ in the lungs and carries it to the tissues through blood circulation and takes out CO₂ from the tissues and carries it to the lungs.

12.4.2 Myoglobin

The myoglobin (Mb) is one of the metalloproteins. The role of myoglobin is to store the oxygen and release it for metabolic processes. In addition, it may serve as an oxygen reservoir from which the organism can draw oxygen during fast metabolic activities.

Myoglobin molecular weight is 17,800 g. The active centre of myoglobin is made of haem unit. It consists of macrocyclic ligands known as porphyrin, and the iron atom is bound in the cavity of porphyrin. The porphyrin macrocycles is made up of four pyrrole rings with conjugated double bonds. The periphery of macrocyclic ring can have several substituents. These substituents can be used to tune the stereoelectronic properties of the complex. The inside of porphyrin ring has four nitrogen atoms, two of which is attached to hydrogen atoms.

These hydrogen atoms can be quite readily deprotonated to generate a dianionic ligand. In this form the ligand can readily bind to a number of divalent metal ions. The distance from centre of the porphyrin ring and nitrogen atom is about 2.0 Å, which corresponds to normal M-N distance involving first row transition metal ion. Because of the delocalization of electron over the entire frame work, the ring is planar and rigid.

However, it can become non-planar to accommodate certain metal ions, which are either too small or too large to be fitted exactly in the cavity of the porphyrin ligand. Because of this versatility, in terms of ligand properties, the porphyrin ligand is present in a variety of biologically important systems.

Myoglobin contains only one haem and one (∝) polypeptide chain. Peptide chain contains 153 amino acids. It is used to store the O₂ molecules.

Function of Haemoglobin and Myoglobin

Haemoglobin and myoglobin exists in two forms, i.e., de-oxy form and oxy form.

Haemoglobin

In de-oxy form, four co-ordinates sites of iron are occupied by four nitrogen atoms, fifth co-ordinate site is occupied by histidine residue. The sixth position is weakly bounded by water molecule. De-oxy haemoglobin is said to be T-state.

On opposite side of the proximal histidine, there is one more histidine group placed near to the Fe ion. It forces the binding of di-oxygen in end on bent conformation.

Haemoglobin co-ordinated to oxygen is called oxy haemoglobin. It is also referred to as R-State. In oxy-haemoglobin state, the sixth co-ordinate site occupied by O₂ in end on bent geometry.

Functioning Mechanism of Haemoglobin

In de-oxy haemoglobin, the porphyrin ring is dome shaped; the Fe (II) is in high spin state and paramagnetic. Its size is 0.78 Å and it occupies above the plane of the porphyrin ring. De-oxy haemoglobin is formed at muscles and tissues to supply oxygen.

However, oxy haemoglobin iron size is reduced to 0.61 Å and can easily fit into the cavity of planar porphyrin ring; hence, it moves into the cavity of porphyrin ring with concomitant dragging of proximal histidine that triggers the conformational changes in other globin subunits and thus by opening other haem sites. As a result, the binding capacity of other haem irons with O₂ is enhanced. This is the best example for co-operativity interaction. At lungs, oxy haemoglobin structure can bind with oxygen easily.

12.4.3 Vitamin B₁₂

Structure Details

Vitamin B₁₂ is the only known biomolecule with a stable organometallic bond (i.e., carbon-metal bond). The molecule contains corrin ring that consists of 4 pyrrole subunits with various side groups. The 4 pyrrole subunits are joined on the opposite side by a C—CH₃ methylene link, on one side by a C—H methylene link and with the two pyrrole subunits joined directly; it is porphyrin but one of the bridging methylene groups is removed. In each pyrrole, the nitrogen is co-ordinated to the central metal atom. The sixth ligand is a nitrogen of 5,6 dimethyl benzimidazole, and it is below the ring. The other nitrogen is linked to a five-carbon sugar, which in turn linked to a phosphate group and then back onto the corrin ring via one of the seven amide groups attached to the periphery of the corrin ring. The base ligand forms a ‘strap’ back onto the corrin ring. When compared to the porphyrin the corrin is more flexible and also less flat. The corrin only has a conjugated chain around the part of the ring system, whereas a porphyrine is delocalised around the whole four pyrrole rings.

Vitamin B₁₂, common name is cynocobalamine, is a water-soluble vitamin with key role in the normal functioning of the brain, nervous system and in the formation of red blood cells (RBC). It contains the biochemically rare element cobalt and 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, and also fatty acid synthesis and energy production.

Medical Uses of Vitamin B₁₂

1. It is used to treat vitamin B₁₂ deficiency.
2. It is used to treat cyanide poisoning.
3. It is used to treat hereditary of transcobalamin II.
4. It is used as a part of the Schilling test for detecting pernicious anaemia.
5. High vitamin B₁₂ level protects the elderly individuals against shrinkage associated with Alzheimer’s disease and impaired cognitive function.
   

Here, R = 5′ deoxyadenosyl, Me, OH, CN

12.4.4 Chlorophyll

Chlorophyll is a green color pigment in cyanobacteria, algae and plants. In plants, the chlorophyll is found in chloroplasts. Chlorophyll is extremely useful for photosynthesis.

Structure of Chlorophyll

Chlorophyll structure is similar to other porphyrin pigments such as haemoglobin. It has Mg containing protoporphyrin in which the Mg metal is placed at the center of the porphyrin ring. Mg metal is bonded with four nitrogen atoms of porphyrin ring. When the metal enters into the ring, it displaces two protons and bonds with four nitrogen atoms of pyrrole ring to form the complex. The four nitrogen atoms are coordinated with Mg²⁺ by two normal covalent bonds and two coordinate covalent bonds. Chlorophyll contains a fused cyclopentanone ring in addition and keto group as substituents. Chlorophyll contains methyl group at 1, 5, 8 positions, Vinyl group at 2nd position, Ethyl group at 4th position and a phytol chain at 7th position.

Different types of chlorophylls like chlorophyll-a, chlorophyll-b and chlorophyll-c are found. Chlorophyll-a and chlorophyll-b differ from each other with respect to the substituents groups at position three of ring B. Chlorophyll-a contains CH₃ group at position 3, whereas chlorophyll-b contains CHO group at position 3.

12.5 METALS AND THEIR TOXICITY

The heavy meals are cumulative and potentially toxic. They cause damages to various organs like kidney and gastrointestinal tract (GIT). The toxic character of metals is either due to:

1. their combination with sulphydryl (-SH) group’s essential enzymes to form ring compacts and thus inactivating the enzymes or
2. complex formation with amino, imidazole, phosphate and carboxyl groups.

Some of the common toxic metals are As, Pb, Hg, Sb, Ag, Au, Cu, Fe, etc.

12.5.1 Toxicity of Arsenic

Arsenic (As) is a well-known poison. As poisoning is mainly due to the following two types:

1. Inorganic arsenicals: These are mainly used as rodenticides, herbicides and insecticides. These inorganic arsenicals cause anaemia, leukaemia, nervous, disorders and syphilis.
2. Organic arsenicals: These are mainly used as chemotherapeutic agents. Trivalent arsenicals are more toxic than the pentavalent arsenicals.

Arsenic Poisoning

Arsenic poisoning is usually homicidal, but it may occur accidently, particularly, in children by ingestion of arsenical herbicides or insecticides. It may be either acute or chronic poisoning.

1. Acute arsenic poisoning: It is characterized by severe gastrointestinal irritation, vomiting, diarrhoea and renal failure. The gastrointestinal symptoms usually begin within 1 hour and sometimes it may be delayed up to the severe poisoning, which in turn leads to death within 24 hours.

   Treatment

   1. The correction of fluid and electrolyte imbalance.
   2. Administration of dimercaprol, also called British anti-Lewisite or British antilewisite.
   3. Prophylactic antibiotic therapy is advanced to prevent secondary infection.
   4. Hemodialysis may be necessary.
2. Chronic arsenic poisoning: It is commonly adopted for homicidal purpose. The symptoms are weight loss, anorexia and diarrhoea. The later symptoms are loss of hair, finger nails become brittle, Jaundice, aplastic anaemia and liver cirrhosis.

   Treatment

   Prolonged administration of dimercaprol. It contains two -SH groups that combine with the metal ions to form relatively non-toxic and easily water-soluble complexes which are excreted via urine.

   

12.5.2 Toxicity of Lead

Metallic lead (Pb) and its soluble compounds are highly toxic. Lead poisoning is an occupational disease, which is usually chronic. Occupations involving lead poisoning are ceramic industries, house painting, copper welding, printing, plumbing and storage battery manufacturing. Flaking, lead-based paints and toys painted with lead compounds are considered as serious health hazards for children. Lead poisoning is also caused by chemotherapy.

Lead Poisoning

Lead poisoning is of the following two types:

1. Acute lead poisoning: It is rare and charaterized by gastrointestinal irritation, muscle cramps and weakness. Acute haemolytic crisis results in severe anaemia and haemoglobinuria.

   Treatment

   1. Prompt gastric lavage.
   2. Use of calcium and phosphate salts and chelating agents for elimination of lead from circulation.
   3. Use to antispasmodics like atropine.
2. Chronic lead poisoning: It is commonly observed in young children, due to licking lead paints or leaded toys, and also found in workers engaged in printing, paint and petroleum industries. The symptoms are metallic taste in mouth, abdominal pains, appearance of greyish lead line along the gingiual margin, muscle weakness, paralysis, irritability, headache and mild renal damage.

   Treatment

   Use of calcium disodium EDTA—the calcium chelate of the disodium salt of EDTA is used for lead poisoning and is excreted via urine.

   

   Dimercaprol is also useful for the treatment of lead poisoning.

12.5.3 Toxicity of Mercury

Inorganic mercurial compounds are used as syphilo therapeutic agents and laxatives. The organic mercurials are used as antiseptics, preservatives, spermicides and diuretics.

Mercury Poisoning

Mercury poisoning is of the following two types:

1. Acute poisoning: It is usually due to accidental intake of mercury chloride and other ionizable mercury salts.

Symptoms

Severe GIT irritation and peripheral circulatory collapse, metalic taste in mouth, excessive salivation and renal impairment

Treatment

1. It is treated by the administration of protiens in the form of raw eggs or milk.
2. By giving medicinal charcoal suspension.
3. By gastric lavage.
4. By administration of dimercaprol within one hour of intake.

12.6 REVIEW QUESTIONS

12.6.1 Fill in the Blanks

1. Hb contains __________ number of polypeptide chains.

   [Ans.: 2α and 2β]

2. α-polypeptide chain contains __________ amino acids in Hb.

   [Ans.: 141]

3. Hb contains __________ rings.

   [Ans.: 4 pyrrole]

4. __________ is the active centre in Hb.

   [Ans.: Fe (iron)]

5. Central metal atom in vitamin B₁₂ is __________.

   [Ans.: Co]

6. Central metal atom present in chlorophyll is __________.

   [Ans.: Mg]

7. In haemoglobin, the protein part is __________ and the non-protein part is __________.

   [Ans.: globin, haem]

8. Haem consists of __________ rings.

   [Ans.: 4 pyrrole]

9. At __________ co-ordinate site, O₂ binds in Hb.

   [Ans.: sixth]

12.6.2 Multiple-choice Questions

1. Elements which required in very small amounts to the body are called
   1. Essential
   2. Non-essential
   3. Macroelement
   4. Micro element

   [Ans.: d]

2. Which of the element involves in electron transfer?
   1. Ca
   2. Fe
   3. Mg
   4. Both (b) and (c)

   [Ans.: d]

3. Number of polypeptide chains present in Hb
   1. 4
   2. 3
   3. 2
   4. 1

   [Ans.: a]

4. Number of haem groups present in myoglobin
   1. 4
   2. 3
   3. 2
   4. 1

   [Ans.: d]

5. In haemoglobin, the protein part is
   1. Haem
   2. Globin
   3. Both (a) and (b)
   4. None of these

   [Ans.: b]

6. Number of amino acid residues present in Hb
   1. 574
   2. 572
   3. 141
   4. 146

   [Ans.: a]

7. Oxidiation state of Fe in oxy-Hb is
   1. +2
   2. +3
   3. +1
   4. Both (a) and (b)

   [Ans.: a]

8. Which of the following are the toxic metal elements?
   1. As, Pb, Hg
   2. Na, K, Cl
   3. H, O, C
   4. None of these

   [Ans.: a]

9. The types of toxicity are
   1. Chronic
   2. Acute
   3. Both (a) and (b)
   4. None of these

   [Ans.: c]

10. The role of chlorophyll is
    1. Oxygen carrier
    2. Photosynthesis
    3. Electron transfer
    4. Neuromuscular irritability

    [Ans.: b]

12.6.3 Short Answer Questions

1. What is the importance of sodium, potasium and chlorine in human body?

   Ans.:

   1. Sodium, potassium and chlorine are used to maintain the normal hydration and osmatic pressure.
   2. They are used to maintain the normal acid–base equilibrium.
   3. They are used to maintained neuromuscular irritability.
2. Give an account on physiological importance of haemoglobin?

   Ans.:

   1. Haemoglobin uptake O₂ from the lungs and carries it to tissues through the blood circulation.
   2. Hb takes out CO₂ from the tissues and carries it to lungs.
3. Describe mercury poisoning?

   Ans.: Gastrointestinal track irritation, peripheral circulatory collapse and renal impairment.

4. Define metal classification in biological system?

   Ans.: The living body contains 29 elements. These can be divided into two main groups:

   1. Essential elements
   2. Non-essential elements

   Essential elements are again classified into two types such as macroelements and micro elements.

5. Draw the structure of haemoglobin?

   Ans.:

   
6. Describe the medicinal uses of vitamin B₁₂?

   Ans.: Vitamin B₁₂ is used to treat vitamin B₁₂ deficiency, cyanide poisoning and hereditary of transcobalamin II.

12.6.4 Descriptive Questions

Q.1 Describe haemoglobin with neat sketch.

Q.2 Give detailed explanation about metal toxicity.

Q.3 Explain structure and function of myoglobin.

Q.4 Write short note on the following:

1. Vitamin B₁₂
2. Chloropyhll

Q.5 Explain the biological importance of metals in detail.

Q.6 Give brief note on essential and non-essential elements and their importance in the human body.