HEME
deoxy and oxy-
and proximal (F8) and distal  (E7) His residues

Java version

HTML 5 version (does not require Java; downloads and moves slowly)

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I. Introduction

Heme consists of tetrapyrole ring structure (protophorphorin IX) with a Fe²⁺ ion bound in the center, coordinated to the nitrogens of the pyrole rings.

The first structure that appears on your screen shows dioxygen bound to the iron in the heme. Notice the Fe²⁺ is in the heme plane.

The proximal His (F8) and distal His (E7) from myoglobin or hemoglobin are also shown.  F8  provides the 5th coordinating ligand to the iron ion.  

For more information see Biochemistry Online: Chapter 5C - Model Binding Systems

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II. General Structure

O₂-heme ball/stick drawing
looking along heme plane

The geometry around the iron ion is octahedral 

O₂-heme drawing facing heme plane

Click on the second structure which shows deoxyheme. The iron ion is out of the heme plane, having square planar geometry. 

Deoxyheme Heme

Notice the square pyramidal geometry around the iron ion.