heme-binding site

Introduction

Globins are heme-containing proteins involved in binding &/or transporting oxygen.

Cytochrome P450's are heme proteins. A conserved Cys in the C-terminal part of cyt P450's is involved in binding the heme iron in the fifth coordination site.

Cytochrome b5 is a membrane-bound hemoprotein which acts as an electron carrier for several membrane-bound oxygenases. Two conserved histidine residues serve as axial ligands for the heme group. The structure of a number of oxidoreductases consists of the juxtaposition of a heme-binding domain homologous to that of b5 & either a flavodehydrogenase or a molybdopterin domain.

In proteins belonging to cytochrome c family, the heme group is covalently attached by thioether bonds to 2 conserved Cys. The consensus sequence for this site is Cys-X-X-Cys-His & the His is one of the 2 axial ligands of the heme iron.

The heme prosthetic group of peroxidases is protoporphyrin IX & the 5th ligand of the heme iron is His (proximal histidine). Another His (distal histidine) serves as an acid-base catalyst in the reaction between hydrogen peroxide & the enzyme. The regions around these 2 active site residues are more or less conserved in a majority of peroxidases.

Each of the subunits of catalase binds one protoheme IX group. A conserved Tyr serves as the heme proximal side ligand. The consensus includes a conserved Arg that participates in heme- binding. A conserved His is important for catalysis.

More general terms

• cofactor-binding site