Subject Chemistry Paper No and Title 15, Bioinorganic Chemistry Module No and Title 19, Oxygen Carriers Module Tag CHE_P15_M19_e-text CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS TABLE OF CONTENTS 1. Learning Outcomes ................................................................................................ 3 2. Introduction ............................................................................................................ 4 3. Oxygen Carriers ....................................................................................................... 4 4. Hemocyanin ............................................................................................................ 4 4.1 Structure ......................................................................................................................4 4.1.1 Active site structure of deoxyhemocyanin ..................................................................... 5 4.1.2 Active site structure of oxyhemocyanin ......................................................................... 5 5. Hemerythrin ........................................................................................................... 6 5.1 Structure ......................................................................................................................6 5.1.1 Active site structure of deoxyhemerythrin .................................................................... 6 5.1.2 Active site structure of oxyhemerythrin ........................................................................ 7 6. Hemoglobin ............................................................................................................ 7 6.1 Structure ......................................................................................................................8 7. Summary .............................................................................................................. 10 CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS 1. Learning Outcomes After studying this module, you shall be able to Know the need for oxygen carriers in living systems. Learn about the different kinds of oxygen carriers in vertebrates and invertebrates. Learn about the structure of oxygen carriers: hemocyanin, hemerythrin and hemoglobin. Understand the role of transition metal ions in these oxygen carriers. CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS 2. Introduction Most living organisms need dioxygen to perform respiration, which is breakdown of food products to release energy. There are two principal ways in which dioxygen is supplied to the cells. One mode of transportation is the blood plasma. However, dioxygen has a very low solubility in water (7.6 mg/L at 20°C and 1atm). The other way of transportation of dioxygen is with help of dioxygen carrying molecules in the blood plasma. This mechanism is capable of transporting large amount of dioxygen. Only about 1.5% of the dioxygen transported by the blood is dissolved directly in the blood plasma, rest is done by oxygen carriers. 3. Oxygen Carriers Oxygen carriers are special types of metalloproteins which have a transition metal from 3d series bound to a protein. Only those transition metals can form dioxygen complexes with O2 which have vacant site/s for binding and exist in higher oxidation states also. The common metal ions are iron, copper and vanadium and the commonly encountered stoichiometries are 1:1 or 1:2 . The metal binds reversibly to dioxygen without the metal or ligand being irreversibly oxidized. More broadly, a complex which can uptake dioxygen and can release it reversibly under ordinary conditions can be called a biological oxygen carrier. The extent of binding will depend on temperature, partial pressure of O2 and pH. nML + O2 ⇌ MLn O2 In the above ML acts as a dioxygen carrier. In order for transition metal complexes to form dioxygen complexes there must be a vacant site (or sites) for binding and an accessible higher oxidation state (or states). There are three known classes of dioxygen transport proteins. These are: 1. Hemocyanins 2. Hemerythrins 3. The Hemoglobin- Myoglobin family 4. Hemocyanin Hemocyanins (or haemocyanins) are oxygen carrying proteins/oxygen carriers in invertebrates such as molluscs (eg octopus, snails, squids) and arthropods (eg scorpions, crabs, lobsters etc). It is extracellular protein and is present in hemolymph. 4.1 Structure Although the oxygen binding sites and function of molluscan and arthropod hemocyanins are quite similar, they are different in molecular structure at all levels. Arthropod hemocyanins are multiples of hexamers, each hexamer made of monomers (Figure 1) Molluscan hemocyanin occur as larger assemblies. CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS Figure 1: Single Oxygenated Functional Unit from the hemocyanin of an octopus 4.1.1 Active site structure of deoxyhemocyanin Hemocyanin is a copper containing metalloprotein. Each monomer contains two cuprous ions [Cu(I)] that reversibly bind one dioxygen. An empty cavity is present between the two cuprous ions to accommodate the dioxygen. The Cu(I)- Cu(I) bond distance is 460 pm. The coordination number of each Cu(I) is three and is satisfied by three histidines residues from the protein. This results in a distorted trigonal pyramidal geometry. Two phenylalanine residues which are in close proximity to the histidines residues provide a hydrophobic environment at the active site. 4.1.2 Active site structure of oxyhemocyanin The binding of dioxygen to the Cu(I) at the active site leads to the following changes Cu(I) is oxidized to Cu(II). 2- O2 is reduced to O2 . Colour of protein changes from colorless to blue. Coordination number of copper changes to five from three. Geometry of copper changes to square pyramidal from trigonal pyramidal. The equatorial plane has two histidyl imidazole nitrogens, the bound oxygens and the third histidyl nitrogen is axially coordinated to copper. The Cu-Cu distance decreases to 360pm. In hemocyanin oxidative addition of dioxygen occurs. Evidence for the peroxo linkage comes from Raman spectroscopy. The v(O-O) stetch is observed at 744 cm-1 confirming the presence of peroxo linkage ( Theoretical stretching frequency for peroxo is 740 to 930 cm-1). Experiments using asymmetrically labeled dioxygen 18O-16O proved conclusively that the coordination of dioxygen to Cu(II) is symmetrical (Figure 2). CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS Figure 2: Binding of Dioxygen to hemocyanin 5. Hemerythrin Hemerythrin (Figure 3) is a reversible oxygen binding metalloprotein found in blood cells of a few marine invertebrates. It is colorless in the deoxy form and on oxygenation the color changes to purple-red. Figure 3: Single Oxygenated Hemerythrin protein 5.1 Structure It is an oligomeric protein generally found in an octomeric form. The dimeric, trimeric and tetrameric forms of hemerythrin are also known. 5.1.1 Active site structure of deoxyhemerythrin Each monomeric unit contains an active site which has two high spin ferrous ions [Fe(II)]. The ferrous ions are bridged together by a hydroxyl group and two carboxyl groups from an aspartate residue and a glutamate residue of the protein chain of 115 amino acid residues. One of the ferrous is hexacoordinated with an octahedral geometry and the other is pentacoordinated with a distorted trigonal bipyramidal geometry. The remaining coordination sites of hexacoordinated ferrous and pentacoordinated ferrous are satisfied by three and two imidazole nitrogens respectively from histidines residues of the protein chain (Figure 4) CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS Figure 4: Active site of deoxyhemerythrin 5.1.2 Active site structure of oxyhemerythrin One monomeric unit of hemerythrin binds one dioxygen. The dioxygen adds to hemerythrin in an oxidative manner resulting in the formation of Fe(III) and peroxide. The dioxygen adds to the coordinatively unsaturated ferrous. The oxidative addition is followed by the shifting of proton - from the bridged OH to the bound peroxide resulting in the formation of hydroperoxo (HO2 ) group. The hydroperoxo group is hydrogen bonded with the µ-oxo group. Figure 5 : Active site of oxyhemerythrin Support for the above structures has been experimentally obtained. By use of radioisotope experiments, it was established that dioxygen binds asymmetrically in oxyhemerythrin. Single crystal X-ray diffraction study of oxyhemerythrin showed end –on coordination of dioxygen to only one iron. 6. Hemoglobin Hemoglobin is a globular, iron containing metalloprotein with a quaternary structure (64*55*50 Å). It is found in red blood cells (RBC) of mammals and other animals. It transports dioxygen from the lungs to the tissues, where it is used to oxidize glucose. This process serves as a source of energy required for cellular metabolic processes. Hemoglobin also plays a role in transport of carbon dioxide and hydrogen ions. Hemoglobin increases the dioxygen carrying capacity of 1L of blood from 5mL to 250mL. The dioxygen binding capacity of hemoglobin is 1.34mL O2 per gram CHEMISTRY PAPER No. 15: BIOINORGANIC CHEMISTRY MODULE No. 19: OXYGEN CARRIERS of hemoglobin. Each RBC has 640 million molecules of hemoglobin. Hemoglobin synthesis occurs in the developing RBCs in the bone marrow. Hemoglobin has many firsts to its credit. It is the first protein to