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Coordination Complexes and Biological Systems

Introduction

Metals are an essential part of the coordination complexes. It is the presence of the metals which provide the surface area for the ligands to coordinate with them. An average adult who has a medium built with a body weight 70 kg contain 43 kg water and 4 kg minerals. About 40 naturally occurring minerals are present in the living bodies such as plants and animals.

The minerals that are essential for the healthy human life must contain.

Essential Metals

  • 1. Na, K, Mg, Ca, Mn, Fe, Co, Cu, Zn, Mo.
  • 2. Some other beneficial metals include : Li, V, Cr, Ni, Sn, and W
  • 3. Non-metals'. H, C, Si, N, О, P, S, Se, F, Cl, Br and I.
  • (a) Out of the above mentioned elements some elements are available in abundance such as : H, О, C, N, Na, K, Ca, Mg, P, S and Cl
  • (b) the less abundant elements are: Mn, Fe, Co, Cu, Zn, and I.
  • (c) The elements occurring in trace amount are: Si, V, Cr, Ni, Sn, F, Br, and Se.

In plants the role of boron, aluminum and barium is also important. The most important function of the metals is their ability to form complexes. Sodium and potassium play the role of charge carries and help to maintain osmotic pressure on either side of cell wall. These ions also maintain the sensitivity of nerves and muscle. Sodium ions are present in the blood plasma of verteberates while the f uid contain КГ ions in the cytoplasm.

There are 50 enzymes which are activated by charged КГ. The reactions catalyzed by K+ are phosphoryation or elimination of—COOH group that are important in protein synthesis and respiration. Na+ inhibits respiration and activate few enzymes.

Magnesium and calcium are required for structure formation and to start reaction Ca2+ maintains the correct rhythm of heart beat and conversion of fibrinogen to fibrin. Calcium is used in cell wall structures which are manifested as shell and bones.

It is also used to stabilize these structures e.g. in thermolysin where there are four Ca2+ ions. Mg2+ ions form complexes with nucleic acids and help the nerve impulse transmissions. In animals the tissues are complexed with ADP and ATP. Mg is also an essential constituent of chlorophyll in green plants.

Manganese activates all those enzymes which magnesium does—e.g. kinases synthases and dehydrogenases. It can activate some RNA and DNA polymerases, guanylate cyclases. It is involved in the synthesis of glycoproteins in the formation of cartelage and bone. The different valencies of manganese are observed in Mn-enzyme function.

There are innumerable iron proteins but to date 70 are isolated. They perform the same function but-differ in molecular mass due to polymerisation.

Iron proteins contain porphyrin, flavins or ptyrins and sometimes other metals such as Mo, Cu or Ni. Iron proteins have been divided into two categories.

  • (a) Proteins that are having sulphide groups. They may be labile or non- labile.
  • (b) Proteins that do not have sulfur. The later groups may be sub divided into (i) Iron complexed by porphyrin (heme) (ii) those group which do not contain heme and are oxo-bridged with dinuclear iron with bridging corboxylates. The Fe-S proteins contain Fe S4, Fe,S, or Fe4S4 or with mixed metals like Fe7MoS8 active sites which accept electrons to reduce Fe(III) to Fe(II) in the Fe-S Cluster.

Some common examples of iron proteins are given below:

  • (/) Наше proteins are: Myoglobine, hemeglobin, cytochromes, catalases.
  • (ii) Fe-S proteins and flavin proteins are: Ferredoxine, rabredoxin, hydrogenase, reductase, e.g. nitrite to sulphide reductase, sulphide oxidase, xanthine oxidase, dehydrogenase, molybdeo ferredoxin etc. There are several kinds of hemes characteristic of hemo proteins. Heme (A) Hemoglobin, myoglobin and catalase.

Heme (B) includes cytochromes and proto heme (Heme B), Fe-proto- porphyrin IX. It is bonded through ionic and hydrophobic interactions.

The а-type cytochromes contain Heme A and the e-type cytochromes contain heme C.

Iii the case of Heme C there are two thio ethers bonds. Their structures are:

Protoporphyrin IX

Fig. 11.1: Protoporphyrin IX

The iron in all hemoproteins is either five or six coordinate etc. Becuase protein donates at least one ligand. The donation of sixth ligand from protein has an important bearing on the chemical reactivity of central iron. In hemoglobin and myoglobin the iron is five coordinate. This gives way for oxygen molecule to add reversibly and make the iron six coordinate. If the goblin protein is absent then ferro heme Fe(II) is oxidized to Fe(III) from heme.

Cyanide and CO Poisoning

Heme A

Fig. 11.2: Heme A

Both hemoglobin and

a-cytochrome, a can bond CN“ and CO to the Fe(II) as the sixth ligand.

The fatal effect of cyanide poisoning are linked to inhibition not of hemoglobin but a-cytochrome.

Cyanide is strongly bound to met-hemoglobin than

a-cytochrome and very weakly to hemoglobin. It is removed as blood when filtered through spleen where erythrocytes containing met hemoglobin are destroyed. The fatal effects of cyanide poisoning are linked to inhibition not of hemoglobin but a-cytochrome. Therefore in cyanide poisoning the treatment

is inhalation of amyl nitrite or injection of NaNO0 which oxidises part of hemoglobin to met hemoglobin Fe(III). Met hemoglobin does not bind CO. Therefore CO poisoning the best remedy is to administer O0 for inhalation to displace CO. In hemoglobin and a-cytochromes the iron center of the respiratory p and у cytochromes are six coordinate. These cytochrome do not interact with cyanide or CO and hence are not autoxidised by oxygen.

Heme C

Fig. 11.3: Heme C

 
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