CH3208 Bioinorganic chemistry

Lecture 1

By

Dr. Moumita Mondal

Transport and storage of dioxygen

All the known O2 transport and storage system employ metalloproteins

Dixoygen carrier Hemoglobin (Hb), Hemocyanin (Hc), Hemerythrin (Hr)

The O2 molecules stored in tissues like muscle by myoglobin (Mb) and myohemerythrin

Hb amd Mb are the only heme protein (i. e. these have an iron- porphyrin group), found in human beings and bigger animals Hr is a non-heme protein (although its name carries the term heme) and

this oxygen carrier is found in lower organisms has two non-heme iron atoms in its functional core Hc has two copper atoms.

Ref P. S. Kalsi, J. P. Kalsi, Bioorganic, Bioinorganic and Supramolecular Chemistry

Structure and function of Mb and Hb

heme containing protein (molecular weight ~ 17, 000) and its protein chain consists of a single polypeptide chain of 153 amino acid residues which folds around the heme prosthic group. In both myoglobin and hemoglobin one has heme b-the iron(II) complex of protoporphyrin IX

v

Ref P. S. Kalsi, J. P. Kalsi, Bioorganic, Bioinorganic and Supramolecular Chemistry

Partially buried in hydrophobic

interior

Hydrophobic nature nonpolar side chains of protein ensures reversible oxygen binding by inhibiting permanent oxidation of Fe

Structure of Myoglobin

Structure of Hemoglobin

Deoxy Mb/Hb

Proximal histidine

nitrogen atom (F8)

Fe(II) high spin

Pseudo Oh geometry Tense or T conformational state

Radii of high spin Fe(II) is 92 pm, too large to fit in porphyrin cavity It lies 42 pm above the plane of porphyrin Electron donating coordinated nitrogen atom prevents oxidation of Fe

Ref P. S. Kalsi, J. P. Kalsi, Bioorganic, Bioinorganic and Supramolecular Chemistry

Oxy Mb/Hb

Pseudo Oh geometry Relaled or R conformational state

Fe(III) low spin

Radii of low spin Fe(III) is 75 pm, fits into porphyrin cavity

Overall result is vibration or breathing of protein structure on binding and dissociation of oxygen

Dioxygen is bonded to Fe by end on bent fashion Fe-O-O bond angle 115 o, νO-O 1105 cm-1

HbO2/MbO2 are best represented as Fe(III) coordinated superoxide

Hydrogen bonding

facilitates O2 binding

Vibrational Spectroscopic Evidence that OxyHb and OxyMb are Formally FeIII–O2

- Species

From resonance Raman spectroscopy the O–O stretch in oxyMb is measured to be ~ 1105 cm-1. The protein is also diamagnetic (d5, Fe(III) and O2

- couple).

Reaction of free heme and O2 in aq. Solution

Release of superoxide anion in presence of Nu

Formation of hematin

μ-peroxo complex

ferryl complex

Acid catalysed release of superoxide in presence of water

Met Hb

O2 binding curve for Mb is hyperbolic O2 binding curve for Hb is sigmoidal i. e. positive cooperitivity of binding

At high pO2 Hb and Mb binds

oxygen equally well

At low pO2 Hb is poor binder than

Mb

Transport of dioxygen

Bohr effect carbamyl Hb

Met Hb

Structure and function of Hemerythrin(Hr)

Oxygen binding mechanism of Hr

O2 binds asymmetrically to Hr by end on coordination to only one iron centre

H atom of OH bridge stabilize peroxo ion

Structure and function of Hemocyanin(Hc)

Oxygen binding mechanism of Hc