Dr. Shumaila Asim Lecture # 1 Structure of Heme Dr. Shumaila Asim Lecture # 1

Hemoproteins Hemoproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+) . The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring.

Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+)

Hemoproteins Hemoglobin (Hb) Myoglobin (Mb) Cytochromes Catalases (decomposition of 2 H2O2 to 2 H2O and O2) Peroxidases

Globular proteins Amino acid chains fold into shapes that resemble spheres are called globular proteins This type of folding increases solubility of proteins in water Polar groups on the protein’s surface Hydrophobic groups in the interior Fibrous proteins are mainly insoluble structural proteins

Four Levels of Protein Structure Primary, 1o the amino acid sequence Secondary, 2o 2-D arrangement of backbone atoms in space Tertiary, 3o 3-D arrangement of all the atoms in space Quaternary, 4o 3-D arrangement of subunit chains

Globin of hemoglobin is a globular protein with a quaternary structure

Globular proteins Hemoglobin: oxygen transport function Myoglobin: oxygen storage/supply function in heart and muscle α1, α2, β-globulins: various functions γ-globulins (immunoglobulins): immune function Enzymes: catalysis of biochemical reactions

Hemoglobin Contains two dimers of αβ subunits A major globular protein in humans Composed of four polypeptide chains: Two α and two β chains Contains two dimers of αβ subunits Held together by non-covalent interactions Each chain is a subunit with a heme group in the center that carries oxygen A Hb molecule contains 4 heme groups and carries 4 molecules of O2

Heme is the prosthetic group of hemoglobin, myoglobin, & cytochromes Heme is the prosthetic group of hemoglobin, myoglobin, & cytochromes. Heme is an asymmetric molecule. E.g., note the positions of methyl side chains around the ring system.

Heme structure Heme is a metaloporphyrine (cyclic tetrapyrrole) Heme contains: conjugated system of double bonds → red colour 4 nitrogen (N) atoms 1 iron cation (Fe2+) → bound in the middle of tetrapyrrole skelet by coordination covalent bonds methenly bridge pyrrole ring

Properties of iron in heme Coordination number of iron in heme = 6 6 bonds: 4x pyrrole ring (A,B,C,D) 1x link to a protein 1x link to an oxygen

Properties of iron in heme

Myoglobin (Mb) is a single-chain globular protein of 153 AA, containing 1 heme group transports O2 in skeletal and heart muscle is found in cytosol within cells is a marker of myocard damage

Iron Iron can bind in the center of the four rings. Fe is in the ferrous state (Fe2+) can form 6 bonds: 4 with the nitrogen of the rings, One (known as the fifth coordinate) with the nitrogen of a histidine imidazole (known as proximal His). One with O2 (the sixth coordinate) Oxidation of iron to the Fe3+, ferric, state makes the molecule incapable of normal O2 binding

Structure-function relationship The planar heme group fits into a hydrophobic pocket of the protein and the myoglobin-heme interaction is stabilized by hydrophobic attractions. The heme group stabilizes the tertiary structure of myoglobin. The hydrophobic interior of myoglobin (or hemoglobin) prevents the oxidation of iron, and so when O2 is released, the iron remains in the Fe(II) state and can bind another O2.

The distal histidine acts as a gate that opens and closes as O2 enters the hydrophobic pocket to bind to the heme

Structure of heme prosthetic group Two hydrophobic side chains on O2 binding site of heme help hold it in place oxygenation changes state of Fe Purple to red color of blood, Fe+3 – brown Oxidation of Fe+2 destroys biological activity of myoglobin Physical barrier of protein is to maintain oxidation state of Fe+2