Structural Biochemistry/Protein function/Heme group/Hemoglobin/Association and Dissociation Constant

General information
The prosthetic group consists of an iron atom in the center of a protoporphyrin which is composed of four pyrrole rings that are linked together by a methane bridge, four methyl groups, two vinyl groups and two propinoic acid side chains. Each pyrrole ring consists of one methyl group. Two of the pyrrole rings have a vinyl group side chain, while the other two rings have a propionate group independently. Heme proteins have some iron-porphyrins such as heme a, heme b, heme c, heme d, heme d1, heme o, etc. They are constituted by tetrapyrrole rings but differ in substituents. For example, heme o contain methyl group while heme a contain methyl group, the rest structure are similar between two groups. The difference between hemes assigned each of them different functions.

Heme of hemoglobin protein is a prosthetic group of heterocyclic ring of porphyrin of an iron atom; the biological function of the group is for delivering oxygen to body tissues, such that bonding of ligand of gas molecules to the iron atom of the protein group changes the structure of the protein by amino acid group of histidine residue around the heme molecule. A holoenzyme is defined to be an enzyme with its prosthetic group, coenzyme, its cofactor, etc. Therefore an example of a holoenzyme is hemoglobin with its iron-containing heme group.



Association Constant
Association constant is the constant at which the bonding affinity between two different molecules, the substrate and the product, is at stable equilibrium. An example of such a bonding constant occurs in the hapten-antibody interaction.

Dissociation Constant
Dissociation constant is the quantifiable constant in which a compound, molecule, or ion dissociates. A type of dissociation constant is acid dissociation constant. This constant is used to calculate the occurrence of a weak and strong acid dissociation.

Function
The Heme group gives myoglobin and hemoglobin the ability to bind oxygen because of the presence of iron atom. It also contributes to the red color found in muscles and blood. Each heme group contains an iron atom that is able to bind to one oxygen (O2) molecule. Each hemoglobin protein can bind four oxygen molecules. The iron atom, usually in the ferrous oxidation state (Fe2+), lies between four pyrrole rings but slightly bends away from the plane (0.4 Angstrom from the plane). The iron ion has two extra binding sites called the fifth and sixth coordination sites on each side of the protoporphyrin plane. Usually, the fifth coordination binds with proximal histidine where the sixth coordination binds to an oxygen. When oxygen binds to iron, the iron becomes slightly smaller allowing it to move into the plane of the porphyrin ring. A distal histidine binds to oxygen to make sure reactive oxygen is not released. The distal histidine will not allow the release of oxygen when the Iron is in the 3+ state.

Mechanistic Functionality
The Iron atom is too large in size to fit perfectly inside the porphyrin ring, and sits outside the ring by 0.4 Angstroms. However, upon binding oxygen, the Iron radius shrinks, facilitating a planar alignment with the porphyrin ring. This change causes the proximal histidine bound the Fe atom to be pulled up and cause a structural change to the alpha helix attached to the histidine residue. This alpha helix's carboxyl terminus interacts with the other alpha-beta dimer, creating a total conformational change in the overall protein. The conformational change facilitates an increased affinity for oxygen, which is shown by a transformation from the T to R state in hemoglobin.